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The electronic spectrum of trimethylene sulfide
JOUILN.\LOF MOLECUL:\RSPECTROSCOPY 29, 1-12 (1969)
The Electronic
Spectrum
of Trimethylene
J. A. B. WHITESIDE~ Cniversiiy
AKD
Sulfide
I’. -1. WUSOP
ofDundee, Dzcndee, Scotlantl
The electronic spectrllm of trimethylene sulfide has been observed in the near and the vacuum ultraviolet. A vibrational analysis of an electronic transition 0 at 2260 .\ shows that the ground state is nollplanar but the excited state is platLar. Apart from a transition at, 1818 % all other electronic transitions lead to nonplanar excit,ed states. Two Rydberg series lead to an ionization potelltial of X.65 eV. Since the Rydberg states are nonplanar the ground state of the ion (CH,),S+ is also nonplanar. A correlation diagram for H,S is drawn and from it the shapes of the states of trimethylene sulfide are predicted. The predicted and observed shapes agree.
Harris
et al.
(I ) observed
t,he microwave
and showed that the ground I’rom t’he microwave of-plane
bending
levels. Borgers Art =
1 and Av =
al. (3) discussed
(2)
3 transitions
electronic
observed
the potential
in the out-of-plane spectra
function
spectrum
bending
for t,hc outthe energy
and identified
frequency.
and assigned
sulfide
t’o inversion.
they calculated
t,he far infrared
and Earnan
of t.rimet.hylene
wit’h a 10~~ barrier
and from the potential
the infrared
of t)he fundamental The
spect’rum they deduced
coordinate
and Strauss
spectrum
state is nonplanar
Scott
values
rf
to most
frequencies. spect’rum
of trimethylene
sulfide A\-aspreviously
reported
b!
l,ee t-i) and by Clark and Simpson (5). A higher resolution and more detailed spectrum of trimethylene sulfide is reported below. The shapes of the ground and excited states are deduced from the vibrational struct.ure of the electronic transitions. The electronic structure is discussed and the shapes of the excited stat,es deduced. The expected \\-ith the observed spect,rum.
shapes
of t,he excited
EXPERIMENTAL
Trimethylene
sulfide \vas obtained
states
are then
correlntcd
RIETHC)l)
from Kodak
I,td. a:ondwas purified
by ga-:
chromat,ogr:tphy. The spectrum between 2500 and 2200 A n-as photographed on an E:bert spectrograph with a reciprocal dispersion of 2.8 Lc/mm and with a resolution of about 3 cm-‘. Absorption pat’hs ranged from 10 mm pressure in a 1%cm cell to 40 mm in a 40-cm cell; temperatures from -35 to 100°C werth used.
1I’rc+nt
address:
I.C.I.
Fibrrs
Lid.,
Pontypool,
Wales
2
WHITESIDE
AND
WARSOP
Spectra in this region were also obtained with a recording spectrophotometer with the temperature ranging from 10 to 150°C. The spectrum at room temperature in the region 2300 to 1300 s was photographed using a l-m normal incidence vacuum spectrograph. Pressures of 0.08 to 3 mm were used in the 2-m path length and a resolution of about 10 cm-’ was obtained. ELECTRONIC
SPECTRUM
The near ultraviolet spectrum consists of two electronic transitions. At 2260 A t’here is a transition with extensive vibrational structure in which about 70 bands have been observed. At 2027 i there is another transition which has very little vibrational structure, only four bands being observed. In the vacuum ultraviolet there are many electronic transitions some of which can be fitted to two Rydberg series. 2260 8 System The spectrum is shown in Fig. 1 and the frequencies of the bands are listed in Table I. The bands consist of a strong central maximum, to which all the measurements refer, with weak wings on either side. On the long wavelengths side a weak head is formed about 20 cm-’ from the central maximum. At first sight the system appears to consist of a progression beginning with the first strong band at 43 919 cm-l with some much weaker bands to longer wavelengths. The vibrational intervals are about 130 cm-’ and the higher frequency bands have two heads. However, closer examination reveals that this is not the correct analysis. I’irstly the longer wavelength bands of the apparent progression have a complex structure with the intervals between them varying irregularly while the remaining bands in the series have a simple structure with the intervals between t,hem varying regularly. Also closer examination of the densitometer traces and the spectra obtained with the spectrophotometer show t.hat the first three bands in the series are slightly but quite definitely temperature dependent. The origin of the system was found by taking intervals between the regular
FIG. 1. 2260 i system
TKIMETHYLENE
SULFIDE
TABLE
I
THE 2260 1%SYSTEM Y
Assignment
(cm-‘)
42 551 42 625 42 676 32 692 42 755 42 786 42 794 42 883 42 900 42 926 42 911 43 010 43 052 43 085 $3 13T 13 179 13 192 -43 291 43 400 43 416
5,” 184” 51° 186’ 51~ 182O 5r0 18e2 510 1831 5,o 5,O 18Z 5,O 18? 5,O 18,’
5,” 1802
Y (cm-l) 43 43 43 43 43 43 13 43 43 43 43 43 43 43 43 43 11 44 44 44
450 483 534 548 578 592 615 G31 71-t 751 767 787 802 853 919 980 016 oti5 084 130
Y (cm-‘) ___~
Assignment 51” 180~
44 44 44 14 44 44 44 44 44 44 44 44 45 45 45 45 45 45 45 45
18x” 183’
18s” 18;’ 182 184” 18:’ 18.20 18i:’ 183’
Assignment
154 172 214 281 294 368 391 410 544 681 820 964 111 151 258 313 408 460 643 788
00° 18a2 18s” 18,’ 18s 18~“~ 1802 18,” 1804 181~ 180” 18,7 70’ 1802 180” iol 181:’ 18,s 7a’ 180” 5,,l 180” 5”’ 18,‘?
