Analysis of the Ã1B1-X̃1A1 electronic transitions of pyrimidine-d0 and d4 vapors

JOURNhL

OF MOLECULAR

31, 76-94 (1969)

of the a’&-?A, Electronic Transitions Pyrimidine-do and d, Vapors’

Analysis

K. K.

SPECTROSCOPY

INNES,~

H. D.

MCSWINEY,

JR.,~ J.

D.

of

SIMMONS,~ AND

S. G. TILFORD~ Department

of Chemistry,

Vanderbilt

University,

Nashville,

Tennessee

The ultraviolet absorption spectra of 1,3-diazabenzene-do and c/d vapors have been studied at high resolution between 2600 and 3500 .&. Each band, including the 0-O band, shows the very strong and sharp Q-branch edge of a type C band. Thus, the electronic transition moment must lie perpendicular to the plane of the molecule. Rotational fine strllctllre of the origin band of the cl0 molecule indicates that t,he transition effects only about a 0.5% increase of the average distance of the atoms from the center of mass. However, interpretation of the vibrational intensities, on both sides of the origin band, by means of the Franck-Condon Principle, suggests that some bond distances in the aromatic ring change by 510%. The two results are made consistent through the assignment of the two intense upper-state progressions to the totally-symmetric (ring-elongation) normal modes, ~6, and YS:,: Atoms on the Cz symmetry axis of the molecrde are little affected by the transition while the off-axis heavy atoms move only slightly away from the center of mass. Bond angles CNC open, but also only slightly. Extensive Fermi resonances, linking ytiBand YX~ in each electronic state, are analyzed. These account for the complexity of vibrational structure. All strong bands are assigned to the single electronic transition ‘&(a*, n)-‘AI . New measurements of the Raman spectrrlm of pyrimidine-da are appended. The

ultraviolet

first observed An absorption observed

absorption

in a redistilled maximum

the ultraviolet

spectrum

of pyrimidine

water solution

Was reported absorption

(1 ,%diazine,

by Heyro$h

at about

Fig. 1) was

and Loofbourow

2330 A. Uber and Winters

of pyrimidine

in the solvents

(I). (6)

methanol,

cyclohexane, and water. They found a second, much weaker maximum; at about 2850 i in cyclohexane, and shifted progressively toward shorter kvavelengths as solvents of increasing polarity Tvere used. The stronger absorption seems to be 1 Based in part upon the Ph.D. theses of H. 1). >IcSwiney, Jr. (1968), J. D. Simmons (1963) and S. G. Tilford (1902). 2 Present address: State University of New York, Binghamton. 3 Present address: Athens College, Athens, Alabama. 4 Present address: National Bureau of Standards, Washington, D.C. 5 DuPont Postgraduate Teaching AssistalIt, 1960-l. Present address: U.S. Naval Kesearch Laboratory, Washington, 1j.C.

ULTRAVIOLET

ABSORPTION

OF 1,3-I)IA%ISE

ii

the analog of the 2400 w system of benzene but the waker has no analog in that isoelectronic molecule. Strong illumination with wavelengths of either region causes pyrimidine to phosphoresce (3) from its lowest-lying triplet skate, which will not, be discussed in the present paper. Recently, wnk fluorescence (.$) 1~1s been found t’o follow illumination in the 260 8 region. Uber (5) \vas the first to study these absorptions of pyrimidine for the vapor phase. He measured some 120 sharp bands near 3200 8 and several diffuw bandi: at shorter wvclengths. The main upper state vibrational difference that lw found near 3200 A was about 1000 cm-’ (5 ), a number strikingI>. higher than t8hrl differences th:tt dominate corresponding spectra of other nzabenzenes. This 1:trgrr interval, as well as a somewhat longer progression of bands than in othrr :G:V bcnzerw accounts, in part, for the rather extensive (-6000 cm- ‘) and sym metrical absorption between 2700 and 3200 8. It is the main purpose of the* present paper to account in detail for the absorption in this region t~hrough vibrational and rotational analysis of a spectrum photographed at high r~solw tion. The most important conclusion will be that t’he strongest bands ~11 :lriw from wily me electw~lic transition, namely X’B,-.X’A41 I. ExPE:ltIIvIi!x’rAL

