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Vibrational analysis of the first ultraviolet band system of aniline
JOURN.lL
OF
MOLECULAR
Vibrational
JOHN
c.
20, 359-380
SPECTROSCOPY
(1966)
Analysis of the First Ultraviolet System of Aniline
D. BRAND, DENIS
Departwzenf of C’hemisiry,
R.
WILLIAMS,
Vanderbilt
AND T~o~rls
CTniversit,y, it’ashville,
Band
J.
COOK
l’ennessee
Vibrational struct,ure associated with the 2940-i hand system of aiiilinc vapor has been analyzed in some detail, aiid resit1t.s are incliided for the -IL? , -d5 , and -d, isot.opes. It. is shown that t.hc w(NH2) vit,ration is strortgly anharmonic in the ground state, having the small-large alteriiation of qrtanta characteristic of a vihratioti occurring in a dotthle-minimttm pot,ential. The data are consistent, with an angle of about Iti0 het,ween the ring-to-x bond and the NH, plane in the electronic groturd state. The same vibration is not harmonic in t,he upper electronic state, thottgh its anharmonic character is less strongly marked there than in the ground state. The data for the upper state are compatible with a douhle-minimrtm pot,ential having a very small maximum for the all-coplanar configuration, bait the evidence is not conclusive. All that can be said definitely is that #V(Q)~$ # 0 for a considerable range of q to eit,her side of q = 0. The term gtasiplanar is proposed to cover this situation, a febv ot.her examples of which can be fotmd in the literat.ure. A complete classification of the vihronic sublevels of aniline is accomplished ttsing a group Gg of order eight; hut the torsional splittings are not resolved in the electronic spectrum so that the practically useful group is GJ , a sttb group of G8 G1 is isomorphous with the point group clzV. The vibronic selection rules are discussed. I. INTROnUCTION Aniline vapor has a rich system of bands in the ultraviolet, running in ahsorption from about 3000 8 towards shorter waves (1). Rotational and vibrational structure are well defined near the origin (294OA) and in hot bands, though the appearance of the spectrum deteriorates t,owards higher frequencies. The f-value is about 0.026 in solution (Z), about 20 times that. of the related ‘Bz,, + ‘4, transition of benzene. In this paper we are concerned mainly with a vibrat’ional analysis of the bands of aniline and the question whether the molecule is planar or nonplanar in the states combined by the transition. It will be shown that there is definite physical evidence for a nonplanar ground state in which the NH2 group is inclined to the ring plane at an angle of about 46”. There are many precedents for taking aniline to be nonplanar in the ground state. For example, dipole moment relaxation studies (3), Kerr-constant meas359
360
BRAND, WILLIAMS,
AND COOK
uren1ent.s (,G), and the dipole moment of phenylene diamine equal to 1.3 II (6) all imply that the NH2 group occupies a plane different from t’hat’ of the phenyl. ring. The microwave spectra of some conjugated amino derivatives point defi-, nitely to pyramidal st)ructure. Formamide (6), cyanamide (7), nitramide (8) all. show inversion doubling. Only for formamide and nitramide, however, has the degree of planarit)y been given quantitatively: in the former the amino group occupies a plane angled at about 10” to t’he CHO plane (twisting is present also)., and in the latter the substituent plane lies at 52” to that of t’he ISO2 fragment Xonplanar aniline is a nonrigid molecule having four symmet’rically-equivalent. conformations converted into one another by int’ernal rotation through r about t,he ring-to-N bond, or by “inversion” of the pyramidal structure. Each conformat,ion gives rise to the same set of energy levels, giving rise to a degeneracy which is lifted to some extent when interconversion by tumlelling is taken into account’. Every energy level of aniline considered as a rigid structure therefore divides int*o four sublevels when nonrigidity is taken into account. n’onplana~ aniline has only one element of point symmetry, a plane perpendicular IO the ring plane, and so belongs to the point group C, ; but in order to label the 4 sub levels arising from tunnelling it is clear in a general way that a higher group of order eight is called for. The paper is therefore developed as follows. We first; examine the group-theoretical classification of the energy levels of nonrigid aniline and derive selection rules for t,he transitions between them. The succaerding sections contain the experimental details and the vibrational analysis. Finally we give a brief summary of the conclusions that can be drawn. A short ncc*ount of this work has already been published (9). IT. DETERMINATION
OF SELECTION
RULES AND
HPMMETRY
81’1~:(‘11~;5
As aniline is a nonrigid molecule, its symmetry classification must take account of split#tings arising from t’he presence of equivalent geometrical conformations. Turmelling between t,hese conformers by inversion and internal rotation divides each rovibronic level into four sublevels of the same species, A’ or A”, in the C’,, point group. To distinguish these sublevels one from another requires a group of order eight,. The sublevel classificat,ion can be performed using the molecular s!ymmetry group (IO) constructed from the operations, (i) all feasible P, inc*luding IS:, (ii) all feasible P*, not necessarily including E”. P is any permutation of the positions and spins of identical nuclei or any product of such permutations; E: is the idcnt)ity, E* the inversion of all particle positions, and P* = PE” = E*P. Which operations are deemed feasible has been discussed by Watson (11). These operations for aniline are given in Fig. 1 and form a group designated Gu , iso-morphous with the D2h point group. When the operations are arranged in the order E, (ab) (AB), (AR), (ab), (ab)*, (LIB)*, (ab) (AI?)‘, and E* the molecular group character table is identical wit,h the usual arrangement of the DC,, point
SPECTRUM
a
361
OF ANILINE
b
X
8
e/
CAB)
A B
b
Y
,’
(ab)
4
‘\
E*
Q
a.
b
Q
(QD)*
b
(d*
FIG. 1. Symmetry elements of Gc+.(ab) exchanges the amino-hydrogens; taneously exchanges O, o’, and m, m’-carbon and hydrogen nuclei. group as given in the Appendix.
The species symbols
for GS but it should
that
be stressed
there
for
(AB)
simul-
Dzh have been retained
is not a one-to-one
correspondence
between the groups. A and B have similar significance in both, but g and u have tot’ally different meaning. (This is emphasized by the species in G8 of vectors parallel to q/z.) The last two columns in Table VI give the reduction of the Gg species to those of the subgroup Gb (isomorphous with C,,) and C, . Reduction to G4 is accomplished by striking out the four central columns in the body of the character table, and to C, by ignoring all columns except E and (ab) (AB)” = rzz . In this section we will consider how the electronic, vibronic, and rovibronic wave functions are classified under the molecular group and how this classification affects the select’ion rules for vibronic transit,ions. Let us first consider the symmetry of the sublevels arising from inversion and
BRAND,
362
WILLIllMS,
AND
COOK
internal rotation. WC choose to examine a st.at.e which is tot,ally symmct ric in C’, symmet.ry and assume t,hat, t.he molecule is in the vihronic ground state so that we need only c*onsider the degrees of freedom associated with rotation, twisting and inversion. If IwisGng and inversion were impossible the molrculc could exist, in any one of four structures,
cwh of which has its own rovihronic wave funotion, symmetric or antisymmelric generate a to the point-symmetry plane or: . Taken toget#her these functions reduc*ibI(t representation of the complete molecular symmetry group. Irving the rwipe of Longuet-Higgins (IO), if R is one of the operations of the group, the caharactcr of R is fn where 11is the number of structures for which R inducw a reflection in the sx plane. The sign takes care of t’he symm&ic or antisymmctrics character of the rovibronia wave function. For an a’ stat’e the characters arc +a for F: and (ab) (AB)*, and zero otherwise, so that twisting and inversion splits il symmc>tric: (a’) level into sublevels A, + &, + Bl,, + BSu ; likewise an antisymmrtric* (a”) level splits into R,, + B,,, + AU + B,,, . Alt,hough this trcntment, qives us the species of the sublevcls collectively it. cannot. tell us whic*h are asso ated with inversion and which with torsion. This information can bc ohi nined by c*onsidcring the effect of inversion and int,ernal rotation separately. We first examine how the coordinate for internal rotation transform> under the operations of group Gx . We choose an angular (soordinate + to measure rotation of the NH? group about the (IN bond, and define 4 on the assumption that, there are dist,inct) ronformations separated by an energy barrier swh that c:orresponds to the pot.ential minima. The symmetry opc’rations 4 = &r/2 tr:tnsform + intjo +#I or &+ + r, as shown below. If 4J
(ob) Ulrz) +
(;I R) Q+r
(ah) 4+r
(ab)* -4
(A4R)* -4
(ab) (AR)* -4+?T
I? -4+S
The cffwt on 4 of the first two and last two operations is to leave the n~olc~&~ in the same conformatiorl-strictly, the same “domain” (11 )-whereas the oi hers exchange one c*onformation for anot’her. The torsional wave frnl~ ions must, thw form tht> basis of a reducible rcprewntation whose char:wter< :lrc 0 for the third through sixth oper&ions, and ~2 otherwise. If the barrier opposing torsion is high the torsional wave fuwtions #(+) can be written as iht, su111 or difl’errnws of harmonic: owillator wave functions fi,. (+ f r/2), anti ihc wnvcfunrtiotw for the pair of sublevels belonging to the 0th state of the torsion MC
SPECTRUM
OF ANILINE
363
where the plus sign refers to the lower sublevel when v is zero or even, and to t’he higher when v is odd. (It is not import,ant, that these wave functions should be accurate, merely that they have the correct transformation properties.) When v is even or zero, the character of t’he representation of $J~(4) is +Z for E’, (ab) MB), (nb) MB)“, and E* and zero otherwise, giving the reducible representation A, + B,,, . For u odd the represent’ation becomes 231, + d,, . As t,he individual furlctions $J~(+) form a fully reduced set the same result can be obtained from each in turn. We next turn to the coordinate for inversion. Let’ .ra , .rb and y0 , y,, be the Cartesian coordinabes of the a and b protons in Fig. 1. Those operations that simultaneously switch .r, to -~l’~ , ya to yb , xb to -rb and yU to yb convert one conformation into another by inversion: they are (ab) (AB), (ab), (AB)*, and E”. The remaining operations leave the molecule in the same domain of inversion. The characters for the transformation of the coordinate arc alternately + 1 and - 1, corresponding to the species B,, . The symmetry species of the sublevels are readily found by the same method used for internal rotat,ion: they are A, and Bz, , t#helatter resulting from the antisymmetrical combinations. We arc now able to give the order of sublevels arising from twisting and inversion in a totally symmetrical (a’) rovibronic state. Inversion divides t,he level into iz,, and B,, components which are t’hen sub-divided by torsion into A!, and B1,, sublevels for the symmetric component of inversion, and to BBgand Bat, for the antisymmetric component. In practice, the inversion splitt’ings are much greater than the torsional; t’hus the sublevels are A, , B1, , B2, , BstL in order of increasing energy. We next consider the classification of electronic states and t’he selection rules. Theories are unanimous in stating that the first excited state of aniline derives from the Bzu state of benzene: in that state the electron coordinates transform as y and thus excited aniline has t’he electronic species B,, in Gg . (The g, u in the Gs species symbols tend to be misleading here.) The electronic ground state is of course A, . As our earlier remarks refer to rovibronic levels we could proceed to discuss the B,, +- A, transit’ion in terms of overall selection rules, but, the rovibronic and vibronic groups are in this case isomorphous and no difficulty arises in using vibronic rules. Transitions contributing to the “origin band” are shown in Fig. 2: there are four, all y-polarized and occurring without crossing between sublevels of the two states. The vibrations of t,he CsH,r\’ fragment’, classed according to Gs , generate the structure lla, + 10b3, + :
364
Fro. 2. Transitions xi-~ for the subgroup
BRAND,
between G,
WILLIAMS,
torsional
AND
alld inversion
COOK
sublevels.