(cm-l)
Assignment
45 936 46 083 46 220 46 363 46 511 46 672 46 709 46 851 46 981 47 108 17 2-L2 47 381 47 5’23 47 WI 47 82;? -47 964 18 103
50’ 180~ 50’ 18,” 5,l 180” 50~ 18,7 50’ 180X 50’ 1819 A* + 18’ Aa + 182 Aa + 18:’ Aa + 181 A:. + 18”
Y
An A* A* ,Q k A:1
+ + + + + f
:LThe assignments of these bands probably involve Ye’ and Y,‘. The superscript q8 assignment is only a running number which is less than the quantum nllmber.
members straight,
of the series
and plotting
line was obtained.
into the progression. weak component
them
against
a running
By means of this graph further
The longest
wavelength
of what was initially
series. This band at 44 1% cm-’
was taken
of the
number
when
a
bands could be fitted
band that could be included
thought
18” 187 18” 1x9 18”’ 18”
to be t#he t’hird member
was a of the
t,o be the origin of the system.
Most
of the long wavelength bands not included in this excit’ed state progression could then be accounted for as hot’ bands provided that (i) the ground stat’e int,ervnls \vere irregular, and (ii) alternate members of t,he excited associated with different, sets of ground state intervals. The
excited
state
progression
arising
from the origin
state
progression
of the system
were
is over-
lapped at its short wavelength end by a number of other bands which can be arranged into progressions with intervals of about l-20 cm-’ (see Table I ). It, is possible
that
these
progressions
belong
to other
electronic
transitions.
However, since the intervals between the first observed bands of the progressions and t,he electronic origin are all small, i.e., 997, 14X9, and 253.5 cm-‘, it is mow probable that all the bands belong to the same electronic transition. The different progressions are due to the excitation of ot’her vibrations as well as the vibration responsible for the long progressions. As we shall see later vibrational
4
WHITESIDE
AND
WARSOP
intervals of about 690 and 1190 cm-’ are observed in the elect’ronic transitions at shorter wavelengths. With this in mind we may tentatively make the following assignments: 997 h” 690 + 2 X 140 and 1489 .X 1190 + 2 X 140. The interval of 2555 cm -’ may involve a combination of 690 and 1190 cm-‘. The first few members of each progression are overlapped by the stronger bands of the progression to longer wavelengths. At about 1200 cm-’ to the red of the origin there is a group of very weak bands wit’h similar structure to the group of bands near the origin. These bands are all due t,o transitions from vibrat,ional levels of the ground state in which one quantum of vg (al) is excited. The value of 1238 cm-’ for V: obtained from the elect,ronic spectrum agrees with the value of 1226 cm-’ obtained from infrared measurements. 2027 8 System The next electronic transibion t,o short wavelengths of the 2260 w system is at 20’27 8. It consists of four broad diffuse bands with intensities decreasing towards shorter wavelengths. These bands are shown in Fig. 2 and their frequencies are given below. Frequency
(cm-‘)
4933 6
50019
II89
50525 51217
The vibrational and 1190 cm-‘.
structure is not very extensive showing intervals of about 690
Region between l&50 and 1650 A In t,his region the spectrum is rather confused and only partly resolved. We have observed about t,hirt,y bands with varying widths, intervals, and intensities
FIG. 2. Region
below 2100 w
TI:IXll~THYLENE
5
SULFII)IZ
which appear to belong t,o several different, electronic transitions. The bands are shown in Fig. 2 and c their frequencies are given in Table II. Fragmenk of two systems have been tJentat,ively assigned. In one system :I series of seven approximatjelg evenly spaced bands form :m excit#ed state progression with a frequency of about8 1X cni’ beginning with the b:mcl at’ .i4 997 cnll ( 1SlS 8 ). At 57 340 cm-l ( 1744 ,&) t,here is the strongest band in this region which is probably the origin of :tnot8hcr electronic transition. Intervals of ciS3, 1’101, and 1201 + GSS cnl? cm be found giving a vibrational structure wry 0 similar to that’ of 2037 A system. After the :tssignmcnt,s given above there still remains a large number of bands. Region below 1650 W Below l&5! i there is a large number of bands which converge to a limit, at, about 1400 A. Some of the longer wavelengths bands can be divided i&o groups which have similar vibrational st8ructures as is shown below. Frequency
Frequency
(cm-‘)
Frequency
(cm-‘)
(cm-’
1
65092
62495 63199
1196
65788
II56
1174
66248
67689
/
66975
62737
65320
696
a=731 ,
‘)
,.