l’yrimidine-rlo was obtained from the Chemical Procurement Company while the pyrimidinc-r14 sample was supplied by Xw3q Sharp and Dohme of Cannd:~. l~:ach compound was used without further purificntior;. Spect’ra of the vapors were obtained between 2600 A and MOO A using :I C’:lrJ model 14 recording spectrophotometer (IXg. L’), :I J: trrcll-Ash Ebert Spectrometer Model S%000, a ,Jnrrell-Ash 3.4 m spectrograph (from I\-hich all wove numbws used in the vibrational analysis and tabulated herein were obtained), :md the Gxtecnth order of a thirty-five foot Ebcrt spectrograph (I;ig. 3 ). Standard liwi: for th(l high resolution measurements were from the iron arc in thcl case of the 3.4 m spectrograph and the iron hollon cathode in the case of the 35-ft spcctrograph. Relative intensities for the sharp bands of the X200 8 region were cst,im:\tetl mainly from the spectrometer tracings taken with the sample at 2.5”, l.iO”, OI ~50°C’. Microphotometer tracings over narrow regions were used for estimates of intensities of weak bands. It is recognized that a major source ot’ error in basing our scale on low resolution recordings is the finite slit-width of the spectrometer. How-tver, Franks (6) has shown that the errors in relatbe intensities of bands of ;in entirely similar system of s-tetrazine are negligible when obtained in this way. liar greater errors are expected to arise from the indeterminant background rel:rabsorption which is evident in Fig. 2. It cannot be expected that intensities tiw t,o the origin band (we below) quoted herein are correct to better than 50”; in most c:ws. On account of the close connection of possible intensity borrowing in th(> electronic spect~rum with Raman intensities, new measurements of the Raman

T ,A\,

_________t_______/_____ _-_-_-Y(d)

./‘\l/c\H

I

Y

1z(b)

Nitrogen

120.4

FIG. 1 (a). Model and axis conventions for the pyrimidine molecule. (lb). Groundstate geometric parameters shown are based on (IO). Arrows represent the geometry change estimated for the electronic transition discussed here. (see Section V). 78

ULTRAVIOLET

ABSORPTION

I

I

79

I

I

34500

37000

OF 1,3-l)IA%INE

30300

32250

cm-’

I

I

I

I

34500

37000

32250

30300

cm-’ FIG. 2. (a) Pyrimidinedo , absorption by the vapor. (h) Pyrimidined~ . In each case, I16b?; 0~0’1

vibrational assignments are noted for the most intense bands. The notations nnd [16b04; 16b$6a01; 6a02] refer to Fermi diads and Fermi triads, respectively.

spectrum photometer

of liquid

pyrimidine-d4

using a helium-neon

pendix, the results are compared citing radiation (7).

were made laser

wit)

(X 6325 A)

with these obtained

a Cary

Model

for excitation.

Sl spectro-

In the Apearlier with Hg 4358 A cx-

80

INN&X

FIG. 3. Rotational

fine structure

ET

AL.

of the 0-O band of pyrimidine-do

II. ROTATIONAL

.

ANALYSIS

The origin0 band (see vibrational analysis) of Fig. 3 is of the same type as that of the 3200 A system of pyrazine (8) so that it is immediately obvious that the electronic transition moment is parallel to the top axis and perpendicular to the aromatic ring. However, the degradation of rotational structure is, in the present case, somewhat greater, and the only features resolved seem to be formed regularly from superpositions of Q-branch transitions. These sub-band Q-branches start with rotational quantum number K = 1 near the band origin and degrade toward lower frequencies. In order to see how the total of several hundred transitions leads to only a few dozen sharp features in the region of high K, we shall assume that all rotational levels are adequately represented by the symmetric rotor formula (9). Thus, Y = VII+

(Bo’ -

Bbl)J(J

+ 1) + [(C,

-

C6’) -

(B,’ -

B;)]K2

(1)

in which J and K are the lower-state quantum numbers and J >r K. Let us substitute a good approximation for the nearly ablate top, namely B = 3C.637 v = vo+

L?(G) -

C’;)J(J

+ 1) -

(C,’ -

C’,‘)K’

(2)

6 It is assumed here and throughout the paper that the pyrimidine molecule is closely planar. See (10) for the microwave and X-ray evidence on this point. 7 Note added in proof: After this paper was submitted for publication, J. M. Brown (Canadian J. Phys. 47, 233 (1969)) published the rotational analysis of similar structure in a O-O band of s-tetrazine. He used an approximation to the asymmetric rotor energy to derive that the second difference should be interpreted as we have done here, namely as 2 (G--d’,

ULTI:A\‘IOLET

ABSORPTION

OF 1,:3-1)1.4%1Nlf

Sl

It is clear from this equation that for J = K the K-spacing will be ex:&l\. half the J-spacing. Since the most intense “Q-branch lines have J = K (9 1, important superpositions ivill be expected for lines of sub-bands of alte~r~~te K-values. For example, for K = 62 the strongest “Q-branch line will be that with .J = 6%. Approximately superimposed wiil be the “Q-line with K = 60 and .J = (il. that with K = T,S and J = 60, etc., but no lines of higher ii, since .I 2 K. \Vc> :~ssume that, all sharp features arise in this \vaJ.. It will be convenient to design:ttcJ wch feature with the label K,,,,, (= J,,,,,). K,,,,, n-ill represent usuall>~ tlics transition contributing greatest intensity to the feature and always the l:rrgc~~t K-value involved in contributions to the f&we. 1Ve arc now in a position to interpret the constant second difference of -0.00 1.i cm I for t’he features of Table I. To the approximation that intensity masim:~ :Lrc interprctnble by .I = K, \VC mny substitute .J = K into (21, n-hereupon w find the wcond differcncc 2(Co’ -