The speries
in parctltheses
but, t#hey cannot appear except through intensity borrowing. Borrowing from a higher x-polarized electronic transition is unknown in aromatic spectra so that :rll contributions to a vibronic BXI + A, t#ransition are very small. T’ibronic* A,, + A!, transit’ions are partly y-polarized but involve crossing also; t,hey are thus “forbidden.” Vibronic A, +- A, transitions can occur with z-polarizat,ion without crossing, but any borrowed intensity is not sufficient for them to be seen in the spectrum. Observations indicate that the splitting due to internal rotation is so small as to leave no definite mark in the spectrum: each pair of torsional sublevels is effectively degenerate. It is then not, essent’ial t’o distinguish between them and the selecation rules discussed above can be re-expressed in terms of t,he subgroup
SPECTRUM
OF ANILINE
365
G4 , isomorphous wit.h CzV. This convention is adopted in the rest of the paper. The splittings due to inversion turn out to be large and the components of t’he origin band are separated by several hundred wave numbers (see Fig. 3). It is convenient to regard only those transitions connecting the lower inversion sublevels as the origin band, and to treat those connecting higher sublevels as a l-l sequence. The higher inversion sublevels are then classed as distinct vibrational states. Finally, it should be said that a vibronic classification according to G8 , G4 , or even C, , does not imply nonmixing vibrat’ional coordinates. Torsion and inversion both take place with large amplit’ude and may couple vibratjions of different symmetry. The theoretical picture is extremely complicated (13). Fortunately, the assigned frequencies closely resemble those of a rigid molecule such as fluorobenzene (14), so that the potential energy of the phenyl group appears not to be heavily dependent upon the nonrigid coordinates. III. EXPERIMENTAL
Spectra of aniline vapor were photographed in the 18-22 orders of a 3.4-m Jarrell-Ash Ebert spectrograph fitted with a 57 OOO-lineHarrison grating. The contour of the individual bands rises rather sharply to a strong peak near the high-frequency edge, then falls more gradually towards the red through a minimum and a broad maximum. Rotational structure is well developed over 60-80 cm-‘. The main peak itself has a substructure of three intense maxima of which the central maximum was arbitrarily chosen for measurements of band posit,ion. The frequency measurements were made on prints enlarged about ten times from the original plates: since the features measured were themselves quit,e broad the accuracy (about 0.2 cm-‘) was not significantly less than by use of n comparator. Our frequency data are generally consistent with those of Ginsburg and Matsen (I), but we find no support for their division into “double” and “single” bands, apparently an artifact of incomplete resolution. Rotational structure is sharp for bands lying to about 1000 cm-’ to the blue of the origin band, but at higher frequencies the bands begin to look diffuse and the spectrum is virtually washed out past 0 + 2000 cm-l. In the sharp region, although different bands show small variations in contour, we found no evidence for the presence of transitions having different polarizat’ion from the origin band; that is, the system appears not to contain any “forbidden” part. This result is broadly in agreement with that of Albrecht and Kalantar (15) who showed that the transition in the condensed phase is quantitatively g-polarized in the neighborhood of the system origin. The isotopes used were aniline-h7 , -dz , -d5 , and -d7 . Aniline-d5 was prepared from benzene-d6 by conversion to nitrobenzene and reduction with SnCl, in D&DCl. Amino-hydrogens were replaced by a few exchanges with DtO. Paths ranged from about 10h3-10-l cm atm at 20°, except for aniline-h, which was also examined in a folded cell (to 1 cm atm) in order to develop “hot” bands. A low-
366
BIIASL),
_ -ig -
-_o -
-
Y_._
_ A
WILLIAMS,
_
AND
COOK
I
I
SPECTRUM
TABLE
I
TOTALLY SYMMETRICAL FUNDAMENTALS IN THE State
“A1
‘H2
7’0, cm-’
2940-ASYSTEM OF ANILINE
CsHsFa CGHBNH2 CGH5ND2 Modeb 6a*
519
528.7
808 1008 1022 1220
822.3 989.6 1029.1 1286.3
796.6 999.9 d d
12* 1 18~ 7*
492.0 797.8 953.3 1183.8 1307.2 1431.5
481.2 786.5 953.2 1104.8 1274.2 1352.1
6a* 12* 1
34 032.1
34 040.7
n See Reference 14. h Asterisk indicates “X-sensitive” c Resonance. ClRegion not examined.
518.7” 511.7
367
OF ANILINE
A 7* 8
CsD,Fa
G,DbNH?
(IN
cm-l)
GDbNDz
Modeb Ga*
505
515.9
504.2
753 880 959
756.3 879.4 957.0 d
d d d d
12* 18a 1
481.3 760.0 823.8
470.0
(ia* 12* 18~ 9a 1 A
844.9 922.6 1181.1 34 196.6
814.6 842.7 890.3
34 025.7
modes.
resolution int’ensity profile was taken with a small recording spectrometer resolving about 2 cm-‘: the t’race (Fig. 3) shows the intensit’y minimum and reddegradation for individual bands, but the substructure in the main peak is not, visible OII this scale. The -dT sample contained approximately 8% hydrogen and its analysis was not pressed beyond the strong bands in the system. A list of principal bands is given in the Appendix. Assignments are indexed by a convention previously described (16) in which a v’-v” transition in the normal mode k is represented by Ic$ . Since we do not have a complete vibrational analysis the numbers allotted to the various modes are somewhat arbitrary, but a provisional scheme is drawn up in Table I. The convention is extended to allow, for instance, ?c”to represent the term value of the vth level of mode k in the excited electronic state. Finally, as shown below, the KHZ out-of-plane rocking mode (bl) occurs in a double-minimum potential so that, it,s energy levels are unequally spaced: because this vibration tends to invert configurat,ion it is symbolized I and, since the inversion splittings are large, its energy levels are numbered 0, 1, 2, 3, . . . so that 0 and 1 correlate with the O+ and O-, etc., in the alternative notation. IV. ANALYSIS
As the vibrational structure in the aniline spectrum has much in common with that found for other derivatives of benzene it is worth while to describe the pattern briefly. Typically, t’he origin band is prominent in these spectra becoming
36S
BRAND,
WILLIAMS,
AND
COOK
one of the strongest bands in the spectrum when the substituent is polar (OH, lc, Cl) (17). Otherwise, only four ring modes occur with really high intensity, namely, the t,hree al substituent sensitive modes (numbered 6a, 7, and 12 in the notation based on Wilson’s classification (18) for benzene) and t,he ring-st,rrtching mode 1. The frequencies assigned to these modes in t#heground :mtl cxcLit,etl states of aniline are given in Table I. Within relatively narrow limits they arc uncQhanged in other monosubstituted benzenes provided that the suhstituelltj is drawn from the first row of the Periodic Table. Surrounding each of the strong bands associated with the fundamentals is a group of v’-u” sequences ant1 (0 f 2 )’ - d transitions in nontotally symmetric fundamentals, the most pronlincnt of which, in the aniline-hi spect’rum, lie at +374, +65, -40 and -?:I2 (‘nP1. These transitions arc identified in the neighborhood of t,he origin hancl itI Fig. :i though the +65crn~’ region is more complex than it appears under low resolution and is actually an envelope of three nearly-coincident transitions. Th(l spectrum develops further through sequences-upon-sequences, and SC)forth. WC do not have sufficient, information for clefinit,e assignments, but $374 is probably the 2-O transition and -232 the l-l sequence in the out-of-plane mode 16a (a?). Although -40 cm-l can be seen to the Z-3 transition and so must :uist from a ground-state fundamental below about 400 cm -I, it is not cble:tr wMher the fundamental is 18b (bz) or 11 (bl). The al modes give rise only to weak sequences. The remarks above refer part,icularly to the ring-& isotope. In the -c/sseries there is no infrared spectrum to check the ground-state fundamentals or c~ollntJeral evidence to guide the assignments for the excit)ed state. (No deuterated monoderivatives of benzene have been analyzed in the ultraviolet region.) The correlation with CsH5F and C&D,F in Table I is largely for convenience, to c~~tablish n shorthand for indexing assignments; thus, by correlating the 10% :nltl 879. cm-’ ground-state fundamentals of aniline-hi and -cl, with 18~ mode of fluorobenzene we do not in either case imply more than a very general connection with the el, mode (18) of benzene it,self. The vibrational coordinates are by no means illsensitive to ring deuteration. The mode A shifts with K-deuteration ant1 may involve KHn bending. Escbepting the -4O-cm-’ sequence, the strongest band to the red of the origin in the -hi spectrum is at -423 cnl-I. It is certainly associated wit’h clisl~lac~cn~cr~ts 1 of the amino-hydrogens, since it remains unchanged in aniline-d5 (123 (‘tn ; Table II) but falls to about 338 am-’ 111t.he ?;-deuterated species. The exc+t,etl state counterpart is at +292 cm-l, marked by an extremely strong band whicgh behaves similarly to the -423 band on isot,opic substitut’ion. (It shifts to +238 cm -I in aniline-&). Both 292 and -423 cm-’ occur in association with the ring fundament’als 1, 6a, 7, and 12 mentioned earlier, as shown in Tables II and 111; and both are surrounded by the usual pattern of sequences involving ring vibr:lt,ions.
423.1 336.8 422.1
423.0 336.8 421.9 324.5
CeH5NHz CsHSNDz
292.6
239.1 279.9 309.4 235.4
292.6
239.4 280.1 -
235.5
CsHsNHz
CsH~ND2
CsDsNDn
GDsNHz
0
6@
Mode
aZ = w(NH,).
GDsNHz CJ&NDt
&I’
0
Xi* 422.7 336.8
III
435.1
TABLE
Ia’)*
l0’6eo’
434.9
131)
337.0
70’
CoH,NDz
-
336.9
IhI+
6%’
B. X$(Z$
336.5
6a$
cnl-1
-
0
422.0
18aa1
I#),
_#(I,’
X$
423.1 336.8
lo’
-
AND
422.9
283.7 303.1 240.4 277.1 236.0
290.4
238.0 308.3 -
120’
6ao’
-
239.2 -
292.4
10’
-
239.1
-
w
-
237.9 -
701
-
238.4 -
-
6u&120
237.6 -
291.5
l&u~’
236.2 -
-
10’12rJ1
102
336.8
237.8 -
238.4 -
-
-
235.3 -
-
6u021201
422.9 336.8 422.0 324.5
Average
l,%ac?
10’6UO~
292.0
102 422.9 336.9
,21X1)
10’120’
(IN
POSITION OF Z1l SEQUENCE RELATIVE TO THE ORIGIN BAND (cm-l)
422.9 336.8 421.9
120’
A. X$(Z#
II Z,‘)
TABLE
CVMBINATI~N DIFFERENCES Y$ (Z,o -
370
BRAN>,
WILLIAMS,
ANI)
COOK
The envelopes of the --223-m-’ band ad the origin band look identical, so that the interval is totally symmetmal: however, it is not a fuudament:4 bec~:~usr the lowest, al subst)ituent mode, 6 (NH?), lies near 1tiOO cm-’ (19) and the lowest it, might rqmsent ring mode, 8a, is well characterized at 528 (WC’. Conceivably, :L O-2 transition in the t’orsional mode’ and, if so, we would attempt to explain The Inttcr +292 (‘I11-’ as either the l-l secluence 0~ the 2-O torsional transition. can be discounted immediately; f292 (*III-~ has as much intensit.y ;LSt tw origin hand (in the case of aniline-d? , 50 percent greatel intensity), i.e., it is ;tt lwst nlk order of magnitude too intense for assigtmerlt to n 2- 0 transition in :I Ilontot:dl\ ~ymrnetric vibration. Its assignment as a l-l I orsional secluencr would :~lso 41 for special pleading to account for the high intensity, hut in my case MII 1)~ rt11~~1 sequences. Likewise, a sitrorig c:w out by the nonappearance of 2-2, :‘,-’.3, t call he mounted against the assignment of --l2:3 cm-’ to the O-2 torsiotlxl transim tlon. The -42:3-cn-” band, after correction to equal populut,ions, 1~s twiw t h int envit’y of the origin band, while the corresponding -337-cm-’ bnrld h:~h almost i hree limes the origin intensity. ,411 this is quite incompatible with tllc* hehaviol txpect,ed for the torsional mode. III the spectrum of phenol, whew the torsiotml sec~u~~~~cescm be identified beyond doubt., the 1 --I .sequencc has intensity 1 IY~:~live to origin = 100, and the O-2 transition is too weak to detect (Z/j 1. Another conjecture is that the bands are O-2 and l-1 transitions iii 11~ out -ofought to tw pl:tnr mode o (NH?), with the implication that a 3-O transitiori prcserlt t awards shorter wavelengths. Xt t,his point a rharacteristi(. featIm> of the +Wand - -l2:3-c111-’ batlds becomes import ant,, namely, that t hew itItcrvals never combine wit,h OIK anot)her. Surrounding the --Il’li-u~~- ’ hnd, for instance, WC fiud t.he usual +W, -40., and -~32-cn-~’ sequences, hut t I~cre iy 110 band of acceptable intensity at or near (292--425) cm-‘. (III the ~pwtrun~ gwerally, the intensities of bands separated by the +Y9%cm-’ inter\-:rl clo not differ by more than a factor of two.) This feature recurs throughout thr s~w(~ttwn. since every strong band has :I waker band 423 cnl- ’ to the ~(1 (‘.i‘w/r/ t I~OSC~ Ixmtls that contain f292 cuC* or higher members of the sa~nc m:uIifoltl. ‘lks inc*idcnce of the -423 interval is shown in Table II, that of +292 in T:rt)l(, III. The former are combinat.ion differences, the latter not. _knalysis of I tt(s rillg-A5 isotopes is more comprehensivct than that of the rillg-r& , for reasons givc.11 cat4iw. The nonoccurrence of a f292 sequerwc attwhed to the --L?:s-NIIhtrtl is explained only if the mode with which these bands are asso(4atcd is atll~artllorlic*, SO thnt the 5-2 sequence does not fall in tke hnmonic position: :ttltl, if 1his i.+ cSorrecbt, t h(> position of the 2-O transition cannot he determined h:~rtll(Jtli~~:lll~. The spwtmm of aniline-rls has a strong band at +.X8 c:111-~(+5139 for wkilitic,-rli) ~vllic*h cxmld be so assigned. This situation is less c*lcar in the -hi sp~~xl~IUII t,ut there is :i hmd at +760 cn-’ which may h:lve this explanation. +7fiO is (~I~wl; 1.~II ilIfrared
vapor
I)slld
st 420 cm-’
has
heel1 assigned
to the overtone
~T~NII,I.