___/
-_
I I 59
700
66023
Each of the* groups represents a separate elect8ronic transition. Beginning with the first members of the groups two Rydberg series have been identified. The observed and calculated frequencies for these two series are given in Table III. E’or series I the frequencies were calculated from v,(cmC”)
= 09 717
R (n -
2.11)”
and for the series IT the ecluat’ion used was v,(cnr’) Thr
average
ionization
= 69 83s
pot~entinl obtained
R from the two series is S.65 eV which
WHITESIDE
AND
TABLE REGION Frequency (cm-‘)
Appearance Broad Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
ms w w w ms s vs s s ms ms
54 54 54 54 54 54 55 55 55 55 55
432 623 675 804 905 9978 1203 241* 365” 4928 614’
BETWEEN
II 1850 AND
1650 A
Frequency (cm-i)
Appearance Sharp Broad Broad Broad Broad Sharp Sharp Broad Sharp Sharp Sharp
WABSOP
w VW ms s w s s vs s s s
55 55 56 56 56 56 56 57 57 57 58
Appearance Broad Broad Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
7458 928 089 278 G30 848 984 340” 727 86G 023b
ms ms ms vs vw VW w VW w w
Frequency (cm-r) 58 541b 59 229b 59 903 60 070 60 146 60 609 60 777 GO 976 61 099 61 293
a Part of the 54 997 cm-r (1818 11) progression. b Part of the 57 340 cm-l (1744 P\) system. TABLE RYDUERG
III SERIES Series II
Series I
It
1 5 6 7 8 9 10 11 12 13
vubh
(cm-l)
44 57 62 65 66 67 67 68 68 68
154 340 495 092 515 380 948 300 600 777
vcl,k
(cm-‘)
37 997 5G 301 62 352 65 071 616 521 67 385 67 941 68 319 68 588 68 786
Yobs
hk
-
(cm-‘) G157 1039 143 21 -6 -5 7 -19 12 -9
It
3 4 5 6 7 8 9
uoba
62 65 G6 67 68 G8
(cm-9
737 320 715 579 087 463
veaic (cm-‘) 57 62 65 66 67 68 68
142 769 341 727 559 097 465
Vobs vedc
(cm-‘)
-32 -21 -12 20 -10 -2
agrees with the value of 8.64 eV obtained by Lee (4). Kane of the Rydberg t.ransitions have extensive vibrational structure all being very similar to the 2027 A transition. DISCUSSION
Grow& State From an analysis of the vibrational structure of the 2260 d systems a set of ground st.at,e energy levels can be obt.ained. Table IV shows the vibrational energy levels of the ground state as obtained from the electronic spectrum as
TRIMETHYLENE
SULFII)I’,
Electronic spectrum Gilirl) (Cl+)
0
0 1
2 3 4
138 151 23i 301 387 479 5i(i Ml
-5 (i
7 x
!I
Calc from microwave spectrum” C&(r) (cm-‘) 0 0.274
140.1 152.3 238 .7 301 .R 3% .9 470.3 584.1 6x7 .U
? illtho~lgh trimethylene sulfide is nor~planar in its growd state the appropriate symmeby group is Czu since inversion effects are not negligible. We have chose11 axes different from Scott el trl. (4) so that otlr axes for trimethylene sulfide rorrelat,e with the accepted system of axes for H2S. The effect, of this difierence it1 axes is to itltercharlgp the species H, alld B*
well as the values deduced from t’he microwave spect,rum. YIany of the differences between t,he energy levels have been observed in t,he far infrared by Borgers and Strauss (2). It can be seen t,hat good agreement can be obtained bct’ween the directly observed vibrational energies obtained from the ultJravioleb spectrum and the energies deduced from the displacement of pure rotation lines in the microwave region. The far infrared result’s of Borgers and St’rauss are also in agreement with Table IV. The vibration involved can only reasonably be assigned to $8 t.he out of plane bending frequency. The irregularity of the vibrational intervals is due to a double miuimum in the potential associated with v18so t,hat the equilibrium position is nonplanar. We have fitted the vibratjional energy levels to a potential function of the form V(Q) = A&” - ae? using t#he tables of energy levels prepared by Coon rt al. (6). By t,aking the CH, groups as point, masses \ve obtained :t barrier height of ‘74 cm-’ and a dihedral angle of 3.3”. Harrison et al. in a more accurate calculation used a potential function of the form V( Q j = d (3” - BQ’ and t,ook into account’ the structure of t,he CH? groups. They obtained a value for the barrier of “74.2 f 2 cn~? and a dihedral angle of about 30”. S1lape.s of Exited
States
In the 3330 ACsystem there is a long progression in I& . The vibrational in tervals increase slightly towards higher energies but, there is no irregular behavior at low energies. Since there is a long progression there is a large difference in the out-of-plane angle between t’he ground state and the excit,ed state. The regular
s
WHITESIDE
AN11 WARSOP
progression is con&ant with the excited state having a very high barrier to inversion so that effects due to inversion are not, observed, or with the excited statmebeing planar. The alternation in the groups of hot bands along the progression shows that the vibrational symmet,ries in the excited state also alternate and that t’he vibration is nont’otally symmetric. From this it follows that the excited st’ate is planar. If the excit’ed states were nonplanar with a, high barrier the vibration would be totally symmetric and the same set of hot bands would be associat’ed with all the bands in the progression. The regularity of the lower members of the excited state progression show t’hat if there is any deviation from planarity it’ must be very small. The vibrational structure of the next system at 2027 B is completely different. There are only a few bands and no progressions in & . The nonappearance of bands involving J& means that’ the dihedral angle in t.he excited state has very nearly the same value as in the ground st’ate. In general the small extent of the vibrational structure shows that there i! very lit’tle change in any of the dimensions. The excited state of the 2027 A system must therefore be nonplanar with almost the same dimensions as the ground state. The only vibrational struct’ure involves frequencies of about 690 and 1190 cm-‘. These must be totally symmetric frequencies and are therefore ~7’and ~5’)respect,ively. The corresponding ground state frequencies are 693 and 1225 cm-‘. These vibrations appear weakly with many of t’he Rydberg transitions. Apart from 08 they are the only vibrations observed in t’he electronic spectrum of trimethylene sulfide. We can now determine the shapes of the upper states associated with the other transit’ions. If there are no progressions in & , then the excited state is nonplanar like the ground &ate. If there is a long progression in & , then the excited state is either planar or much further from planar than the ground state. However as shown below we do not expect to observe any transitions to excited states that are furt,her from planar than the ground state. Therefore if we observe a long progression in v;, the excit’ed state is most probably planar. Using the above criterion \ve can see that t#he only transition below 2000 d which has a planar excited state is the one whose first observed band is at 181s A. All other transitions are to nonplanar st’ates. Since the Rydberg stat,es are nonplanar the ground state of the ion (CH2)&3+ must also be nonplanar. Electronic
Structures