Cl’,

= B’o’ -

BI) = -0,OOl;i

cm-’

Th(> absolute numbering of Table I was fixed by a long extrapolation to the measured edge of the QQ-branches (31072.6 cnlP1) and may be in error b), one or t\\-o. However the change of constant is largely independent of such an error. Since it is known that the ground state Ci’-value is 0.103 cm-’ (IO), we may b(x confident that the efiect of the electronic transition is to increase the largest moment of inertia I’,’ by 0.7 ‘; . No other rotational structure has been analyzed. III.

VIBRATIONAL

ANALYHlS

For both pgrimidine-do and (7, , each of the approximately SO0 measured bands< exhibits a sharp edge of the Q-branch similar to that of E‘ig. 3. Hence, it is :w sumed that all bands are of type C and that all vibrational differences may btl assigned to totally-symmetric fundamental, combination or overtone levels of the ground or excited electronic states. Diffuseness becomes noticeable in the (1, spectrum :tt frequencies above 32500 cm-’ and is quite marked above 33000 cm-‘. In the & spectrum, diffuseness begins at about 33000 cm-’ and becomes marked less rapidly than in dl . It is assumed that this diffuseness of energy levels is responsible for the underlying “continuum” evident in Fig. 2. Available evidence is fully consistent with the point group C,, for the ground state pprimidine molecule (10). Thus, there are nine totally-symmetric fundamental vibrations for each electronic state (9). Three of the nine are carborlP hydrogen stretching modes. The similarity of Figs. ?a (do) and 2b (r&1)indicates that these are of minor importance in the spectrum. Five of the remaining six vibrations are ring modes which range in frequencies from 659 cm-’ to 1570 cm-~’ j Lists of wave numbers and estimat’ed intensit,ies of all observed bands are given in the Ph.1). t,heses of H. L>. XlcSwiney, Jr. (1968), J. 11. Simmons (1963) and S. G. Tilford (1962). Each of these theses is on file in the library of T’anderhilt University.

INNES

82

ET AL.

TABLE

I

FREQUENCIES OF REGULAR ROTATIONAL FEATURES OF THE O-O BAND OF PYRIMIDINE-&I (vY,, , cm-l) YO= 31072.32 cm-’

KIUX 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

'z?Q 31070.55 70.43 70.34 70.21 70.08 69.96 69.83 69.71 69.57 69.43 69.29 69.14 68.99 68.84 68.69 68.53 68.37 68.21 68.04 67.88 67.70 67.52 67.34 67.16 66.98 66.81 66.61 66.42

K max 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

qQ

31066.23 66.03 65.83 65.62 65.41 65.19 64.98 64.77 64.55 64.32 64.10 63.88 63.63 63.40 63.16 62.91 62.67 62.32 62.17 61.92 61.66 61.42

for the ground electronic state. The sixth is a carbon-hydrogen bending frequency which falls near the middle of this range. It is clear that, even if six, lowlying, totally-symmetric frequencies can be identified, there will be some arbitrariness in assigning them to particular normal modes. We shall make use of the recent infrared and Raman assignments of Sbrana, Adembri and Califano (II) to assign many lower (electronic) state vibrational differences, determined from measurements of “hot” bands in the electronic spectra. The numbering notation for the normal modes is that of (11). An assignment in the electronic spectrum referenced as h$ signifies a v’-vN transition in mode k accompanying the electronic jump, k”’ indexes the vth level of k in the upper electronic state, and so forth. The electronic origin band of the system, near 31000 cm-‘, is most convincingly