S~~IICY
SPECTRUM
OF ANILINE
371
resolved from the first -40 sequence attached to +797 (mode 12) in the highresolution spectra t’hough not in the profile given in Fig. 3. Our reservation concerning the +760-cm-’ assignment is not connected with the near-coincidence with 797-40 so much as the fact that mode 12 is in resonance with a combination of two out-of-plane fundamentals in certain other benzene derivatives, notably phenol-d1 (61),’ so that we cannot be sure that 760 cm-’ is associated with the w(NH,) mode. Granted t,his uncertainty we give very little weight to the 2-O assignment in the -h7 spectrum. In the -d? spectrum the 3-l transition can be identified for t’he following reason: The strong sequences at -40 cm-l in the -hT spectrum are usually seen at -45 cm? wit’h aniline-d2 , but occur in that spect#rurnat -37 cm-’ when attached to bands arising from the U” = 1 of w(NHZ). (The S-cm-’ shift’ is characterist’ic of bands from the U” - 1 level of w(NH,) and must originate in some Fermi-t)ype resonance in the ground state, but as we cannot definit,ely assign the -45.cm-’ sequence we cannot identify the resonance either.) The 3-l transition (at +860 cm-‘) is one of the -d, bands singled out by this anomalously small spacing in the sequences. It happens that the 3-l transition can also be identified for aniline-h? , again as the result of an accident’. The Oransition 1201111 (where I symbolizes the w(NH2) mode) does not occur at t’he harmonically calculated frequency of 797 + 293 = 1090 cm-‘; instead two equally intense bands are found at 1080 and 1100 cK1, displaced to either side. We know there is no bl ring fundamental in this region, since all bl ring modes lie below 1000 cm-’ , so t,hat, the probable explanation is
position
BB1 : 12l1’ +-p--+ I3 (cm-‘) expected: 1090 observed: 1081.4 1100.8 (cm-‘).
Since the 12’1’ level is forced down by about 9 cm-’ the unperturbed 3w (NHz)’ level I3 must’ lie near 1092 cm-‘. Another type of resonance, present in the aniline-d6 spectrum, strongly supports the II’ assignmenbs. Aniline-d6 has strong bands at +280 and 309 cm-’ (see row 3 in the body of Table III) symmetrically placed relative to the expectation for III of +293 cm-‘. We know that an out-of-plane ring fundamental falls in t’his region, namely, the mode 4 which occurs at 306 cm-’ in the Bt,, state of benzenedg (W2). Strong coupling bet,ween I’ and 4l leads to the appearance of two transitions II’ and 110401.With roughly equal intensities observed we expect the unperturbed II’ t.ransition to be close to the mean position, 294 cm-‘. This is essent’ially the sanle as its position in the -1~~spectrum, where the ring fundamental has shifted to about 365 cm-1.3 The import,ant point is that the vibrational symmetry 2 Phenol-d1 shows strong bands at 0 + 773 and 0 + 789 cm-’ with the intensity ratio 2: 1 (21). 773 cm-’ is the perturbed fundamental 12; 789 cm-l is probably a combination of bl fundamentals 429 + 356 cm-’ (20). 3 This is the frequency observed for the ring mode 4 in t,he B 2u stat,e of benzene (82). III t,he eR2 st,ate of phenol-hs the fimdamental frequency is 357 cm-’ (20).
372
BRAND,
.
.
1;
0
WILLIAMS,
1:
1:
1:
AND
I .
I
1 1
.
’
b,
0
A,
3
B,
2
A,
b
2,
1:
. W
COOK
Fro. 4. Transitions involving levels of w(NIIn). The scale applies however, the II3 transition is not observed. The oibrafional symmetry of the figklre.
to aniline-h7 where, is given on the left
of the upper state of the 111 transition is confirmed as “bl , as the assigmnent requires. As companions to the 12’ transitions we expect t’o find IS1 transit’ions emanating from the 3w(NH2)” level of the ground state. It will be shown lat’er that these transitions are weak and we have actually assigned only two of them, both in the spectrum of aniline-& where they occur 4% cnl’ to the red of t,he transitions II’ and 6a01Z1’.The combination differences II’ - Iso and 6a01[,T1’- 1311shown :lt the foot of Table II then isolate the V” = 3-1 interval of w(NH*) in t(he ground stat,e of this isotope. The t’ransitions emanating from I3 are weak so that their identification is somewhat tentative, but’ a band-by-band search failed t)o give nny alternative assignments; and their identification for aniline-& is not, surprising since bhis isotope has t’he best-developed spectrum. A survey of transit ions involving the w(NH2) mode is shown in Fig. 4. V. DISCUSSION
The transitions between levels of w(NH2) fall into two sets-one combining V-41 levels, the other combining ‘B1 levels-without crossing bet,ween them,
SPECTRUM
373
OF ANILINE
so that the analysis does not position the B1 levels relative to t’he ground state. Their relationship can be calculat,ed by treating the vibration as occurring in a double-minimum potential, V(q)/hcvo = Yiq2 + CYexp( -pq”),
(2)
where p is the (dimensionless) normal coordinate and a, /3,and ~0are parameters. The eigenvalues and eigenfunctions of this problem are available from tables (,=). The ground-state combination differences in Table II, namely, G(2)” - G (0)” for aniline-h, and -d2 together with G(3)” - G(1)” for aniline-& , are sufficient to determine the potential constants and hence the term values Gus where u is odd. This is done in Table IV. The barrier height., I’,
= Y~(~LY~ - In 2ap -
l)/Zp
(3)
calculated in this way is considerably smaller than the well-known ammonia barrier (-2000 cm-‘), though comparable with the values obt,ained for structures in which a NH2 substituent’ is conjugated with an unsaturated group: these include formamide (V, = 300 f 50 cm-‘) and nitramide (6,8). TABLE
State
LEVELS OF w(NH*)
-
-
-i-
T
AND
Go(3)) cm-’
i_
0
IV
IN THE A,
Obs
--
1VU cm-’
B
a
Bz STATES
Calc
Barrier
4Jm
Cm-'
_-
em
degrees
-I-
A, (Ground stat,e)
C,H,NH,
0
0
CcHsNDe
1 2 3 0 1 2 3
(33)s 422.9 665” 0 -
0 1 2 3 0 1 2 3
0 326
336.8 4401
0 34 423 665 0 7.1 337.0 441.9
7.9568
0.1145
579
562
5.2402
46
10.8717
0.0838
429
568
7.1599
48
0.5497
1.6575
406
24
0.6153
1.4807
317
27
B? (Excited state)
C~HENHZ
CsHsNDz
(760) 1126 0 246 548 868
= From the infrared
0 326 730 1124 0 247 561 868
/I
spectrum
1 (19).
1
-
I
-
-
HIIANI),
37-l The
WILLIAMS,
ANI)
COOK
angle bet’ween the planes of the substituent
and t’he phenyl
ring in the
ground state was cakulated assuming t,hat the reduced mass reyuired is a funcaCon of t)he C-NH? fragment alone. As the data for aniline-d? are more reliable than
those for aniline-h7 we performed
1:Ltter as :I caherk. The formula 8,’
the calculation
= Qm’C =
8, and qm are, respectively,
T
where (: is the appropriut,e
wit,h the geometry, are typical
kinetic
energy
matrix
amines),
and
L DKD
c+nlent which This procedure
:i:i”, keeping
a planar
potent,ial;
configuration
130”. We thus obtain
L DSD
bond lengths
a value
is a poor approsima-
by allowing
0 to be nonzero
equal to an nrbit,rary
the same,
wc calculate
v:~lue
a new G
may be used to obtain :L new value for L DKD and thus C. was c*ontinued until the self-consistent’ values L DND = 113”
:n1tl 0, = 48” were obt,ained. lar results
=
of the fragment
of G and this was rectified
to cbhangc from 120”. Choosing
of lo!)” and I!?, =
(18).
the hindering
G by assuming
of B,, = 5.5”. It is obvious t)hat the planarity and i DKD
element
= 1.36 A (these values taken from ILeferenre 2~
R(C---N)
tion for the computation
(5)
used t)o describe
We first est,imatzcl
for aromatic
plane angle
= &’ = (;--‘#,
from the parameters
G must) be calculated.
plane-ring
This formula is obt’ainetl by on bending can bc ncglcc+ctl
is given by 2’J
Y,~ is obtained
(1)
t’hc values of the subst)ituent
energy
using t,he
(h,‘4&o)y,‘C:.
and t’he normal coordinate at the potential minima. assuming that changes in the S--D bond di&ance and that the kinetic
on the former
recluired is
(see Table
IV);
n~ss are :it least compatible Since the available
A calculation
on the aniline-h7 molecule
thus the assumption<
made to calculate
with the experimental data. G(2)” - C(0)"(twice)
information,
led to simt,he rrtluc~etl
and Ci(:$)” -
G(l)‘,
in the iq suffi(+ient only to calculate LY,8, and ~g, there in no surplus information c~le~tronic~bands to check the results. In the infrared vapor spectrum, however,
transitions assigned to the w(NH2) mode appear at ti(iF, and 440 c’n~‘, rcspc+ t ively, for aniline-h7 and -tlr (19). There is no evidence bearing directly upon their polarization
but they are presumably
Y-0 transitions
since
OIK
cxprrls on
gmerl~l
grounds 1hat (dp/dq)o will have ti larger component along s than along z, :mcl c~orlsequently t)hat. the infrared 1ransitions are :3--O and 2-1, as in the an~monis s~~~(*trul11. Their
c~omputcd po&ions are at 66% :Lnd 44% cm-’ as shown in T:rble IV. WC :11so expect to find cvidetice for the ‘I’--1 t,ransition whose c*al(~ulatctl position is at 388 caniPI in the infrared spect,runl of aniline-h7 . Here the xit uat ion is less c*lear. ,4 vapor infrared band observed near 390 cnC1 has been assigned to XI cc? ring fundamental (19), but this assignment contradicts the selection rules and the band in question ~oultl represent the 2-l trarlsition. If this is corrcc*t the observed value of f& (1)” is 33 cni-‘. Over-all agreement between the ~alc~ulntc~tl nnd observed infrared transitions is as good as could be expected considering
SPECTRUM
375
OF ANILINE
that the calculation assumes that the w(NHz) mode is not coupled to any other vibration. The 2-l transition predicted for aniline-& lies at the limit of the recorded spectrum and no definite conclusion can be reached. Once the term value Go(l)” has been determined for the m(NH2) mode the excited state terms Go(v)’ follow from the vibrational analysis. They are collected in t’he lower portion of Table IV. It emerges that the excited state mode is not harmonic and the concept of a double-minimum potential may again be called for. This potential does indeed give a reasonable fit to the known term values (see Table IV), though whet,her t,his analysis is realistic is anot’her question. The computed G(0)’ is 140 cm-l for aniline-h, ; t’hus the calculated barrier height of ~25 cm-’ is . a small fraction only of the zero-point energy in this coordinate. Although there cannot be much doubt that t,he reason for the anharmonic character of the w(NHz) levels is the same in both electronic states, the excited state anharmonicity could be account’ed for by a potential with parabolic walls and a single minimum, perturbed near the foot so t’hat a3v(q)/8q3 # 0 for a considerable range around p = 0. The double-minimum is not an essential feature of t’he excit’ed-state potential as it is for the ground state, and we earlier proposed the term quasi~2anar to cover this situation (9). Similar behavior is known for the excited state (‘A”) of propynal ($2;) and t,he ground state of ketone and diazomethane (%). As a final check we have calculated the intensity of the 11’ and 12’ transitions in t,he electronic spectrum and compared them wit#h experimental values from Fig. 3. The calculation was performed by first expanding the wave functions of the inversion levels in terms of harmonic oscillator functions using t,he coefficients tabulated by Coon, Naugle and McKenzie (23). Assuming that the transition moment is given by M(#Vt 1#V(r) w h cre M is the electronic transition moment, and Gv,, #v,, are the vibrational wave functions for the states V’ and V”, the int,ensrty relative to the origin band is given by
These ratios for I’,‘, IZo, I:,
INTENSITIESOF I:,
and 1: are given in Table V for aniline& I
TABLE
and -I& .