In order to understand trimethylene sulfide we will first consider molecular orbitals of the related molecule H&3.” From these considerations we will then 3 Clark and Simpson discussed the electronic structure of sulfides (6) and produced an energy level diagram which is labeled with symmetry symbols appropriate to the CZV point group. However, in constructing their diagram they have used, at least in part, valence bond arguments so that some of the lines on the diagram do not represent the energies of transitions cannot be correct molecular orbitals. Also their discussion of “quasiforbidden”
TRIMETHYLENE
NJLFII)E
9
derive our expect8at8ions for the spectrum of trimethylene sulfide. Walsh (7) has constructed a correlation diagram for AH, molecules which \ve will extend for our puqwses (see Fig. 3). Using the methods of Walsh we see that as \vell as four lo\vest energy orbitals (1~ - la, , 16, - lu,, , 2~ - la, and 1O1 - L’S,,) there :w tn-o antibonding orbitale; :I 3al - %J, orbital which lies above the lb, - ?a,, orbital and at :L higher energy st’ill :L Zb, - ST,, orbit)al. Both thee orhitnls haw a lower energy in the linear configuration. C&relation of t hc molecular orbitals of HZS with the atomic orbitals of t hc united atom argon sho1t.s that 3al - L’u, is another description for the As KJdbcrg orbital and Yb, - %,, is :L 4-1’ Rytlberg orbital. There are also two other -I-p orbit&. The energies of these orbit:& are difficulty to c&mate. They should lie at lower energies t,han t 11~ 3~1 :mtl .is orbit& and at higher energies than the 3al - ?a, orbital. The\- XC rcprewnted on t,he diagram as a horizontal broken line. This line is not, int,endctl to represent, the accurate position of the orbit& but to indicate their approxim:li o position and t,he fact that their energies do not depend very much on the angle of the molecule. Above these orbit& are the M and 5s and higher energ). 12ytlberg orbit&. The configuration of trimet)hylene sulfide is dctertnined by, an equilibrium bet\veen two opposing forces. Ring strain tends to make t)he molecule I)lanur while nonbonded int,eract’ion of the hydrogen atoms on adjacent carbon atoms tends to m:tlw the ring nonplanar with the CH, groups twisted relative to wch other. In the ground state the forces are such that the equilibrium configuration is non~~l:m:w. If during an elect’ronic transition there is XII increase in the ring htr:lin then the excited state will be nearer to planar than the louver state. WIen for :I tmnsition in H,S, Tve expect :I large increase in the bond angle thc,n for trimc~thylene sulfide we expect a large increase in the ring strain and therel’cwe :L planar excited stat’e. However if for a t)ransition in H,S there is little change of angle then for the corresponding transition of trimethylene sulfide wc clxpc>ct littk change in ring strain and :I nonplanar excited ststc. As nho\vn in l’ig. 15 wc do not expect any low lying transitions of H,S to hnvc ;L dccreasc in :~ngl(~. Re therefore do not espect any low l\kg transitions of trimrthylcne sulfido t I) devintc further from planarity than does the ground Gtatcl.
In the ground state of kimethylene sulfide the 1~ , lb1 , 2u1 and lb1 orbitals ‘I of A1 . :\I& of the will each be occupied by two electrons giving :I configuration electronic transit’ions are expected t,o be due to the excitation of one of t,he most _~._____ ______ since thr orbit& with which they make linear comhitlatiolls are treatment. we have used molecular orbital concepts through~)llt diagram is 1m1y qualitative we feel that the conclusions derived the elect rollic structure are substantially rorrrct particldarly account changes of shape during excitation.
it1 fact tlot related. III O,W Although our energy level from the consideratiolt of sillrc we have takeIt illto
WHITESIDE
10
AND WARSOP
3 d,5s
3d,5s
4p(TTu)
4pb,,b,) 2 b2 za,
,
I Bent FIG. 3. Correlation
diagram
TRIMETHYLENE
SULFII )E
11
weakly bound electrons i.e., an electron from the 16, orbital. The lowest energS transit’ion will be one in which a lb1 electron is excited to t’he 3al orbital. The transition will give rise to a planar excited state and will be associated with long progressions in & . The next transit’ion from the lb1 orbital will bc to t,he 26, orbital and will also give rise to a planar excited state. However this is :I forbidden transition and is not expected t’o be observed. All the remaining transitions from the lb1 orbital will be to Rydberg orbit:& and will give rise to lit!tIe change of shape. None of t’he transitions will have extensive vibrational structure. Transitions from t)he 2~ orbital may also he observed. The 2al clect,rons have :I much lolver energy in t,he nonplanar configuration. Hence excitation of :L L’n, electron .should give rise to planar excited state s irrespective of whether the upper orbital is an intravalence or a Rydberg orbital. Therefore all transitions from the ?a1 orbital will be associated lvith long progressions in vLx. Excitation of :I 1~ or a lb, electron would probably produce continuous ahsorptjion since these are strongly bonding electrons. The 2260 .t transition is the lowest energy transition and gives rise t#o a planar excited state. There can be little doubt that the WiO Li system is due to trawlsition i 1 1. . . . (2aJ(lb1)(3aJ
R, +- ( laI)“( 16~)~(,2aI)‘( Ml)” ‘A1
(1)
Seglccting for the t,ime being the 1‘31s 8 progression, all the remaining elcct~ronic transitions have little vibrational st’ructure associated with them. Therefore they must all he due to the excitation of a 161 electron. The term values of trimethylene sulfide and of argon ( the unit’ed atom of H,S ) I\-ere compared and found to agree quite closely. The comparison showed that the transitions near 1600 Ll \vere to As and 3d Rydberg orbitals. The transitions to 5s and St/ orbitals should he near to 1800 8. The strong transition at 174-I .I is probably t,hc 5s Rydherg transition. There should be four allowed transitions to 3rl orbitals which will probably show appreciable splitting since these are transitions to the lowest c/: orbit& These 3d Rydberg transitions probably account for many of the WIassigned bards in the 1SOO A region. The onlv transit’ion from a lb1 orbital yet to be assigned is the strong transition 0. at 2027 A. This is in t,he region where transitions to the 4p Rydhcrg orbit8als should be seen. There are three -1-7~ orbit,als but only transit,ions to the al and bl orbital!: are allowed. The 1O1orbital is almost a pure 3~ orbital and the transitions correspond to forbidden p-p transitions in the united atom. It would not be surprising if a11 transitions to 7, Rydberg orbitals \\-ere very weak. Indeed no transitions to 5p and higher (J Iiydberg orbit& have been identified. However one of the 41’ orbitals has the same symmetry as the 3~ orbital and has near1J the same energy. Thus the 4p( al ) orbit’al may be perturbed by the %L~orbital and hence give rise to a relatively intense transition. The S/J( al) and higher energ>- orbitals will not be sufficient Iv close in energy to the 3u, orbit,al to be
WHITESIDE
12 significantly transition
perturbed.