ULTRA\‘IOLET

ABHOIIPTIOS

OF 1,3-DIAZINE

s:s

established by the appearance of progressions in known lower state differences terminating (crossing) at a common-and intrinsically relatively weak-type C band assumed to be the O&Oband. In addition, our studies of the temperature dependence of the intensities of bands to lower frequencies, V, than that labeled 0-O in Fig. 2 show that none originates from a vibrational level lower than Y,,--O- V, that is that there is no f~_~eorigin at frequencies 200-1500 cm-’ lower than that labeled O-0. almost by The other assignments shown in l:igs. ?a and 2b are reasonable inspection. Note that the most prominent upper-state progression based on the origin is, for each molecule, the one labeled Sao” and exhibiting intervals of about progression, exhibiting intervals near 600 cm-l, is in 1000 cm-’ . A less prominent each case labelled by brackets to indicate a Fermi multiplet, with all intrinsic intensity arising from 6~~“. The 6~: progression is harder to follow because of the doubling, tripling, etc. for successive n-values, but if the intensity of a diad is of 6a01 is comintegrated it is evident from Figs. 2 that the intrinsic intensity parable to that for 8u01 and that both are 1.3-2.0 times stronger than the origin band. Moreover, the maximum intensity in the system is found for combinuiion bands with ~)z + n either 2 or 3, that is for bands 1600 or 2600 cm-’ from the origin. The rest of this section is intended to confirm the assignments of the normal modes vi, _N 600 cm-’ and vi, E 1000 cm-’ and to summarize assignments of some less important bands. Analysis of the Fermi resonance will be presented in the following section. In Section V the change of geometry will be estimated through combination of the vibrational intensities, the Franck-Condon Principle, and the results of the rotational analysis. vcn . Frequencies of this mode for pyrimidine-cl0 and cl4 in the ground state are available from infrared (a shoulder on a stronger band) and Raman (See Appendix ) spectra (7,ll). These are measured more precisely in the high-dispersion ultraviolet spectra as differences between the origin and strong “hot” bands (7). Indeed, with this higher resolution, it is revealed that the 6~1’ bands are Fermi doublets (see below). When the intensities of the diads are compared with those of the origin bands, it may be estimated, after correction for the Roltzmann factor, that the intrinsic intensity of each 6~1’ band is two to three times that of the respective 04 band. All of this is confirmed by observation of 6uZotriads. It follows that a prominent progression in vi, must be found to higher frequencies than each origin. Since, for each isotopic species, one of the two progressions found exhibits a spacing less than 10 % smaller than & , we assign the progression difference to Y& , as shown in Fig. 2. (It was noted earlier that the intrinsic intensity of each But diad is nearly twice that of the O-0 band.) Q,, . The second upper-state progression, which shows the difference of about uncomplicated by Fermi 1000 cm-‘, is, for each isotopic species, relatively resonance. The 1-O band again shows about twice the intensity of the O-O band. We refer as before to “hot” bands of intrinsic intensity greater than that of the

84

INNES

ET

TABLE

AL. II

DESLANDRES TABLES FOR THE veoVIBRATIONAL MODES A. Pryimidine-do

1669.8 0

31072.7(500) 1012.4

1

32085.1(675) 1015.0

29502.9(0.5) 1012.6 1569.6 30515.5(O)

33098.1(532) B. Pyrimidine-d4 1547.6 31188.3(500) 1002.5

29650.7(-) 1002.5 1537.8

32190.8 (860) 1oog.J$ 2 &Frequencies

30653.0(-)

33195.2 (888) in cm-l.

Estimated

relative

intensities

in parentheses.

origin band : The remaining ones are the 1%’ and Sal0 bands. Only the latter, with its lower-state difference of 1570 cm-’ (da) forms convincing combination tables with the 1000 cm-’ upper-state difference. On the basis of these tables (see Tables IIAB), the choice Sa,” is made for the labels of Figs. 2. Further verification is found in Section V. Other assignments. It has been remarked already that the remaining active lower-state mode is v12. No colcl band is so active as 121’ except those already discussed. The strongest band unconnected with vca or vg, or both is 941 cm-’ (do) [929 cm-’ (&)I to the high-frequency side of the origin. It has only about half the intensity of the O-O band and no 220 band is established. We tentatively assign it as 12o1and mention that 1%” and 13; may owe part or all of their intensities to mixing with another Bl-Al transition. The absence of Au = f2 bands is consistent with the Herzberg-Teller selection rule Av = &l. So also are the Raman intensities given in t,he Appendix for pyrimidine-d4 : Albrecht (12) has emphasized that those normal modes which are most responsible for vibronic (“forbidden”) intensity in allowed electronic transitions should show greatest activity in Raman scattering, especially as resonance conditions are approached. For excitationby Hg 43% A, which is of course closer to resonance than 6328 A, we find v:‘, somewhat enhanced compared to V& _ (The latter is only faintly active

ULTRAVIOLET 4%b ?)b.+2G,b 2%.