V
TRANSITIONSRELATIVE TO THE ORIGIN BAND
Transition
13l
ZP
I?
IO2
Relative intensity Aniline-h,
Aniline-d?
Obs Calc
0.04
0.26 0.17
1.01 l.OG
co.851 0.99
Obs Calc
0.05 0.31
0.40 0.83
1.50 1.93
0.50 2.79
376
BRAND,
WILLIAMS, TABLE
CHARACTER
G8
e
(ab)(AB)
MB)
TABLE
--I_-
“L
1
1
1
1
Au
1
1
1
1
B,,
1
1
-1
-1:
B,, I&,
I 1
1
-1
-1
-1
R.‘,‘
1
-1
H,,
1
-1
-1
1
HI,,
1
-1
-1
1
1
-1
1
-1
VI
FOR THE GROUP
Cab)’
CabI
AN11 COOK
WV*
1 -1 -1
(ab)(AB)*
1 -1
1 :
-
GS
-1
-1 1
1
-1
1
1
-1
1
-I/
1 -1
-1
-1
E*
1 -1
1
- -
1 -1 1
-1
1
-1 1
1
-1
-
1
As t)he spectrum is crowded the observed values are relative peak intensities instead of integrated intensit’ies which would have involved large and uncertain corrections for overlapping. dlthough agreement is by no mems perfect the calculated intensit,y ratios predict the general trend of observed intensities reasonably well. This is encouraging when we consider that, the calculation of intensit,ies is a more severe test, of t,he wave funcbions used t,o describe a system f,h:m the calculation of energies.
The aniline molecule is nonplanar in its ground state, the NH, plane being angled at about 46’ to the plane of the remainder of t.he molecule. This struct)ure is consistent with that proposed by Tyler (27) from a microwave study of aniline. The excited-state geometry canriotj bc uniquely determined from the potetkial hindering inversion in the molecule. The potential remains approximately constant over a large range of q near q = 0; the molecule is then q~La+s~pZnnnr. The t,heoret’ical interpretation of the groundMate geomet’ry can be formulntetl in t.he following manner: The various substit uent pert,urbations of the amino group cancel each ot’her to a large extent and allow the KHZ to take up the ammonia-like configuration. The charge-transfer and ground st’at,es are well separated so the mesomeric stabilizat’ion is small [0.3 eV in the Rlurrell c*al(~ul:ttion (28)]. This mesomeric effect may be cancelled by the inductive effect of the KHZ group which dest,abilizes the a-electrons of the hydrocarbon (29). This tlcstabilization is t’hough to arise from exchange repulsion; t,he amino and phenyl a-clect,rons occupy orbitals which overlap to some extent; thus electrons of the same spin occupy the same region of space and repulsion results.
Tnhle VII contains frequency measurements for bands discussed in the paper. It is not a comprehensive listing of all bands observed, t)hough all strong hnr& are included. Relative peak irnensities, based on 0 = 100, are given in pnren theses for aniline-h7 and -dz .
SPECTRUM
OF ANILINE
Table
Assignment
0
Aniline-h7
377
VII
Aniline-g2 --._
Aniline-d
-5
Ani'line-d
32
745.8
1201° l-2
32
785.8
6~;
32
975.0 (0.8)
33
164.5
182:
33
003.0
(0.3)
33
317.5
I0 I
33
042.5
(0.f)
33
040.7
(0.8)
33
239.6
00 6"l_!2
33
083.6
(3)
33
186.1
(5)
33
261.2
12;
33
210.8
(4)
33
244.1
(1)
33
440.3
6~;
33
503.5
(12)
:;
;;;$)
33
680.7
33
701.5
33
609.0
(26)
33
703.9
33
774.7
33
871.1
33
796.1
(8)
33
960.6
33
937.0
34
032.1
(100)
34
205.7
34
100.9
(10)
34
324.7
(lO1)
34
44v.1
3b9.1)
(14)
34
675.7
34
744.6
34
909.6
71
0 12 6a'l'
-1-I
(0.2) (0.8)
1 '3 0
(Origin)
10 63,L2 1 I1 II @d3 2
(34
L2 12'1° e-2
(13) M-0)
5;
;:8':,03(18)
35
845.1
(5)
34
040.7
(io0)
34
1115.0
m)
34
2sc.o
(l50)
34
324.7
(5)
34
490.3
(10)
9a'IO -Q-2 I %I AL;
34
524.1
(80)
34
521.9
34
562.3
(6)
34
657.1
(25)
34
666.9
(10)
6a210 u2 (34
792.2)
(85)
05)
34
567.5
(50)
6a'l' -c-l
34
814.4
(87)
34
759.8
(Q)
12;
34
829.7
(q@
34
827.2
(60)
G
hidden
34
196.6
34
255.7
34
476.5
34
506.0
34
554'.7
34
619.5
34
b77.3
34
736.0
34
956.0
34
986.2
34
956.6
I
-7
BRBND,
378
Table
VII
Assicnment
WILLIAMS,
AN11 COOK
(continued) Anilinr-h. _--.---
154 971.1
Aniline-d -? -_____.
Aniline-d5
Aniline-d_
35
020.4
35
GE.2
35
041.5
35
04b.7
'15.2
(5)
34
Y77.b
(10)
34
9t5.4
(66)
34
993.Y
(42')
35
35
015.8
(?o)
35
003.4
(20)
35
156.4
35
09j.Y
35 ~5
300.0 326.6 I
3:
255.5
35
233.7
35
319.5 .-
33
204.7
35
055.1
Iq)
35
135.1
(12)
35
"3.5
(25)
35
067.6
(f5)
35
132.9
(16)
34
901.1
ISJ)
35
215.9
(30)
35
145.4
(25)
35
217.9
(5yq
35
233.1
(36)
35
304.0
(23)
35
240.1
(4)
35
319.8
(30)
35
az.1
(25)
35
359.9(42)
35
355.2
(5)
35
471.9
(J;$)
35
471.4
(32)
35
514.2
(3)
35
bUb.l
(4)
35
546.4
Czo)
35
55i.r
16)
35
626.2
(30)
35
377.7
35
433.2
35
501.7
35
522.2
35
55Y.t:
3
SPECTRUM
OF ANILINE
379
Table VII (continued) Assignment
Aniline-F,
Aniline-c2
'2 'c-h
35 746.' (181
35 540.6 (?I
"6a'l' o-u-'
35 169.3 <=-I
35 709.5
(35)
35 614.2
(3)
d6a 35 782.1 (24)
‘o’*o 1 1 6a2'2' -00 '3 'CL'
Arliline-47
,35 780.1 (24) 35 788.0 (8) 35 855.' (16)
412;
35 930.5 CaI
l2 0
35 937.2 (15)
12 '0%
35 945.0 (14) 35 951.0
‘O’2& I 1 1.
(20)
36 016.4 (3‘S)
I21’
O-l '2 lo6s
36 '82.8
(6)
36 '89.4
(5)
1;6&
36 328.4
(=I
This
Aniline-g5 -P
work was supported
RECEIVED:
by the National
Science
Foundation.
l\lay :3, 1966 REFERENCES
1. N. GINSBCRG ;\ND F. A. M~TsEN, J. C’heul. Phys. 13, 167 (1945). 2. 11. BOB.%, Bull. Chem. Sot. Japan 34, 76 (1961). S. T. J. BH.\TT.XH~RYY.~, Indian J. Phys. 36, 533 (1962). 4. M. AKONEY .\ND R. J. W. LEFEVRE, J. Chem. Sot. p. 2775 (1956). 5. A. WEISSBEKGER ‘\ND R. SANGEW.LLD, Z. Physik. Chem. 36, 237 (1929). 6. C. C. COSTIIN .IND J. M. DOU’LING, J. C’hem. Phys. 33, 158 (1960). 7. I>. J. MILLEN, G. TOPPING, .IND D. R. LIDE, J. Mol. Spectry. 8, 153 (1962). 8. J. K. TYLER, J. Mol. Spectry. 11, 39 (1963). 9. J. C. D. BKIND, D. R. WILLLZMS, AND T. J. Coos, J. Mol. Spectry. (in press). 10. 11. C. LONGL-ET-HIGGINS, Mol. Phys. 6, 445 (1963). 11. J. Ii. G. W.&TSOS, Can. J. Phjys. 43, 1996 (1965). 12. G. HEI~ZBEI~G AND E. TELLER, 2. Physik. Chem. 21B, 410 (1933). 13. J. T. HUTGEN, Can. J. Phys. 43, 935 (1965). 14. D. STEELE, E. R. LIPpINcoTT, AND J. XAVIER, J. Chem. Phys. 33, 1242 (1960). 15. A. H. K.\L.INT.zR *ENDA. C. ALBKECHT, Bw. Bunsenges. Physik. Chem. 68, 361 (1964). f6. J.C.D.BR.IND,J.H.CALLOMON,D.C.M~ULE,J.TYRRELL,.~NDT.H.~~OODWIN,T~~~~. Fax&y
Sot. 61, 2365 (1965).
17. H. SPOXER, Rev.
Mod. Phys. 18. E. B. WILSON, J. C. DECKS, New York.
1955.
14, 224 (1942). .\ND P. C. CROSS, “Molecular
Vibrations.”
McGraw-Hill,
30
BRAND,
WILLIAMS,
AND
COOK
19. J. C. EV.1NS,SPeCI~ochi/lL. ;Icla 16, 428 (lQ60). 20. H. I). BIST .IND J. C. D. BK.IXD (to he pLll)lished). 91. W. W. ROBERTSON, A. J. SCOTT, -\NDF. A. M.\TSEX, J. .J VI. (‘hem. Snc. 72, 1539 (19501. it?. F. M. GARFOKTH, C. K. INGOLD, AND H. C;. Po~)LE, J. Chenz. Sot. p. 401 (191X). ZS. J. B. COON, W. W. N.\~~GI,E,.IND R. L). MVKENZIE, Report Air Force 0911 /C’OII~ract AF JQ(638) -5931 (1963). 24. L. E. SITTOn-, “Int,eratomic I)ist,ances.” The Chemical Society, Lo!ldoll, 195X. lQli5. _‘5. J. C. D. BRDD, J. H. C:.~LLI~~OS, .XED J. K. (.i. W.\‘I’SI)N, Disc. Parar/rr.!g Sot. 35, 175 (lQ(i3). 26’. C. B. MOOHE .\ND (+. C. PIMENTEL, J. Ch,em. Phys. 40, 1529 (lQti4). 27. J). (:. LISTEK AND J. K. TYLER, Chem. (‘ommun. No. 152 (1966). AX. J. N. MCRRELL -\NDM. (;UDF&EY, Proc. Roy Sot. A278, 71 (19(X). 9. J. N. ,MUKKELLr\w M. (;ODFI~ET, Pwc. Roy.SW. A278, (i-l- (lQ(i-I).