Whatever
AND WARSOP
the reason for there being only one strong
near to 2000 8 it is most probable that the 2027 8 transition can be
assigned to transition
(2).
* * . (2a,)“( 1&)(4p)
+
* . . (2a,(“( lb1)2 ‘Al.
(2)
The only transition yet to be assigned is the progression at 1818 8. It is most probable that this represents transition (3).
. . . (2~1)(&)~(3a1)
‘A, t
. . - (2~)~(lb1)~
‘Al.
(3)
This is the lowest energy transition in which a 2u1electron is excited. It is possible that the 181s i progression is due to transition (4).
- . - (2~1>~(lb1)(2b2) ‘A2 t It does not seem probable
. . . (2c~l)~(lbl)~ ‘A,.
since this is a forbidden
spectra it may be possible to differentiate
(4) since for (3) the selection rule Au18= 0, f2, selection rule will be Aqs = fl, Comparison
+3,
transition.
between transition
(4) With improved
(3 ) and transition
. . . will hold while for (4) the
...
of the term values of the Rydberg
the levels in argon shows that the two Rydberg
series with the term values of
series which have been identified
are s and d series. The two series are too close together for the positions of the bands to be used to differentiate
between s and d but it seems probable that the
stronger series is the s series. If this is so then the s series has a quantum defect of 2.14 and the d series a quantum defect of 0.06. RECEIVED: May
8, 196s REFERENCES
1. D. 0. HARRIS, H. W. HARRINGTON,A. C. LUNTZ, AND W. D. GWINN, J. Chew. Phys. 44, 3467 (1966). 2. T. It. BORGERSANDH. L. STRAUSS, J. Chem. Phys. 46, 947 (1966). 3. D. W. SCOTT,H. L. FINKE, W. N. HUBBARD,J. P. MCCULLOUGH,C. KATZ, M. E. CROSS, J. F. MESSERLY,R. E. PENNINGTON,AND G. W~DDINGTON,J. Am. Chenz. Sot. 76, 2795 (1953). 4. J. LEE, thesis, University of Leeds (1955). 5. L. B. CLARK AND W. T. SIMPSON,J. Chem. Phys. 43, 3666 (1965). 6. J. B. COON,N. W. NXJGLE, AND R. D. MCKENZIE,d. Mol. Spectry. 20, 107 (1966). 7. A. 1). WALSH, J. Ch,em. Sot. 1963, 2260.
The Electronic
Spectrum
of Trimethylene
J. A. B. WHITESIDE~ Cniversiiy
AKD
Sulfide
I’. -1. WUSOP
ofDundee, Dzcndee, Scotlantl
The electronic spectrllm of trimethylene sulfide has been observed in the near and the vacuum ultraviolet. A vibrational analysis of an electronic transition 0 at 2260 .\ shows that the ground state is nollplanar but the excited state is platLar. Apart from a transition at, 1818 % all other electronic transitions lead to nonplanar excit,ed states. Two Rydberg series lead to an ionization potelltial of X.65 eV. Since the Rydberg states are nonplanar the ground state of the ion (CH,),S+ is also nonplanar. A correlation diagram for H,S is drawn and from it the shapes of the states of trimethylene sulfide are predicted. The predicted and observed shapes agree.
Harris
et al.
(I ) observed
t,he microwave
and showed that the ground I’rom t’he microwave of-plane
bending
levels. Borgers Art =
1 and Av =
al. (3) discussed
(2)
3 transitions
electronic
observed
the potential
in the out-of-plane spectra
function
spectrum
bending
for t,hc outthe energy
and identified
frequency.
and assigned
sulfide
t’o inversion.
they calculated
t,he far infrared
and Earnan
of t.rimet.hylene
wit’h a 10~~ barrier
and from the potential
the infrared
of t)he fundamental The
spect’rum they deduced
coordinate
and Strauss
spectrum
state is nonplanar
Scott
values
rf
to most
frequencies. spect’rum
of trimethylene
sulfide A\-aspreviously
reported
b!
l,ee t-i) and by Clark and Simpson (5). A higher resolution and more detailed spectrum of trimethylene sulfide is reported below. The shapes of the ground and excited states are deduced from the vibrational struct.ure of the electronic transitions. The electronic structure is discussed and the shapes of the excited stat,es deduced. The expected \\-ith the observed spect,rum.
shapes
of t,he excited
EXPERIMENTAL
Trimethylene
sulfide \vas obtained
states
are then
correlntcd
RIETHC)l)
from Kodak
I,td. a:ondwas purified
by ga-:
chromat,ogr:tphy. The spectrum between 2500 and 2200 A n-as photographed on an E:bert spectrograph with a reciprocal dispersion of 2.8 Lc/mm and with a resolution of about 3 cm-‘. Absorption pat’hs ranged from 10 mm pressure in a 1%cm cell to 40 mm in a 40-cm cell; temperatures from -35 to 100°C werth used.
1I’rc+nt
address:
I.C.I.