1303.6 1240.5 1185.1

2%b

645.4

*u,.s

ABSORPTION

OF

1,X-DIAZINE

M.i

595.4

0

23;. 2);.+ 2q,b 4Qb

via 2%b

1318.1 1303.1 1286.9

658.3 644.0

0

FIG. 4. Lwcls nrisiug from anharmol\ic (Fermi) terms of the type kQ&&, ilr t.he :I’& ad .fl.al electronic states of pgrimitline-tl, Observed transitions are shoa-n also with illtrusitics in parentheses. Energies we in crt-‘. The ahsetlre uf “l-l” haflds is ass~~mrtl to result from Frnnck-Condos> rancellatio~~ of itltcckty (see, e.g., b’. I,. Smith, IJwc. I%!/.~. Sot. lB, 8’3 (l!M)).

the ultraviolet spectrum.) However, the main value of the Rnmnn data are to ~lwm-that there is no intensity effect as dramatic as that found in one of the &isymmetric vibrations of pyrazinc (13) and therefore that there is no compellir\g argument for a large amount of borrowed intensi@ in the pyrimidine absorption. The strongest sequence bands of the system are found at distances from t,hc origin band (and other strong bands) of -155.0 (-137.4 in cl,) and +?2.4 (65.4 in (/.I) Cm-‘, and may be seen in Fig. 2. Section IV (see Fig. 1) ahon-a t,h:Lt neither of these arises from the lowest-lying vibration Vlfib . Accordingly, we assign thr strongest l-l bands to vlEr,, for lvhich the Appendix gives for pyrimidine-(1, liquid I&, = 373 crL’ so t,hat v:~~ is about 236 cm-‘. Seither 2-O nor O-2 assignments could be established for this mode. in

It has been mentioned doublets. The transitions

already that are illustrated

601~ and 6~; are each in fact Fermi for pyk~idine-d.l in Fig. 4; excrpt for

86

INNES

ET AL.

slightly higher vibrational frequencies, and more nearly equal intensities for the Fermi diad components, the situation is the same in pyrimidine-do . On account of the complexity of the spectrum, the Fermi triads encountered near the expected bands 6~2’ and 6a02are difficult to assign uniquely unless we predict them by careful analysis of the observed properties of the diads. These predictions have been made in the following simple steps: (1) We assume that the term in the potential energy which produces this anharmonic resonance is a cubic one of the form kq,qb’ = Vanh. (2) The perturbed vibrational term values of a set of near-degenerate resonating levels are then obtained as the eigenvalues of a matrix whose diagonal elements are the unperturbed term-values and whose off-diagonal elements are of the form E,; = Ei, = $ \En*VanhSPi dr. (3) In those forms the integrals have been tabulated to the harmonic oscillator approximation (14). The yl”& of (14) equals our q. (4) Thus, for the diads, we have E,’ - E

E21 EbO-E

E12

in which E represents the two perturbed 1’2

El, = &I cc [

2

11

21, +

Assuming that v6ais interacting tion, v, = vb = 0, and we find

=

0

(observed)

term values and

x % I(% + l)(Vb +

2)l 1/Z

with the first overtone of some low-lying vibra-

E,’ -

E

xk E:-E

?!ik

=

0

which gives term values for the interacting 1-O and 220 levels. (5) Similarly, for the triads (2, 0) : (1, 2) : (0, 4), we find E:

- E

(#‘2k

0

($&“2k

E,’ - E

@$>““k

0

(x)‘12k

Et - E

= 0

in which the potential constant k is as defined in (1). (6) If we define p as the ratio of intensities of the components of a diad, I~_~/12-o, it can be shown (9)g that, when all of the 12-o is “borrowed”, AE’=E,‘-E;= ‘Our p equals the a2/b2 of (9).

(&-Eb)s.

ULTRAVIOLET

ABSORPTION

87

OF 1,3-DIAZINE

Thus, the experimental information about the diads of Fig. 4 enables us to calculate the E” and the k-values for the upper and lower states. In each case, we then may set up the equation for the triad and, if we take EI’, Ez” and Es0 as harmonic values, solve the cubic equation. The results are, for the upper state: Assignmcnls

6a’ 6aL 16b” 16b4

Observed

Calculated

1173.0 1236.4 1313.1

1175.4(20) or 11&5.1(250) 1240.5(400) 1310.2(40) or 1303.6(60)

1284.4 1300.2 1322.3

1286.9 1303.1 1322.8 (or 1324.4)

and for the lower state: 16b, 6a1 16b2 tja,

In view of the harmonic approximation to the energies that we have used, the agreement confirms satisfactorily the origin of the interaction and enables us to make reasonable assignments in the triad. They are unambiguous only for the lower state. In order to remove all ambiguity, and to make assignments to the 6a03 t.etrad, we should have to determine the anharmonic constants in the vibrational energy expressions. The identification of the perturbing level 16b2is certain because of the observation of 16bl in the infrared spectrum. Identification of 16b2 is by analogy and is therefore not as certain. T’. FRANC%CONDON