OF
MOLECULAR
Vibrational
JOHN
c.
20, 359-380
SPECTROSCOPY
(1966)
Analysis of the First Ultraviolet System of Aniline
D. BRAND, DENIS
Departwzenf of C’hemisiry,
R.
WILLIAMS,
Vanderbilt
AND T~o~rls
CTniversit,y, it’ashville,
Band
J.
COOK
l’ennessee
Vibrational struct,ure associated with the 2940-i hand system of aiiilinc vapor has been analyzed in some detail, aiid resit1t.s are incliided for the -IL? , -d5 , and -d, isot.opes. It. is shown that t.hc w(NH2) vit,ration is strortgly anharmonic in the ground state, having the small-large alteriiation of qrtanta characteristic of a vihratioti occurring in a dotthle-minimttm pot,ential. The data are consistent, with an angle of about Iti0 het,ween the ring-to-x bond and the NH, plane in the electronic groturd state. The same vibration is not harmonic in t,he upper electronic state, thottgh its anharmonic character is less strongly marked there than in the ground state. The data for the upper state are compatible with a douhle-minimrtm pot,ential having a very small maximum for the all-coplanar configuration, bait the evidence is not conclusive. All that can be said definitely is that #V(Q)~$ # 0 for a considerable range of q to eit,her side of q = 0. The term gtasiplanar is proposed to cover this situation, a febv ot.her examples of which can be fotmd in the literat.ure. A complete classification of the vihronic sublevels of aniline is accomplished ttsing a group Gg of order eight; hut the torsional splittings are not resolved in the electronic spectrum so that the practically useful group is GJ , a sttb group of G8 G1 is isomorphous with the point group clzV. The vibronic selection rules are discussed. I. INTROnUCTION Aniline vapor has a rich system of bands in the ultraviolet, running in ahsorption from about 3000 8 towards shorter waves (1). Rotational and vibrational structure are well defined near the origin (294OA) and in hot bands, though the appearance of the spectrum deteriorates t,owards higher frequencies. The f-value is about 0.026 in solution (Z), about 20 times that. of the related ‘Bz,, + ‘4, transition of benzene. In this paper we are concerned mainly with a vibrat’ional analysis of the bands of aniline and the question whether the molecule is planar or nonplanar in the states combined by the transition. It will be shown that there is definite physical evidence for a nonplanar ground state in which the NH2 group is inclined to the ring plane at an angle of about 46”. There are many precedents for taking aniline to be nonplanar in the ground state. For example, dipole moment relaxation studies (3), Kerr-constant meas359
360
BRAND, WILLIAMS,
AND COOK
uren1ent.s (,G), and the dipole moment of phenylene diamine equal to 1.3 II (6) all imply that the NH2 group occupies a plane different from t’hat’ of the phenyl. ring. The microwave spectra of some conjugated amino derivatives point defi-, nitely to pyramidal st)ructure. Formamide (6), cyanamide (7), nitramide (8) all. show inversion doubling. Only for formamide and nitramide, however, has the degree of planarit)y been given quantitatively: in the former the amino group occupies a plane angled at about 10” to t’he CHO plane (twisting is present also)., and in the latter the substituent plane lies at 52” to that of t’he ISO2 fragment Xonplanar aniline is a nonrigid molecule having four symmet’rically-equivalent. conformations converted into one another by int’ernal rotation through r about t,he ring-to-N bond, or by “inversion” of the pyramidal structure. Each conformat,ion gives rise to the same set of energy levels, giving rise to a degeneracy which is lifted to some extent when interconversion by tumlelling is taken into account’. Every energy level of aniline considered as a rigid structure therefore divides int*o four sublevels when nonrigidity is taken into account. n’onplana~ aniline has only one element of point symmetry, a plane perpendicular IO the ring plane, and so belongs to the point group C, ; but in order to label the 4 sub levels arising from tunnelling it is clear in a general way that a higher group of order eight is called for. The paper is therefore developed as follows. We first; examine the group-theoretical classification of the energy levels of nonrigid aniline and derive selection rules for t,he transitions between them. The succaerding sections contain the experimental details and the vibrational analysis. Finally we give a brief summary of the conclusions that can be drawn. A short ncc*ount of this work has already been published (9). IT. DETERMINATION
OF SELECTION
RULES AND
HPMMETRY
81’1~:(‘11~;5
As aniline is a nonrigid molecule, its symmetry classification must take account of split#tings arising from t’he presence of equivalent geometrical conformations. Turmelling between t,hese conformers by inversion and internal rotation divides each rovibronic level into four sublevels of the same species, A’ or A”, in the C’,, point group. To distinguish these sublevels one from another requires a group of order eight,. The sublevel classificat,ion can be performed using the molecular s!ymmetry group (IO) constructed from the operations, (i) all feasible P, inc*luding IS:, (ii) all feasible P*, not necessarily including E”. P is any permutation of the positions and spins of identical nuclei or any product of such permutations; E: is the idcnt)ity, E* the inversion of all particle positions, and P* = PE” = E*P. Which operations are deemed feasible has been discussed by Watson (11). These operations for aniline are given in Fig. 1 and form a group designated Gu , iso-morphous with the D2h point group. When the operations are arranged in the order E, (ab) (AB), (AR), (ab), (ab)*, (LIB)*, (ab) (AI?)‘, and E* the molecular group character table is identical wit,h the usual arrangement of the DC,, point
SPECTRUM
a
361
OF ANILINE
b
X
8
e/
CAB)
A B
b
Y
,’
(ab)
4
‘\
E*
Q
a.
b
Q
(QD)*
b
(d*
FIG. 1. Symmetry elements of Gc+.(ab) exchanges the amino-hydrogens; taneously exchanges O, o’, and m, m’-carbon and hydrogen nuclei. group as given in the Appendix.
The species symbols
for GS but it should
that
be stressed
there
for
(AB)
simul-
Dzh have been retained
is not a one-to-one
correspondence
between the groups. A and B have similar significance in both, but g and u have tot’ally different meaning. (This is emphasized by the species in G8 of vectors parallel to q/z.) The last two columns in Table VI give the reduction of the Gg species to those of the subgroup Gb (isomorphous with C,,) and C, . Reduction to G4 is accomplished by striking out the four central columns in the body of the character table, and to C, by ignoring all columns except E and (ab) (AB)” = rzz . In this section we will consider how the electronic, vibronic, and rovibronic wave functions are classified under the molecular group and how this classification affects the select’ion rules for vibronic transit,ions. Let us first consider the symmetry of the sublevels arising from inversion and
BRAND,
362
WILLIllMS,
AND
COOK
internal rotation. WC choose to examine a st.at.e which is tot,ally symmct ric in C’, symmet.ry and assume t,hat, t.he molecule is in the vihronic ground state so that we need only c*onsider the degrees of freedom associated with rotation, twisting and inversion. If IwisGng and inversion were impossible the molrculc could exist, in any one of four structures,
cwh of which has its own rovihronic wave funotion, symmetric or antisymmelric generate a to the point-symmetry plane or: . Taken toget#her these functions reduc*ibI(t representation of the complete molecular symmetry group. Irving the rwipe of Longuet-Higgins (IO), if R is one of the operations of the group, the caharactcr of R is fn where 11is the number of structures for which R inducw a reflection in the sx plane. The sign takes care of t’he symm&ic or antisymmctrics character of the rovibronia wave function. For an a’ stat’e the characters arc +a for F: and (ab) (AB)*, and zero otherwise, so that twisting and inversion splits il symmc>tric: (a’) level into sublevels A, + &, + Bl,, + BSu ; likewise an antisymmrtric* (a”) level splits into R,, + B,,, + AU + B,,, . Alt,hough this trcntment, qives us the species of the sublevcls collectively it. cannot. tell us whic*h are asso ated with inversion and which with torsion. This information can bc ohi nined by c*onsidcring the effect of inversion and int,ernal rotation separately. We first examine how the coordinate for internal rotation transform> under the operations of group Gx . We choose an angular (soordinate + to measure rotation of the NH? group about the (IN bond, and define 4 on the assumption that, there are dist,inct) ronformations separated by an energy barrier swh that c:orresponds to the pot.ential minima. The symmetry opc’rations 4 = &r/2 tr:tnsform + intjo +#I or &+ + r, as shown below. If 4J
(ob) Ulrz) +
(;I R) Q+r
(ah) 4+r
(ab)* -4
(A4R)* -4
(ab) (AR)* -4+?T
I? -4+S
The cffwt on 4 of the first two and last two operations is to leave the n~olc~&~ in the same conformatiorl-strictly, the same “domain” (11 )-whereas the oi hers exchange one c*onformation for anot’her. The torsional wave frnl~ ions must, thw form tht> basis of a reducible rcprewntation whose char:wter< :lrc 0 for the third through sixth oper&ions, and ~2 otherwise. If the barrier opposing torsion is high the torsional wave fuwtions #(+) can be written as iht, su111 or difl’errnws of harmonic: owillator wave functions fi,. (+ f r/2), anti ihc wnvcfunrtiotw for the pair of sublevels belonging to the 0th state of the torsion MC
SPECTRUM
OF ANILINE
363
where the plus sign refers to the lower sublevel when v is zero or even, and to t’he higher when v is odd. (It is not import,ant, that these wave functions should be accurate, merely that they have the correct transformation properties.) When v is even or zero, the character of t’he representation of $J~(4) is +Z for E’, (ab) MB), (nb) MB)“, and E* and zero otherwise, giving the reducible representation A, + B,,, . For u odd the represent’ation becomes 231, + d,, . As t,he individual furlctions $J~(+) form a fully reduced set the same result can be obtained from each in turn. We next turn to the coordinate for inversion. Let’ .ra , .rb and y0 , y,, be the Cartesian coordinabes of the a and b protons in Fig. 1. Those operations that simultaneously switch .r, to -~l’~ , ya to yb , xb to -rb and yU to yb convert one conformation into another by inversion: they are (ab) (AB), (ab), (AB)*, and E”. The remaining operations leave the molecule in the same domain of inversion. The characters for the transformation of the coordinate arc alternately + 1 and - 1, corresponding to the species B,, . The symmetry species of the sublevels are readily found by the same method used for internal rotat,ion: they are A, and Bz, , t#helatter resulting from the antisymmetrical combinations. We arc now able to give the order of sublevels arising from twisting and inversion in a totally symmetrical (a’) rovibronic state. Inversion divides t,he level into iz,, and B,, components which are t’hen sub-divided by torsion into A!, and B1,, sublevels for the symmetric component of inversion, and to BBgand Bat, for the antisymmetric component. In practice, the inversion splitt’ings are much greater than the torsional; t’hus the sublevels are A, , B1, , B2, , BstL in order of increasing energy. We next consider the classification of electronic states and t’he selection rules. Theories are unanimous in stating that the first excited state of aniline derives from the Bzu state of benzene: in that state the electron coordinates transform as y and thus excited aniline has t’he electronic species B,, in Gg . (The g, u in the Gs species symbols tend to be misleading here.) The electronic ground state is of course A, . As our earlier remarks refer to rovibronic levels we could proceed to discuss the B,, +- A, transit’ion in terms of overall selection rules, but, the rovibronic and vibronic groups are in this case isomorphous and no difficulty arises in using vibronic rules. Transitions contributing to the “origin band” are shown in Fig. 2: there are four, all y-polarized and occurring without crossing between sublevels of the two states. The vibrations of t,he CsH,r\’ fragment’, classed according to Gs , generate the structure lla, + 10b3, + :
364
Fro. 2. Transitions xi-~ for the subgroup
BRAND,
between G,
WILLIAMS,
torsional
AND
alld inversion
COOK
sublevels.