Fibrrs
Lid.,
Pontypool,
Wales
2
WHITESIDE
AND
WARSOP
Spectra in this region were also obtained with a recording spectrophotometer with the temperature ranging from 10 to 150°C. The spectrum at room temperature in the region 2300 to 1300 s was photographed using a l-m normal incidence vacuum spectrograph. Pressures of 0.08 to 3 mm were used in the 2-m path length and a resolution of about 10 cm-’ was obtained. ELECTRONIC
SPECTRUM
The near ultraviolet spectrum consists of two electronic transitions. At 2260 A t’here is a transition with extensive vibrational structure in which about 70 bands have been observed. At 2027 i there is another transition which has very little vibrational structure, only four bands being observed. In the vacuum ultraviolet there are many electronic transitions some of which can be fitted to two Rydberg series. 2260 8 System The spectrum is shown in Fig. 1 and the frequencies of the bands are listed in Table I. The bands consist of a strong central maximum, to which all the measurements refer, with weak wings on either side. On the long wavelengths side a weak head is formed about 20 cm-’ from the central maximum. At first sight the system appears to consist of a progression beginning with the first strong band at 43 919 cm-l with some much weaker bands to longer wavelengths. The vibrational intervals are about 130 cm-’ and the higher frequency bands have two heads. However, closer examination reveals that this is not the correct analysis. I’irstly the longer wavelength bands of the apparent progression have a complex structure with the intervals between them varying irregularly while the remaining bands in the series have a simple structure with the intervals between t,hem varying regularly. Also closer examination of the densitometer traces and the spectra obtained with the spectrophotometer show t.hat the first three bands in the series are slightly but quite definitely temperature dependent. The origin of the system was found by taking intervals between the regular
FIG. 1. 2260 i system
TKIMETHYLENE
SULFIDE
TABLE
I
THE 2260 1%SYSTEM Y
Assignment
(cm-‘)
42 551 42 625 42 676 32 692 42 755 42 786 42 794 42 883 42 900 42 926 42 911 43 010 43 052 43 085 $3 13T 13 179 13 192 -43 291 43 400 43 416
5,” 184” 51° 186’ 51~ 182O 5r0 18e2 510 1831 5,o 5,O 18Z 5,O 18? 5,O 18,’
5,” 1802
Y (cm-l) 43 43 43 43 43 43 13 43 43 43 43 43 43 43 43 43 11 44 44 44
450 483 534 548 578 592 615 G31 71-t 751 767 787 802 853 919 980 016 oti5 084 130
Y (cm-‘) ___~
Assignment 51” 180~
44 44 44 14 44 44 44 44 44 44 44 44 45 45 45 45 45 45 45 45
18x” 183’
18s” 18;’ 182 184” 18:’ 18.20 18i:’ 183’
Assignment
154 172 214 281 294 368 391 410 544 681 820 964 111 151 258 313 408 460 643 788
00° 18a2 18s” 18,’ 18s 18~“~ 1802 18,” 1804 181~ 180” 18,7 70’ 1802 180” iol 181:’ 18,s 7a’ 180” 5,,l 180” 5”’ 18,‘?
(cm-l)
Assignment
45 936 46 083 46 220 46 363 46 511 46 672 46 709 46 851 46 981 47 108 17 2-L2 47 381 47 5’23 47 WI 47 82;? -47 964 18 103
50’ 180~ 50’ 18,” 5,l 180” 50~ 18,7 50’ 180X 50’ 1819 A* + 18’ Aa + 182 Aa + 18:’ Aa + 181 A:. + 18”
Y
An A* A* ,Q k A:1
+ + + + + f
:LThe assignments of these bands probably involve Ye’ and Y,‘. The superscript q8 assignment is only a running number which is less than the quantum nllmber.
members straight,
of the series
and plotting
line was obtained.
into the progression. weak component
them
against
a running
By means of this graph further
The longest
wavelength
of what was initially
series. This band at 44 1% cm-’
was taken
of the
number
when
a
bands could be fitted
band that could be included
thought
18” 187 18” 1x9 18”’ 18”
to be t#he t’hird member
was a of the
t,o be the origin of the system.
Most
of the long wavelength bands not included in this excit’ed state progression could then be accounted for as hot’ bands provided that (i) the ground stat’e int,ervnls \vere irregular, and (ii) alternate members of t,he excited associated with different, sets of ground state intervals. The
excited
state
progression
arising
from the origin
state
progression
of the system
were
is over-
lapped at its short wavelength end by a number of other bands which can be arranged into progressions with intervals of about l-20 cm-’ (see Table I ). It, is possible
that
these
progressions
belong
to other
electronic
transitions.
However, since the intervals between the first observed bands of the progressions and t,he electronic origin are all small, i.e., 997, 14X9, and 253.5 cm-‘, it is mow probable that all the bands belong to the same electronic transition. The different progressions are due to the excitation of ot’her vibrations as well as the vibration responsible for the long progressions. As we shall see later vibrational
4
WHITESIDE
AND
WARSOP
intervals of about 690 and 1190 cm-’ are observed in the elect’ronic transitions at shorter wavelengths. With this in mind we may tentatively make the following assignments: 997 h” 690 + 2 X 140 and 1489 .X 1190 + 2 X 140. The interval of 2555 cm -’ may involve a combination of 690 and 1190 cm-‘. The first few members of each progression are overlapped by the stronger bands of the progression to longer wavelengths. At about 1200 cm-’ to the red of the origin there is a group of very weak bands wit’h similar structure to the group of bands near the origin. These bands are all due t,o transitions from vibrat,ional levels of the ground state in which one quantum of vg (al) is excited. The value of 1238 cm-’ for V: obtained from the elect,ronic spectrum agrees with the value of 1226 cm-’ obtained from infrared measurements. 2027 8 System The next electronic transibion t,o short wavelengths of the 2260 w system is at 20’27 8. It consists of four broad diffuse bands with intensities decreasing towards shorter wavelengths. These bands are shown in Fig. 2 and their frequencies are given below. Frequency
(cm-‘)
4933 6
50019
II89
50525 51217
The vibrational and 1190 cm-‘.
structure is not very extensive showing intervals of about 690
Region between l&50 and 1650 A In t,his region the spectrum is rather confused and only partly resolved. We have observed about t,hirt,y bands with varying widths, intervals, and intensities
FIG. 2. Region
below 2100 w
TI:IXll~THYLENE
5
SULFII)IZ
which appear to belong t,o several different, electronic transitions. The bands are shown in Fig. 2 and c their frequencies are given in Table II. Fragmenk of two systems have been tJentat,ively assigned. In one system :I series of seven approximatjelg evenly spaced bands form :m excit#ed state progression with a frequency of about8 1X cni’ beginning with the b:mcl at’ .i4 997 cnll ( 1SlS 8 ). At 57 340 cm-l ( 1744 ,&) t,here is the strongest band in this region which is probably the origin of :tnot8hcr electronic transition. Intervals of ciS3, 1’101, and 1201 + GSS cnl? cm be found giving a vibrational structure wry 0 similar to that’ of 2037 A system. After the :tssignmcnt,s given above there still remains a large number of bands. Region below 1650 W Below l&5! i there is a large number of bands which converge to a limit, at, about 1400 A. Some of the longer wavelengths bands can be divided i&o groups which have similar vibrational st8ructures as is shown below. Frequency
Frequency
(cm-‘)
Frequency
(cm-‘)
(cm-’
1
65092
62495 63199
1196
65788
II56
1174
66248
67689
/
66975
62737
65320
696
a=731 ,
‘)
,.