PRINCIPLE

AND

THE

CHANGE

OF GEOMETRY

With the establishment of assignments for all of the strong peaks of Figs. 2a, b, it is possible to use intensities relative to that of the origin band to estimate the effect of the electronic change on the equilibrium geometry of the molecule. A quantitative application of the Franck-Condon Principle does not seem promising because nine normal coordinates would be needed for each electronic state and because accurate experimental intensities are difficult to obtain. (Bands are badly overlapped even at high resolution and Fermi resonance and the underlying continua evident in Figs. 2a, b are further complications.) We may illustrate the latter problem, and at the same time gain confidence in a qualitative application of the Franck-Condon Principle, by considering the estimated relntive intensities (in parentheses) for the pyrimidine-d4 bands, O-O (l.O), 6a01: 16bu” (l.S ), Xal (1 .X ) and 6al8ai (2.Fi). To the extent that the normal coordinates are independent of bhe electronic state, we should have predicted that the intensity of the combination band would be (1.8)2 or 3.2 rather than 2.5. Clearly, the agreement with Franck-Condon expectation is impressive, but only qualitatively. iiccordingly, we shall infer the geometry change on the basis; (1) of smoothed intensities 1.0, 1.7, 1.7, and 2.S of the bands noted, and (2) of the atomic dis-

88

INNES

6.9

FIG. 5. Normal sults (15).

modes most active

ET AL

aa in the pyrimidine

12 transition.

Based

on pyridine

re-

placements for V&L and ~8, determined by Long and Thomas (15) for the ground state of pyridine. (Franks (6) has determined the normal coordinates of the totally-symmetric vibrations of s-tetraazabenzene and has found V6aand vg, to be entirely similar to their analogues in pyridine.) These displacements are shown in Fig. 5. Maximum intensity in Fig. 2b is exhibited by the 6~4~8~~ (or possibly the 6ao18ao2) band. Thus, a sum of the two relevant sets of displacements in Fig. 5 must best represent the change of equilibrium geometry effected by the electronic transition. The choice of sign in such a sum can never be made on the basis of the Franck-Condon Principle alone. Fortunately, here the choice isseverely restricted by the result that the moment of inertia, I, , increases by only 0.7 “?o (see Section II). Reference to the molecular model in Fig. 1, as well as to Fig. 5, indicates that excitation of either V6aor vg, separately can increase I, , mainly by net displacement of the four atoms on the C2-axis, from the center of mass. However, it seems likely that the combined effects of these displacements on I, would be larger than that observed and that therefore the displacements along the &-axis must nearly cancel, as shown in Fig. 5. In that case it is implied that the electronic transaction effects little change in the position ofany hydrogen atom 01 in the positions of the carbon atoms on the C&axis. The appreciable changes must be in the positions of the four, heavy, off-axis atoms, either as shown by the arrows of Fig. lb or in the directions opposite to those. The latter choices are eliminated by the fact that they would reduce I, rather than increasing it. .By qualitative comparison with existing exact Franck-Condon treatments for benzene and s-tetrazine (8), we may estimate that the lengths of the arrows of Fig. 1 should represent very roughly 0.07 A. The most important change of geometric parameter is therefore a reduction of 0.09 8 in the length of those CN bonds that are nearly parallel to the &-axis. The CC bonds and the other two CN bonds are each lengthened by about 0.07 A. Bond angles are little affected. A somewhat different approach to the geometry change is to determine the directims of the arrows of Fig. lb from the sum of vti and vgadisplacements of Fig. 5, and the lengths of the arrows from the observed chtnge in the moment of inertia 1, . This procedure fixes the arrow lengths% 0.10 A.

ULTRAVIOLET

ABSORPTION

OF l,S-DIAZINE

s9

TABLE III Vacuumwave numbers,relativepeak intensities, and assf.gnmcnts of the importantQ-branch edges of pyrimidinc-d0 and d4.%+ 1($)I

d4

29502.9

1

29650.7

8ay

29669.4

1

29699.1

1

29301.4

16b;

29710.5

2

29885.2

6aTlGhi

29730.5

0

29865.5

6ai

29934.7

5

30351.9

9a!j

30001.6

8

30140.7

12;

30023.9

6

4;16b;

30079.1

5

6ayl6ag

30082.2

5

30171.9

1

8ay16bi

30232.8

8

16bil6a:

30240.2

9

30282.6

1

30388.6

42

30544.3

161,;

30395.4

42

30530.0

6ay

30444.4

3

30579.2

8a0121 10

30515.6

1

30653.0

8a:

30548.2

3

dn

*

30214.1

30393.6

Assignment

lo 1

6ay16ai 01 19al6a,,

9a06al 10

It is estimatedthat these assignmentsaccountfor 60 to 702 of the intensity. The estimateis uncertainmainly on account of the backgroundabsorptionevidentin Fig. 2.

:'Onthe evidenceof the rotationalylysis, Q-branchedges representband centersto + 0.2 an . Intensities are those of pyrimidine-do exceptwhen only the d4 band has been found.