The speries
in parctltheses
but, t#hey cannot appear except through intensity borrowing. Borrowing from a higher x-polarized electronic transition is unknown in aromatic spectra so that :rll contributions to a vibronic BXI + A, t#ransition are very small. T’ibronic* A,, + A!, transit’ions are partly y-polarized but involve crossing also; t,hey are thus “forbidden.” Vibronic A, +- A, transitions can occur with z-polarizat,ion without crossing, but any borrowed intensity is not sufficient for them to be seen in the spectrum. Observations indicate that the splitting due to internal rotation is so small as to leave no definite mark in the spectrum: each pair of torsional sublevels is effectively degenerate. It is then not, essent’ial t’o distinguish between them and the selecation rules discussed above can be re-expressed in terms of t,he subgroup
SPECTRUM
OF ANILINE
365
G4 , isomorphous wit.h CzV. This convention is adopted in the rest of the paper. The splittings due to inversion turn out to be large and the components of t’he origin band are separated by several hundred wave numbers (see Fig. 3). It is convenient to regard only those transitions connecting the lower inversion sublevels as the origin band, and to treat those connecting higher sublevels as a l-l sequence. The higher inversion sublevels are then classed as distinct vibrational states. Finally, it should be said that a vibronic classification according to G8 , G4 , or even C, , does not imply nonmixing vibrat’ional coordinates. Torsion and inversion both take place with large amplit’ude and may couple vibratjions of different symmetry. The theoretical picture is extremely complicated (13). Fortunately, the assigned frequencies closely resemble those of a rigid molecule such as fluorobenzene (14), so that the potential energy of the phenyl group appears not to be heavily dependent upon the nonrigid coordinates. III. EXPERIMENTAL
Spectra of aniline vapor were photographed in the 18-22 orders of a 3.4-m Jarrell-Ash Ebert spectrograph fitted with a 57 OOO-lineHarrison grating. The contour of the individual bands rises rather sharply to a strong peak near the high-frequency edge, then falls more gradually towards the red through a minimum and a broad maximum. Rotational structure is well developed over 60-80 cm-‘. The main peak itself has a substructure of three intense maxima of which the central maximum was arbitrarily chosen for measurements of band posit,ion. The frequency measurements were made on prints enlarged about ten times from the original plates: since the features measured were themselves quit,e broad the accuracy (about 0.2 cm-‘) was not significantly less than by use of n comparator. Our frequency data are generally consistent with those of Ginsburg and Matsen (I), but we find no support for their division into “double” and “single” bands, apparently an artifact of incomplete resolution. Rotational structure is sharp for bands lying to about 1000 cm-’ to the blue of the origin band, but at higher frequencies the bands begin to look diffuse and the spectrum is virtually washed out past 0 + 2000 cm-l. In the sharp region, although different bands show small variations in contour, we found no evidence for the presence of transitions having different polarizat’ion from the origin band; that is, the system appears not to contain any “forbidden” part. This result is broadly in agreement with that of Albrecht and Kalantar (15) who showed that the transition in the condensed phase is quantitatively g-polarized in the neighborhood of the system origin. The isotopes used were aniline-h7 , -dz , -d5 , and -d7 . Aniline-d5 was prepared from benzene-d6 by conversion to nitrobenzene and reduction with SnCl, in D&DCl. Amino-hydrogens were replaced by a few exchanges with DtO. Paths ranged from about 10h3-10-l cm atm at 20°, except for aniline-h, which was also examined in a folded cell (to 1 cm atm) in order to develop “hot” bands. A low-
366
BIIASL),
_ -ig -
-_o -
-
Y_._
_ A
WILLIAMS,
_
AND
COOK
I
I
SPECTRUM
TABLE
I
TOTALLY SYMMETRICAL FUNDAMENTALS IN THE State
“A1
‘H2
7’0, cm-’
2940-ASYSTEM OF ANILINE
CsHsFa CGHBNH2 CGH5ND2 Modeb 6a*
519
528.7
808 1008 1022 1220
822.3 989.6 1029.1 1286.3
796.6 999.9 d d
12* 1 18~ 7*
492.0 797.8 953.3 1183.8 1307.2 1431.5
481.2 786.5 953.2 1104.8 1274.2 1352.1
6a* 12* 1
34 032.1
34 040.7
n See Reference 14. h Asterisk indicates “X-sensitive” c Resonance. ClRegion not examined.
518.7” 511.7
367
OF ANILINE
A 7* 8
CsD,Fa
G,DbNH?
(IN
cm-l)
GDbNDz
Modeb Ga*
505
515.9
504.2
753 880 959
756.3 879.4 957.0 d
d d d d
12* 18a 1
481.3 760.0 823.8
470.0
(ia* 12* 18~ 9a 1 A
844.9 922.6 1181.1 34 196.6
814.6 842.7 890.3
34 025.7
modes.
resolution int’ensity profile was taken with a small recording spectrometer resolving about 2 cm-‘: the t’race (Fig. 3) shows the intensit’y minimum and reddegradation for individual bands, but the substructure in the main peak is not, visible OII this scale. The -dT sample contained approximately 8% hydrogen and its analysis was not pressed beyond the strong bands in the system. A list of principal bands is given in the Appendix. Assignments are indexed by a convention previously described (16) in which a v’-v” transition in the normal mode k is represented by Ic$ . Since we do not have a complete vibrational analysis the numbers allotted to the various modes are somewhat arbitrary, but a provisional scheme is drawn up in Table I. The convention is extended to allow, for instance, ?c”to represent the term value of the vth level of mode k in the excited electronic state. Finally, as shown below, the KHZ out-of-plane rocking mode (bl) occurs in a double-minimum potential so that, it,s energy levels are unequally spaced: because this vibration tends to invert configurat,ion it is symbolized I and, since the inversion splittings are large, its energy levels are numbered 0, 1, 2, 3, . . . so that 0 and 1 correlate with the O+ and O-, etc., in the alternative notation. IV. ANALYSIS
As the vibrational structure in the aniline spectrum has much in common with that found for other derivatives of benzene it is worth while to describe the pattern briefly. Typically, t’he origin band is prominent in these spectra becoming
36S
BRAND,
WILLIAMS,
AND
COOK
one of the strongest bands in the spectrum when the substituent is polar (OH, lc, Cl) (17). Otherwise, only four ring modes occur with really high intensity, namely, the t,hree al substituent sensitive modes (numbered 6a, 7, and 12 in the notation based on Wilson’s classification (18) for benzene) and t,he ring-st,rrtching mode 1. The frequencies assigned to these modes in t#heground :mtl cxcLit,etl states of aniline are given in Table I. Within relatively narrow limits they arc uncQhanged in other monosubstituted benzenes provided that the suhstituelltj is drawn from the first row of the Periodic Table. Surrounding each of the strong bands associated with the fundamentals is a group of v’-u” sequences ant1 (0 f 2 )’ - d transitions in nontotally symmetric fundamentals, the most pronlincnt of which, in the aniline-hi spect’rum, lie at +374, +65, -40 and -?:I2 (‘nP1. These transitions arc identified in the neighborhood of t,he origin hancl itI Fig. :i though the +65crn~’ region is more complex than it appears under low resolution and is actually an envelope of three nearly-coincident transitions. Th(l spectrum develops further through sequences-upon-sequences, and SC)forth. WC do not have sufficient, information for clefinit,e assignments, but $374 is probably the 2-O transition and -232 the l-l sequence in the out-of-plane mode 16a (a?). Although -40 cm-l can be seen to the Z-3 transition and so must :uist from a ground-state fundamental below about 400 cm -I, it is not cble:tr wMher the fundamental is 18b (bz) or 11 (bl). The al modes give rise only to weak sequences. The remarks above refer part,icularly to the ring-& isotope. In the -c/sseries there is no infrared spectrum to check the ground-state fundamentals or c~ollntJeral evidence to guide the assignments for the excit)ed state. (No deuterated monoderivatives of benzene have been analyzed in the ultraviolet region.) The correlation with CsH5F and C&D,F in Table I is largely for convenience, to c~~tablish n shorthand for indexing assignments; thus, by correlating the 10% :nltl 879. cm-’ ground-state fundamentals of aniline-hi and -cl, with 18~ mode of fluorobenzene we do not in either case imply more than a very general connection with the el, mode (18) of benzene it,self. The vibrational coordinates are by no means illsensitive to ring deuteration. The mode A shifts with K-deuteration ant1 may involve KHn bending. Escbepting the -4O-cm-’ sequence, the strongest band to the red of the origin in the -hi spectrum is at -423 cnl-I. It is certainly associated wit’h clisl~lac~cn~cr~ts 1 of the amino-hydrogens, since it remains unchanged in aniline-d5 (123 (‘tn ; Table II) but falls to about 338 am-’ 111t.he ?;-deuterated species. The exc+t,etl state counterpart is at +292 cm-l, marked by an extremely strong band whicgh behaves similarly to the -423 band on isot,opic substitut’ion. (It shifts to +238 cm -I in aniline-&). Both 292 and -423 cm-’ occur in association with the ring fundament’als 1, 6a, 7, and 12 mentioned earlier, as shown in Tables II and 111; and both are surrounded by the usual pattern of sequences involving ring vibr:lt,ions.
423.1 336.8 422.1
423.0 336.8 421.9 324.5
CeH5NHz CsHSNDz
292.6
239.1 279.9 309.4 235.4
292.6
239.4 280.1 -
235.5
CsHsNHz
CsH~ND2
CsDsNDn
GDsNHz
0
6@
Mode
aZ = w(NH,).
GDsNHz CJ&NDt
&I’
0
Xi* 422.7 336.8
III
435.1
TABLE
Ia’)*
l0’6eo’
434.9
131)
337.0
70’
CoH,NDz
-
336.9
IhI+
6%’
B. X$(Z$
336.5
6a$
cnl-1
-
0
422.0
18aa1
I#),
_#(I,’
X$
423.1 336.8
lo’
-
AND
422.9
283.7 303.1 240.4 277.1 236.0
290.4
238.0 308.3 -
120’
6ao’
-
239.2 -
292.4
10’
-
239.1
-
w
-
237.9 -
701
-
238.4 -
-
6u&120
237.6 -
291.5
l&u~’
236.2 -
-
10’12rJ1
102
336.8
237.8 -
238.4 -
-
-
235.3 -
-
6u021201
422.9 336.8 422.0 324.5
Average
l,%ac?
10’6UO~
292.0
102 422.9 336.9
,21X1)
10’120’
(IN
POSITION OF Z1l SEQUENCE RELATIVE TO THE ORIGIN BAND (cm-l)
422.9 336.8 421.9
120’
A. X$(Z#
II Z,‘)
TABLE
CVMBINATI~N DIFFERENCES Y$ (Z,o -
370
BRAN>,
WILLIAMS,
ANI)
COOK
The envelopes of the --223-m-’ band ad the origin band look identical, so that the interval is totally symmetmal: however, it is not a fuudament:4 bec~:~usr the lowest, al subst)ituent mode, 6 (NH?), lies near 1tiOO cm-’ (19) and the lowest it, might rqmsent ring mode, 8a, is well characterized at 528 (WC’. Conceivably, :L O-2 transition in the t’orsional mode’ and, if so, we would attempt to explain The Inttcr +292 (‘I11-’ as either the l-l secluence 0~ the 2-O torsional transition. can be discounted immediately; f292 (*III-~ has as much intensit.y ;LSt tw origin hand (in the case of aniline-d? , 50 percent greatel intensity), i.e., it is ;tt lwst nlk order of magnitude too intense for assigtmerlt to n 2- 0 transition in :I Ilontot:dl\ ~ymrnetric vibration. Its assignment as a l-l I orsional secluencr would :~lso 41 for special pleading to account for the high intensity, hut in my case MII 1)~ rt11~~1 sequences. Likewise, a sitrorig c:w out by the nonappearance of 2-2, :‘,-’.3, t call he mounted against the assignment of --l2:3 cm-’ to the O-2 torsiotlxl transim tlon. The -42:3-cn-” band, after correction to equal populut,ions, 1~s twiw t h int envit’y of the origin band, while the corresponding -337-cm-’ bnrld h:~h almost i hree limes the origin intensity. ,411 this is quite incompatible with tllc* hehaviol txpect,ed for the torsional mode. III the spectrum of phenol, whew the torsiotml sec~u~~~~cescm be identified beyond doubt., the 1 --I .sequencc has intensity 1 IY~:~live to origin = 100, and the O-2 transition is too weak to detect (Z/j 1. Another conjecture is that the bands are O-2 and l-1 transitions iii 11~ out -ofought to tw pl:tnr mode o (NH?), with the implication that a 3-O transitiori prcserlt t awards shorter wavelengths. Xt t,his point a rharacteristi(. featIm> of the +Wand - -l2:3-c111-’ batlds becomes import ant,, namely, that t hew itItcrvals never combine wit,h OIK anot)her. Surrounding the --Il’li-u~~- ’ hnd, for instance, WC fiud t.he usual +W, -40., and -~32-cn-~’ sequences, hut t I~cre iy 110 band of acceptable intensity at or near (292--425) cm-‘. (III the ~pwtrun~ gwerally, the intensities of bands separated by the +Y9%cm-’ inter\-:rl clo not differ by more than a factor of two.) This feature recurs throughout thr s~w(~ttwn. since every strong band has :I waker band 423 cnl- ’ to the ~(1 (‘.i‘w/r/ t I~OSC~ Ixmtls that contain f292 cuC* or higher members of the sa~nc m:uIifoltl. ‘lks inc*idcnce of the -423 interval is shown in Table II, that of +292 in T:rt)l(, III. The former are combinat.ion differences, the latter not. _knalysis of I tt(s rillg-A5 isotopes is more comprehensivct than that of the rillg-r& , for reasons givc.11 cat4iw. The nonoccurrence of a f292 sequerwc attwhed to the --L?:s-NIIhtrtl is explained only if the mode with which these bands are asso(4atcd is atll~artllorlic*, SO thnt the 5-2 sequence does not fall in tke hnmonic position: :ttltl, if 1his i.+ cSorrecbt, t h(> position of the 2-O transition cannot he determined h:~rtll(Jtli~~:lll~. The spwtmm of aniline-rls has a strong band at +.X8 c:111-~(+5139 for wkilitic,-rli) ~vllic*h cxmld be so assigned. This situation is less c*lcar in the -hi sp~~xl~IUII t,ut there is :i hmd at +760 cn-’ which may h:lve this explanation. +7fiO is (~I~wl; 1.~II ilIfrared
vapor
I)slld
st 420 cm-’
has
heel1 assigned
to the overtone
~T~NII,I.