___/
-_
I I 59
700
66023
Each of the* groups represents a separate elect8ronic transition. Beginning with the first members of the groups two Rydberg series have been identified. The observed and calculated frequencies for these two series are given in Table III. E’or series I the frequencies were calculated from v,(cmC”)
= 09 717
R (n -
2.11)”
and for the series IT the ecluat’ion used was v,(cnr’) Thr
average
ionization
= 69 83s
pot~entinl obtained
R from the two series is S.65 eV which
WHITESIDE
AND
TABLE REGION Frequency (cm-‘)
Appearance Broad Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
ms w w w ms s vs s s ms ms
54 54 54 54 54 54 55 55 55 55 55
432 623 675 804 905 9978 1203 241* 365” 4928 614’
BETWEEN
II 1850 AND
1650 A
Frequency (cm-i)
Appearance Sharp Broad Broad Broad Broad Sharp Sharp Broad Sharp Sharp Sharp
WABSOP
w VW ms s w s s vs s s s
55 55 56 56 56 56 56 57 57 57 58
Appearance Broad Broad Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
7458 928 089 278 G30 848 984 340” 727 86G 023b
ms ms ms vs vw VW w VW w w
Frequency (cm-r) 58 541b 59 229b 59 903 60 070 60 146 60 609 60 777 GO 976 61 099 61 293
a Part of the 54 997 cm-r (1818 11) progression. b Part of the 57 340 cm-l (1744 P\) system. TABLE RYDUERG
III SERIES Series II
Series I
It
1 5 6 7 8 9 10 11 12 13
vubh
(cm-l)
44 57 62 65 66 67 67 68 68 68
154 340 495 092 515 380 948 300 600 777
vcl,k
(cm-‘)
37 997 5G 301 62 352 65 071 616 521 67 385 67 941 68 319 68 588 68 786
Yobs
hk
-
(cm-‘) G157 1039 143 21 -6 -5 7 -19 12 -9
It
3 4 5 6 7 8 9
uoba
62 65 G6 67 68 G8
(cm-9
737 320 715 579 087 463
veaic (cm-‘) 57 62 65 66 67 68 68
142 769 341 727 559 097 465
Vobs vedc
(cm-‘)
-32 -21 -12 20 -10 -2
agrees with the value of 8.64 eV obtained by Lee (4). Kane of the Rydberg t.ransitions have extensive vibrational structure all being very similar to the 2027 A transition. DISCUSSION
Grow& State From an analysis of the vibrational structure of the 2260 d systems a set of ground st.at,e energy levels can be obt.ained. Table IV shows the vibrational energy levels of the ground state as obtained from the electronic spectrum as
TRIMETHYLENE
SULFII)I’,
Electronic spectrum Gilirl) (Cl+)
0
0 1
2 3 4
138 151 23i 301 387 479 5i(i Ml
-5 (i
7 x
!I
Calc from microwave spectrum” C&(r) (cm-‘) 0 0.274
140.1 152.3 238 .7 301 .R 3% .9 470.3 584.1 6x7 .U
? illtho~lgh trimethylene sulfide is nor~planar in its growd state the appropriate symmeby group is Czu since inversion effects are not negligible. We have chose11 axes different from Scott el trl. (4) so that otlr axes for trimethylene sulfide rorrelat,e with the accepted system of axes for H2S. The effect, of this difierence it1 axes is to itltercharlgp the species H, alld B*
well as the values deduced from t’he microwave spect,rum. YIany of the differences between t,he energy levels have been observed in t,he far infrared by Borgers and Strauss (2). It can be seen t,hat good agreement can be obtained bct’ween the directly observed vibrational energies obtained from the ultJravioleb spectrum and the energies deduced from the displacement of pure rotation lines in the microwave region. The far infrared result’s of Borgers and St’rauss are also in agreement with Table IV. The vibration involved can only reasonably be assigned to $8 t.he out of plane bending frequency. The irregularity of the vibrational intervals is due to a double miuimum in the potential associated with v18so t,hat the equilibrium position is nonplanar. We have fitted the vibratjional energy levels to a potential function of the form V(Q) = A&” - ae? using t#he tables of energy levels prepared by Coon rt al. (6). By t,aking the CH, groups as point, masses \ve obtained :t barrier height of ‘74 cm-’ and a dihedral angle of 3.3”. Harrison et al. in a more accurate calculation used a potential function of the form V( Q j = d (3” - BQ’ and t,ook into account’ the structure of t,he CH? groups. They obtained a value for the barrier of “74.2 f 2 cn~? and a dihedral angle of about 30”. S1lape.s of Exited
States
In the 3330 ACsystem there is a long progression in I& . The vibrational in tervals increase slightly towards higher energies but, there is no irregular behavior at low energies. Since there is a long progression there is a large difference in the out-of-plane angle between t’he ground state and the excit,ed state. The regular
s
WHITESIDE
AN11 WARSOP
progression is con&ant with the excited state having a very high barrier to inversion so that effects due to inversion are not, observed, or with the excited statmebeing planar. The alternation in the groups of hot bands along the progression shows that the vibrational symmet,ries in the excited state also alternate and that t’he vibration is nont’otally symmetric. From this it follows that the excited st’ate is planar. If the excit’ed states were nonplanar with a, high barrier the vibration would be totally symmetric and the same set of hot bands would be associat’ed with all the bands in the progression. The regularity of the lower members of the excited state progression show t’hat if there is any deviation from planarity it’ must be very small. The vibrational structure of the next system at 2027 B is completely different. There are only a few bands and no progressions in & . The nonappearance of bands involving J& means that’ the dihedral angle in t.he excited state has very nearly the same value as in the ground st’ate. In general the small extent of the vibrational structure shows that there i! very lit’tle change in any of the dimensions. The excited state of the 2027 A system must therefore be nonplanar with almost the same dimensions as the ground state. The