INNES ET AL. TABLE III(Continued) 30996.7

ga@b;

30603.7

3

30610.9

1

19aOlZl 10

30612.8

10

8a09al 10

30614.9

13

30735.9

12'6a' 10

30670.9

2

30785.9

12;16b;

30682.1

1

30695.7

2

30809.8

19a08a1 10 01 116ao

30751.3

2

1;16h;

2

30891.9

8a06a116b2 10 0

30762.2

44

30913.2

16ai

30833.1

0

30875.8

0

8a:16bi 01 9a11Z0

15

31029.9

01 119ao

140

31050.9

16a:

3

31143.0

01 11120

10

31168.8

31072.7

500

31188.3

Origin

31095.1

100

31253.7

6b;

1

31354.2

2

31380.9

3

31400.1

20

31459.2

4

31510.7

31407.1

2

31532.5

31505.0

20

30916.8

31293.1

31336.9

1Z06a116b2 10 0 0 2 116a0 01 6al12" 8a01Z2 10 01 6a18a0 01 6a19a0

ULTRAVIOLET

ABSORPTION TABLE

III

91

OF 1.3-DIAZINE

(Continued)

saf

31527.7

2

31535.6

160

31651.5

6a;lba;

31686.1

560

31783.7

6a;

31742.0

570

31833.7

260

31847.9

1 1 6a06hl

20

32074.6

1o122 1 0

32014.3

440

32117.6

12;

32085.1

680

32190.8

Sa;

32181.8

420

32004.3

250

32257.8

8a16b1 0 1

30

32353.5

9a08a2 1 l-l

32323.9

270

32373.4

32364.2

620

32428.8

32402.6

30

32491.9

32420.7

16b;,

6a;16b; 16b; 6a18a0 0 2

2 430

32601.5

32618.6

310

32703.6

32677.0

400

32702.0

650

32757.4

520

6a09a0 1 1 6a1121 0 0 12;16b;

32790.5

6a18al 0 0 8ail6bi 6a09a0 1 1

32795.9

410

32932.7

210

32897.6

6”:

32977.9

405

33024.4

6ai16hi

33022.1

370

33120.7

8a1121 0 0

33035.4

340

33092.8

_

6+6bIf

INNES ET AL.

92

TABLE III (Continued) 33076.3

60

33224.4

16b;

33098.1

530

33189.5

8a;

33184.9

400

33297 .o

400

32813.6

33375.2

530

33398.3

33579.1

410

33715.1

610

33778.1

370

8ail6bi

33915.0

220

21 2 6a012$6b0

33988.2

360

21 2 6a08a016b0

34106.5

450

34304.3

400

34365.9

430

34382.6

500

34723

550

34788

550 550

1 1 8a09a0

6a18a116b2 00 0 6az16bi

33781.6

34207.5

1 2 6a08a0

8ai 8$ai

34384.1

2 2 6a08a0 12 2 6a08aO16b0

34784.7

1 3 6a08a0 8ail6b;

35368.6

2 3 6a08a0

It is important to realize that the effect on inertial constants A and B of the structure change of Fig. lb will be greater than the effect on C. In particular, the asymmetry parameter, K = 2B - A - C/A - C, may be expected to increase appreciably from the ground-state value of 0.87 (do) [0.67 for &I, perhaps by enough that the a and b axes of Fig. la are interchanged by the transition in the case of pyrimidine-do . In Section III, it was remarked that this reasoning about the change of geometry confirmed the conclusion that 8a01,rather than 120’, is the strong band near ~~~ + 1000 cm-‘. According to Long and Thomas, ~12does not show atomic displacements similar enough to those of vti that large changes of bond distances could be made consistent with the observed small change in I, . (see Fig. 5). It must be admitted that inspection of low resolution photographs of fluorescence from the xlB1 state, made by Logan and Ross (4), indicates a longer progression in V& (1071 cm-‘) than in & (1048 cm-‘). If this comparison survives quantitative intensity measurements, our assignments of v:, and vi, might need

ULTRAVIOLET

ABSORPTION

OF

1,3-DIAZIKE

93

to be interchanged. This would be difficult to understand in light of the rotational analysis, unless normal coordinates of pyrimidine turned out to be quite different from those of the related molecules. However, it could account for the fact that, of the three diazines and s-tetrazine, only pyrimidine shows an important upperstate progression in a 1000 cm-’ difference. The normal mode ~12is totally symmetric (and therefore allowed in first approximation) only in the case of pyrimidine. Aloreover, it may be that our difficulty in forming tables of combinations between V” = 1071 cm-l and Y’ = 1013 cm-’ arises from the known l;ermi resonance of the former (11). 1-r. CONCLUSIONS All strong bands between 3600 and 3500 8 are assigned in Table III to the single electronic transition, L~‘B,,~‘A1 . It is usually assumed that the orbital configuration for the excited state is . . (x, )’ (T,)*~&~T& , where a represents a node through atoms and b a node that cuts through bonds (17). However, the geometry change inferred mainly from the Franc&Condon Principle is consistent instead with . . . (a,)2(m,)2n,,Ln~~~. The geometry change is quite large and accounts for the fact that the maximum of absorption intensity lies 2500 cm:’ to higher frequencies than the origin band. APPENJ)IS,