S~~IICY
SPECTRUM
OF ANILINE
371
resolved from the first -40 sequence attached to +797 (mode 12) in the highresolution spectra t’hough not in the profile given in Fig. 3. Our reservation concerning the +760-cm-’ assignment is not connected with the near-coincidence with 797-40 so much as the fact that mode 12 is in resonance with a combination of two out-of-plane fundamentals in certain other benzene derivatives, notably phenol-d1 (61),’ so that we cannot be sure that 760 cm-’ is associated with the w(NH,) mode. Granted t,his uncertainty we give very little weight to the 2-O assignment in the -h7 spectrum. In the -d? spectrum the 3-l transition can be identified for t’he following reason: The strong sequences at -40 cm-l in the -hT spectrum are usually seen at -45 cm? wit’h aniline-d2 , but occur in that spect#rurnat -37 cm-’ when attached to bands arising from the U” = 1 of w(NHZ). (The S-cm-’ shift’ is characterist’ic of bands from the U” - 1 level of w(NH,) and must originate in some Fermi-t)ype resonance in the ground state, but as we cannot definit,ely assign the -45.cm-’ sequence we cannot identify the resonance either.) The 3-l transition (at +860 cm-‘) is one of the -d, bands singled out by this anomalously small spacing in the sequences. It happens that the 3-l transition can also be identified for aniline-h? , again as the result of an accident’. The Oransition 1201111 (where I symbolizes the w(NH2) mode) does not occur at t’he harmonically calculated frequency of 797 + 293 = 1090 cm-‘; instead two equally intense bands are found at 1080 and 1100 cK1, displaced to either side. We know there is no bl ring fundamental in this region, since all bl ring modes lie below 1000 cm-’ , so t,hat, the probable explanation is
position
BB1 : 12l1’ +-p--+ I3 (cm-‘) expected: 1090 observed: 1081.4 1100.8 (cm-‘).
Since the 12’1’ level is forced down by about 9 cm-’ the unperturbed 3w (NHz)’ level I3 must’ lie near 1092 cm-‘. Another type of resonance, present in the aniline-d6 spectrum, strongly supports the II’ assignmenbs. Aniline-d6 has strong bands at +280 and 309 cm-’ (see row 3 in the body of Table III) symmetrically placed relative to the expectation for III of +293 cm-‘. We know that an out-of-plane ring fundamental falls in t’his region, namely, the mode 4 which occurs at 306 cm-’ in the Bt,, state of benzenedg (W2). Strong coupling bet,ween I’ and 4l leads to the appearance of two transitions II’ and 110401.With roughly equal intensities observed we expect the unperturbed II’ t.ransition to be close to the mean position, 294 cm-‘. This is essent’ially the sanle as its position in the -1~~spectrum, where the ring fundamental has shifted to about 365 cm-1.3 The import,ant point is that the vibrational symmetry 2 Phenol-d1 shows strong bands at 0 + 773 and 0 + 789 cm-’ with the intensity ratio 2: 1 (21). 773 cm-’ is the perturbed fundamental 12; 789 cm-l is probably a combination of bl fundamentals 429 + 356 cm-’ (20). 3 This is the frequency observed for the ring mode 4 in t,he B 2u stat,e of benzene (82). III t,he eR2 st,ate of phenol-hs the fimdamental frequency is 357 cm-’ (20).
372
BRAND,
.
.
1;
0
WILLIAMS,
1:
1:
1:
AND
I .
I
1 1
.
’
b,
0
A,
3
B,
2
A,
b
2,
1:
. W
COOK
Fro. 4. Transitions involving levels of w(NIIn). The scale applies however, the II3 transition is not observed. The oibrafional symmetry of the figklre.
to aniline-h7 where, is given on the left
of the upper state of the 111 transition is confirmed as “bl , as the assigmnent requires. As companions to the 12’ transitions we expect t’o find IS1 transit’ions emanating from the 3w(NH2)” level of the ground state. It will be shown lat’er that these transitions are weak and we have actually assigned only two of them, both in the spectrum of aniline-& where they occur 4% cnl’ to the red of t,he transitions II’ and 6a01Z1’.The combination differences II’ - Iso and 6a01[,T1’- 1311shown :lt the foot of Table II then isolate the V” = 3-1 interval of w(NH*) in t(he ground stat,e of this isotope. The t’ransitions emanating from I3 are weak so that their identification is somewhat tentative, but’ a band-by-band search failed t)o give nny alternative assignments; and their identification for aniline-& is not, surprising since bhis isotope has t’he best-developed spectrum. A survey of transit ions involving the w(NH2) mode is shown in Fig. 4. V. DISCUSSION
The transitions between levels of w(NH2) fall into two sets-one combining V-41 levels, the other combining ‘B1 levels-without crossing bet,ween them,
SPECTRUM
373
OF ANILINE
so that the analysis does not position the B1 levels relative to t’he ground state. Their relationship can be calculat,ed by treating the vibration as occurring in a double-minimum potential, V(q)/hcvo = Yiq2 + CYexp( -pq”),
(2)
where p is the (dimensionless) normal coordinate and a, /3,and ~0are parameters. The eigenvalues and eigenfunctions of this problem are available from tables (,=). The ground-state combination differences in Table II, namely, G(2)” - G (0)” for aniline-h, and -d2 together with G(3)” - G(1)” for aniline-& , are sufficient to determine the potential constants and hence the term values Gus where u is odd. This is done in Table IV. The barrier height., I’,
= Y~(~LY~ - In 2ap -
l)/Zp
(3)
calculated in this way is considerably smaller than the well-known ammonia barrier (-2000 cm-‘), though comparable with the values obt,ained for structures in which a NH2 substituent’ is conjugated with an unsaturated group: these include formamide (V, = 300 f 50 cm-‘) and nitramide (6,8). TABLE
State
LEVELS OF w(NH*)
-
-
-i-
T
AND
Go(3)) cm-’
i_
0
IV
IN THE A,
Obs
--
1VU cm-’
B
a
Bz STATES
Calc
Barrier
4Jm
Cm-'
_-
em
degrees
-I-
A, (Ground stat,e)
C,H,NH,
0
0
CcHsNDe
1 2 3 0 1 2 3
(33)s 422.9 665” 0 -
0 1 2 3 0 1 2 3
0 326
336.8 4401
0 34 423 665 0 7.1 337.0 441.9
7.9568
0.1145
579
562
5.2402
46
10.8717
0.0838
429
568
7.1599
48
0.5497
1.6575
406
24
0.6153
1.4807
317
27
B? (Excited state)
C~HENHZ
CsHsNDz
(760) 1126 0 246 548 868
= From the infrared
0 326 730 1124 0 247 561 868
/I
spectrum
1 (19).
1
-
I
-
-
HIIANI),
37-l The
WILLIAMS,
ANI)
COOK
angle bet’ween the planes of the substituent
and t’he phenyl
ring in the
ground state was cakulated assuming t,hat the reduced mass reyuired is a funcaCon of t)he C-NH? fragment alone. As the data for aniline-d? are more reliable than
those for aniline-h7 we performed
1:Ltter as :I caherk. The formula 8,’
the calculation
= Qm’C =
8, and qm are, respectively,
T
where (: is the appropriut,e
wit,h the geometry, are typical
kinetic
energy
matrix
amines),
and
L DKD
c+nlent which This procedure
:i:i”, keeping
a planar
potent,ial;
configuration
130”. We thus obtain
L DSD
bond lengths
a value
is a poor approsima-
by allowing
0 to be nonzero
equal to an nrbit,rary
the same,
wc calculate
v:~lue
a new G
may be used to obtain :L new value for L DKD and thus C. was c*ontinued until the self-consistent’ values L DND = 113”
:n1tl 0, = 48” were obt,ained. lar results
=
of the fragment
of G and this was rectified
to cbhangc from 120”. Choosing
of lo!)” and I!?, =
(18).
the hindering
G by assuming
of B,, = 5.5”. It is obvious t)hat the planarity and i DKD
element
= 1.36 A (these values taken from ILeferenre 2~
R(C---N)
tion for the computation
(5)
used t)o describe
We first est,imatzcl
for aromatic
plane angle
= &’ = (;--‘#,
from the parameters
G must) be calculated.
plane-ring
This formula is obt’ainetl by on bending can bc ncglcc+ctl
is given by 2’J
Y,~ is obtained
(1)
t’hc values of the subst)ituent
energy
using t,he
(h,‘4&o)y,‘C:.
and t’he normal coordinate at the potential minima. assuming that changes in the S--D bond di&ance and that the kinetic
on the former
recluired is
(see Table
IV);
n~ss are :it least compatible Since the available
A calculation
on the aniline-h7 molecule
thus the assumption<
made to calculate
with the experimental data. G(2)” - C(0)"(twice)
information,
led to simt,he rrtluc~etl
and Ci(:$)” -
G(l)‘,
in the iq suffi(+ient only to calculate LY,8, and ~g, there in no surplus information c~le~tronic~bands to check the results. In the infrared vapor spectrum, however,
transitions assigned to the w(NH2) mode appear at ti(iF, and 440 c’n~‘, rcspc+ t ively, for aniline-h7 and -tlr (19). There is no evidence bearing directly upon their polarization
but they are presumably
Y-0 transitions
since
OIK
cxprrls on
gmerl~l
grounds 1hat (dp/dq)o will have ti larger component along s than along z, :mcl c~orlsequently t)hat. the infrared 1ransitions are :3--O and 2-1, as in the an~monis s~~~(*trul11. Their
c~omputcd po&ions are at 66% :Lnd 44% cm-’ as shown in T:rble IV. WC :11so expect to find cvidetice for the ‘I’--1 t,ransition whose c*al(~ulatctl position is at 388 caniPI in the infrared spect,runl of aniline-h7 . Here the xit uat ion is less c*lear. ,4 vapor infrared band observed near 390 cnC1 has been assigned to XI cc? ring fundamental (19), but this assignment contradicts the selection rules and the band in question ~oultl represent the 2-l trarlsition. If this is corrcc*t the observed value of f& (1)” is 33 cni-‘. Over-all agreement between the ~alc~ulntc~tl nnd observed infrared transitions is as good as could be expected considering
SPECTRUM
375
OF ANILINE
that the calculation assumes that the w(NHz) mode is not coupled to any other vibration. The 2-l transition predicted for aniline-& lies at the limit of the recorded spectrum and no definite conclusion can be reached. Once the term value Go(l)” has been determined for the m(NH2) mode the excited state terms Go(v)’ follow from the vibrational analysis. They are collected in t’he lower portion of Table IV. It emerges that the excited state mode is not harmonic and the concept of a double-minimum potential may again be called for. This potential does indeed give a reasonable fit to the known term values (see Table IV), though whet,her t,his analysis is realistic is anot’her question. The computed G(0)’ is 140 cm-l for aniline-h, ; t’hus the calculated barrier height of ~25 cm-’ is . a small fraction only of the zero-point energy in this coordinate. Although there cannot be much doubt that t,he reason for the anharmonic character of the w(NHz) levels is the same in both electronic states, the excited state anharmonicity could be account’ed for by a potential with parabolic walls and a single minimum, perturbed near the foot so t’hat a3v(q)/8q3 # 0 for a considerable range around p = 0. The double-minimum is not an essential feature of t’he excit’ed-state potential as it is for the ground state, and we earlier proposed the term quasi~2anar to cover this situation (9). Similar behavior is known for the excited state (‘A”) of propynal ($2;) and t,he ground state of ketone and diazomethane (%). As a final check we have calculated the intensity of the 11’ and 12’ transitions in t,he electronic spectrum and compared them wit#h experimental values from Fig. 3. The calculation was performed by first expanding the wave functions of the inversion levels in terms of harmonic oscillator functions using t,he coefficients tabulated by Coon, Naugle and McKenzie (23). Assuming that the transition moment is given by M(#Vt 1#V(r) w h cre M is the electronic transition moment, and Gv,, #v,, are the vibrational wave functions for the states V’ and V”, the int,ensrty relative to the origin band is given by
These ratios for I’,‘, IZo, I:,
INTENSITIESOF I:,
and 1: are given in Table V for aniline& I
TABLE
and -I& .
V
TRANSITIONSRELATIVE TO THE ORIGIN BAND
Transition
13l
ZP
I?
IO2
Relative intensity Aniline-h,
Aniline-d?
Obs Calc
0.04
0.26 0.17
1.01 l.OG
co.851 0.99
Obs Calc
0.05 0.31
0.40 0.83
1.50 1.93
0.50 2.79
376
BRAND,
WILLIAMS, TABLE
CHARACTER
G8
e
(ab)(AB)
MB)
TABLE
--I_-
“L
1
1
1
1
Au
1
1
1
1
B,,
1
1
-1
-1:
B,, I&,
I 1
1
-1
-1
-1
R.‘,‘
1
-1
H,,
1
-1
-1
1
HI,,
1
-1
-1
1
1
-1
1
-1
VI
FOR THE GROUP
Cab)’
CabI
AN11 COOK
WV*
1 -1 -1
(ab)(AB)*
1 -1
1 :
-
GS
-1
-1 1
1
-1
1
1
-1
1
-I/
1 -1
-1
-1
E*
1 -1
1
- -
1 -1 1
-1
1
-1 1
1
-1
-
1
As t)he spectrum is crowded the observed values are relative peak intensities instead of integrated intensit’ies which would have involved large and uncertain corrections for overlapping. dlthough agreement is by no mems perfect the calculated intensit,y ratios predict the general trend of observed intensities reasonably well. This is encouraging when we consider that, the calculation of intensit,ies is a more severe test, of t,he wave funcbions used t,o describe a system f,h:m the calculation of energies.
The aniline molecule is nonplanar in its ground state, the NH, plane being angled at about 46’ to the plane of the remainder of t.he molecule. This struct)ure is consistent with that proposed by Tyler (27) from a microwave study of aniline. The excited-state geometry canriotj bc uniquely determined from the potetkial hindering inversion in the molecule. The potential remains approximately constant over a large range of q near q = 0; the molecule is then q~La+s~pZnnnr. The t,heoret’ical interpretation of the groundMate geomet’ry can be formulntetl in t.he following manner: The various substit uent pert,urbations of the amino group cancel each ot’her to a large extent and allow the KHZ to take up the ammonia-like configuration. The charge-transfer and ground st’at,es are well separated so the mesomeric stabilizat’ion is small [0.3 eV in the Rlurrell c*al(~ul:ttion (28)]. This mesomeric effect may be cancelled by the inductive effect of the KHZ group which dest,abilizes the a-electrons of the hydrocarbon (29). This tlcstabilization is t’hough to arise from exchange repulsion; t,he amino and phenyl a-clect,rons occupy orbitals which overlap to some extent; thus electrons of the same spin occupy the same region of space and repulsion results.
Tnhle VII contains frequency measurements for bands discussed in the paper. It is not a comprehensive listing of all bands observed, t)hough all strong hnr& are included. Relative peak irnensities, based on 0 = 100, are given in pnren theses for aniline-h7 and -dz .
SPECTRUM
OF ANILINE
Table
Assignment
0
Aniline-h7
377
VII
Aniline-g2 --._
Aniline-d
-5
Ani'line-d
32
745.8
1201° l-2
32
785.8
6~;
32
975.0 (0.8)
33
164.5
182:
33
003.0
(0.3)
33
317.5
I0 I
33
042.5
(0.f)
33
040.7
(0.8)
33
239.6
00 6"l_!2
33
083.6
(3)
33
186.1
(5)
33
261.2
12;
33
210.8
(4)
33
244.1
(1)
33
440.3
6~;
33
503.5
(12)
:;
;;;$)
33
680.7
33
701.5
33
609.0
(26)
33
703.9
33
774.7
33
871.1
33
796.1
(8)
33
960.6
33
937.0
34
032.1
(100)
34
205.7
34
100.9
(10)
34
324.7
(lO1)
34
44v.1
3b9.1)
(14)
34
675.7
34
744.6
34
909.6
71
0 12 6a'l'
-1-I
(0.2) (0.8)
1 '3 0
(Origin)
10 63,L2 1 I1 II @d3 2
(34
L2 12'1° e-2
(13) M-0)
5;
;:8':,03(18)
35
845.1
(5)
34
040.7
(io0)
34
1115.0
m)
34
2sc.o
(l50)
34
324.7
(5)
34
490.3
(10)
9a'IO -Q-2 I %I AL;
34
524.1
(80)
34
521.9
34
562.3
(6)
34
657.1
(25)
34
666.9
(10)
6a210 u2 (34
792.2)
(85)
05)
34
567.5
(50)
6a'l' -c-l
34
814.4
(87)
34
759.8
(Q)
12;
34
829.7
(q@
34
827.2
(60)
G
hidden
34
196.6
34
255.7
34
476.5
34
506.0
34
554'.7
34
619.5
34
b77.3
34
736.0
34
956.0
34
986.2
34
956.6
I
-7
BRBND,
378
Table
VII
Assicnment
WILLIAMS,
AN11 COOK
(continued) Anilinr-h. _--.---
154 971.1
Aniline-d -? -_____.
Aniline-d5
Aniline-d_
35
020.4
35
GE.2
35
041.5
35
04b.7
'15.2
(5)
34
Y77.b
(10)
34
9t5.4
(66)
34
993.Y
(42')
35
35
015.8
(?o)
35
003.4
(20)
35
156.4
35
09j.Y
35 ~5
300.0 326.6 I
3:
255.5
35
233.7
35
319.5 .-
33
204.7
35
055.1
Iq)
35
135.1
(12)
35
"3.5
(25)
35
067.6
(f5)
35
132.9
(16)
34
901.1
ISJ)
35
215.9
(30)
35
145.4
(25)
35
217.9
(5yq
35
233.1
(36)
35
304.0
(23)
35
240.1
(4)
35
319.8
(30)
35
az.1
(25)
35
359.9(42)
35
355.2
(5)
35
471.9
(J;$)
35
471.4
(32)
35
514.2
(3)
35
bUb.l
(4)
35
546.4
Czo)
35
55i.r
16)
35
626.2
(30)
35
377.7
35
433.2
35
501.7
35
522.2
35
55Y.t:
3
SPECTRUM
OF ANILINE
379
Table VII (continued) Assignment
Aniline-F,
Aniline-c2
'2 'c-h
35 746.' (181
35 540.6 (?I
"6a'l' o-u-'
35 169.3 <=-I
35 709.5
(35)
35 614.2
(3)
d6a 35 782.1 (24)
‘o’*o 1 1 6a2'2' -00 '3 'CL'
Arliline-47
,35 780.1 (24) 35 788.0 (8) 35 855.' (16)
412;
35 930.5 CaI
l2 0
35 937.2 (15)
12 '0%
35 945.0 (14) 35 951.0
‘O’2& I 1 1.
(20)
36 016.4 (3‘S)
I21’
O-l '2 lo6s
36 '82.8
(6)
36 '89.4
(5)
1;6&
36 328.4
(=I
This
Aniline-g5 -P
work was supported
RECEIVED:
by the National
Science
Foundation.
l\lay :3, 1966 REFERENCES
1. N. GINSBCRG ;\ND F. A. M~TsEN, J. C’heul. Phys. 13, 167 (1945). 2. 11. BOB.%, Bull. Chem. Sot. Japan 34, 76 (1961). S. T. J. BH.\TT.XH~RYY.~, Indian J. Phys. 36, 533 (1962). 4. M. AKONEY .\ND R. J. W. LEFEVRE, J. Chem. Sot. p. 2775 (1956). 5. A. WEISSBEKGER ‘\ND R. SANGEW.LLD, Z. Physik. Chem. 36, 237 (1929). 6. C. C. COSTIIN .IND J. M. DOU’LING, J. C’hem. Phys. 33, 158 (1960). 7. I>. J. MILLEN, G. TOPPING, .IND D. R. LIDE, J. Mol. Spectry. 8, 153 (1962). 8. J. K. TYLER, J. Mol. Spectry. 11, 39 (1963). 9. J. C. D. BKIND, D. R. WILLLZMS, AND T. J. Coos, J. Mol. Spectry. (in press). 10. 11. C. LONGL-ET-HIGGINS, Mol. Phys. 6, 445 (1963). 11. J. Ii. G. W.&TSOS, Can. J. Phjys. 43, 1996 (1965). 12. G. HEI~ZBEI~G AND E. TELLER, 2. Physik. Chem. 21B, 410 (1933). 13. J. T. HUTGEN, Can. J. Phys. 43, 935 (1965). 14. D. STEELE, E. R. LIPpINcoTT, AND J. XAVIER, J. Chem. Phys. 33, 1242 (1960). 15. A. H. K.\L.INT.zR *ENDA. C. ALBKECHT, Bw. Bunsenges. Physik. Chem. 68, 361 (1964). f6. J.C.D.BR.IND,J.H.CALLOMON,D.C.M~ULE,J.TYRRELL,.~NDT.H.~~OODWIN,T~~~~. Fax&y
Sot. 61, 2365 (1965).
17. H. SPOXER, Rev.
Mod. Phys. 18. E. B. WILSON, J. C. DECKS, New York.
1955.
14, 224 (1942). .\ND P. C. CROSS, “Molecular
Vibrations.”
McGraw-Hill,
30
BRAND,
WILLIAMS,
AND
COOK
19. J. C. EV.1NS,SPeCI~ochi/lL. ;Icla 16, 428 (lQ60). 20. H. I). BIST .IND J. C. D. BK.IXD (to he pLll)lished). 91. W. W. ROBERTSON, A. J. SCOTT, -\NDF. A. M.\TSEX, J. .J VI. (‘hem. Snc. 72, 1539 (19501. it?. F. M. GARFOKTH, C. K. INGOLD, AND H. C;. Po~)LE, J. Chenz. Sot. p. 401 (191X). ZS. J. B. COON, W. W. N.\~~GI,E,.IND R. L). MVKENZIE, Report Air Force 0911 /C’OII~ract AF JQ(638) -5931 (1963). 24. L. E. SITTOn-, “Int,eratomic I)ist,ances.” The Chemical Society, Lo!ldoll, 195X. lQli5. _‘5. J. C. D. BRDD, J. H. C:.~LLI~~OS, .XED J. K. (.i. W.\‘I’SI)N, Disc. Parar/rr.!g Sot. 35, 175 (lQ(i3). 26’. C. B. MOOHE .\ND (+. C. PIMENTEL, J. Ch,em. Phys. 40, 1529 (lQti4). 27. J). (:. LISTEK AND J. K. TYLER, Chem. (‘ommun. No. 152 (1966). AX. J. N. MCRRELL -\NDM. (;UDF&EY, Proc. Roy Sot. A278, 71 (19(X). 9. J. N. ,MUKKELLr\w M. (;ODFI~ET, Pwc. Roy.SW. A278, (i-l- (lQ(i-I).