TABLE

TABLE

I\‘

I\’

(dppenrlis)

RAMAN SPECTR~TMOF LIQL-IDPYISIMIDINI~+& Frequenciesa

(uyRC, cm-l) for excitation

by:

;\ssignment” 6328 A

4358 .i

_... ..____.__ vifia ia?P PC’6 (hl) Y&I((II) vi(a,) Y15(b?) u:j(bz) Y:,a((Cl) YIY((11) VlBO(ai)

v%l(u1) vx(ai) vzil,cal) Yih(bi)

373 (0) SOS(O.5) 6600.3) 808? W(O.5) 8X5(0.5) 915(O) 989 (0) 978(15) 1045 (8) 1278 (0) 1531(0.5) 2266CO.51 2280(0.5) 2300 (1)

659 (2) 824(l) 885(l)

977 (15) 1045(11) 1530 (1) 2264 (3) 2299 (4)

R Intensities as measured by peak heights are given iu parentheses. In the second (6328 b), int,ensities for frequencies higher than 2000 cm--i are unreliable. b See (7) and (11). c It is interesting that Foglizzo and Sovak recently observed this forbidden mental in the infrared spectrum of crystalline pyrimidine-do (11).

columii

furrda-

INNES

94

ET AL.

ACKNOWLEDGMENTS We gratefully acknowledge support of this work by the National Science Foundation Grants GP-402 and GP-5126, and, in the earlier stages, by the Office of Ordnance Research with funds supplied by the Advanced Research Projects Agency. An NSF instrument grant to the Vanderbilt Chemistry Department made the Raman spectrometer available. We are indebted to Dr. M. Kroll for obtaining the Raman spectrum, to Professor J. Brand for the use of his single-beam spectrometer, and to Dr. A. E. Douglas for the use of the thirtyfive foot Ebert spectrograph. REFERENCES 1. F. F. HEYROTH AND J. R. LOOFBOURO~, J. Am. Chem. Sot. 66.1728 (1934). 2. F. M. UBER AND R. WINTERS, J. Am. Chem. Sot. 63, 137 (1941). 3. (a). V. G. KRISHNA AND L. GOODMAN, J. Chem. Phys. 36, 2217 (1962). (b). R. SHIMADA, Spectrochim. Acta 17, 30 (1961). 4. B. J. COHEN, H. BABA, AND L. GOODMAN, J. Chem. Phys. 43, 2902 (1965), and L. M. LOGAN AND I. G. Ross, (private communication), 1968. 5. F. M. UBER, J. Chem. Phys. 9, 777 (1941). 6. L. FRANKS, Ph.D. Thesis, Vanderbilt University, 1968. 7. J. D. SIMMONS AND K. K. INNES, J. Mol. Spectry. 13, 435 (1964). 8. J. A. MERRITT AND K. K. INNES, Spectrochim. Acta 16, 945 (1960); J. Mol. &e&y. 23, 280 (1967). Van Nostrand, Princeton, New Jersey, 9. G. HERZBERG, “Infrared and Raman Spectra,” 1945. 1959. (b). P. J. WHEATLEY, 10. (a). R. F. SCHNEIDER, Ph.D. Thesis, Columbia University, Acta Cry&. 13, 80 (1960). 11. G. SBRANA, G. ADEMBRI, AND S. CALIFANO, Spectrochim. A& 22, 1831 (1966) ; (See also, R. FOGLIZZO AND A. NOVAK, J. Chim. Phys. 64, 1484 (1967)). la. A. C. ALBRECHT, J. Chem. Phys. 34, 1479 (1961). 19. K. K. INNES, J. D. SIMMONS, AND S. G. TILFORD, J. Mol. Speck-y. 11, 257 (1963). 14. E. B. WILSON, J. C. DECIUS, AND P. C. CROSS, “Molecular Vibrations,” McGraw-Hill, New York, 1955. 15. D. A. LONG AND E. L. THOMAS, Trans. Faraday Sot. 69,783 (1963). 16. A. J. MERER AND K. K. INNES, Proc. Roy. Sot. 302A. 271 (1968). 17. K. K. INNES, J. P. BYRNE, AND I. G. Ross, J. Mol. Spectry. 22,125 (1967). Note that the tentative vibrational assignments given in this review have been altered in the assignments of the present work. RECEIVED:

November 23, 1968