only vibrational struct’ure involves frequencies of about 690 and 1190 cm-‘. These must be totally symmetric frequencies and are therefore ~7’and ~5’)respect,ively. The corresponding ground state frequencies are 693 and 1225 cm-‘. These vibrations appear weakly with many of t’he Rydberg transitions. Apart from 08 they are the only vibrations observed in t’he electronic spectrum of trimethylene sulfide. We can now determine the shapes of the upper states associated with the other transit’ions. If there are no progressions in & , then the excited state is nonplanar like the ground &ate. If there is a long progression in & , then the excited state is either planar or much further from planar than the ground state. However as shown below we do not expect to observe any transitions to excited states that are furt,her from planar than the ground state. Therefore if we observe a long progression in v;, the excit’ed state is most probably planar. Using the above criterion \ve can see that t#he only transition below 2000 d which has a planar excited state is the one whose first observed band is at 181s A. All other transitions are to nonplanar st’ates. Since the Rydberg stat,es are nonplanar the ground state of the ion (CH2)&3+ must also be nonplanar. Electronic
Structures
In order to understand trimethylene sulfide we will first consider molecular orbitals of the related molecule H&3.” From these considerations we will then 3 Clark and Simpson discussed the electronic structure of sulfides (6) and produced an energy level diagram which is labeled with symmetry symbols appropriate to the CZV point group. However, in constructing their diagram they have used, at least in part, valence bond arguments so that some of the lines on the diagram do not represent the energies of transitions cannot be correct molecular orbitals. Also their discussion of “quasiforbidden”
TRIMETHYLENE
NJLFII)E
9
derive our expect8at8ions for the spectrum of trimethylene sulfide. Walsh (7) has constructed a correlation diagram for AH, molecules which \ve will extend for our puqwses (see Fig. 3). Using the methods of Walsh we see that as \vell as four lo\vest energy orbitals (1~ - la, , 16, - lu,, , 2~ - la, and 1O1 - L’S,,) there :w tn-o antibonding orbitale; :I 3al - %J, orbital which lies above the lb, - ?a,, orbital and at :L higher energy st’ill :L Zb, - ST,, orbit)al. Both thee orhitnls haw a lower energy in the linear configuration. C&relation of t hc molecular orbitals of HZS with the atomic orbitals of t hc united atom argon sho1t.s that 3al - L’u, is another description for the As KJdbcrg orbital and Yb, - %,, is :L 4-1’ Rytlberg orbital. There are also two other -I-p orbit&. The energies of these orbit:& are difficulty to c&mate. They should lie at lower energies t,han t 11~ 3~1 :mtl .is orbit& and at higher energies than the 3al - ?a, orbital. The\- XC rcprewnted on t,he diagram as a horizontal broken line. This line is not, int,endctl to represent, the accurate position of the orbit& but to indicate their approxim:li o position and t,he fact that their energies do not depend very much on the angle of the molecule. Above these orbit& are the M and 5s and higher energ). 12ytlberg orbit&. The configuration of trimet)hylene sulfide is dctertnined by, an equilibrium bet\veen two opposing forces. Ring strain tends to make t)he molecule I)lanur while nonbonded int,eract’ion of the hydrogen atoms on adjacent carbon atoms tends to m:tlw the ring nonplanar with the CH, groups twisted relative to wch other. In the ground state the forces are such that the equilibrium configuration is non~~l:m:w. If during an elect’ronic transition there is XII increase in the ring htr:lin then the excited state will be nearer to planar than the louver state. WIen for :I tmnsition in H,S, Tve expect :I large increase in the bond angle thc,n for trimc~thylene sulfide we expect a large increase in the ring strain and therel’cwe :L planar excited stat’e. However if for a t)ransition in H,S there is little change of angle then for the corresponding transition of trimethylene sulfide wc clxpc>ct littk change in ring strain and :I nonplanar excited ststc. As nho\vn in l’ig. 15 wc do not expect any low lying transitions of H,S to hnvc ;L dccreasc in :~ngl(~. Re therefore do not espect any low l\kg transitions of trimrthylcne sulfido t I) devintc further from planarity than does the ground Gtatcl.
In the ground state of kimethylene sulfide the 1~ , lb1 , 2u1 and lb1 orbitals ‘I of A1 . :\I& of the will each be occupied by two electrons giving :I configuration electronic transit’ions are expected t,o be due to the excitation of one of t,he most _~._____ ______ since thr orbit& with which they make linear comhitlatiolls are treatment. we have used molecular orbital concepts through~)llt diagram is 1m1y qualitative we feel that the conclusions derived the elect rollic structure are substantially rorrrct particldarly account changes of shape during excitation.
it1 fact tlot related. III O,W Although our energy level from the consideratiolt of sillrc we have takeIt illto
WHITESIDE
10
AND WARSOP
3 d,5s
3d,5s
4p(TTu)
4pb,,b,) 2 b2 za,
,
I Bent FIG. 3. Correlation
diagram
TRIMETHYLENE
SULFII )E
11
weakly bound electrons i.e., an electron from the 16, orbital. The lowest energS transit’ion will be one in which a lb1 electron is excited to t’he 3al orbital. The transition will give rise to a planar excited state and will be associated with long progressions in & . The next transit’ion from the lb1 orbital will bc to t,he 26, orbital and will also give rise to a planar excited state. However this is :I forbidden transition and is not expected t’o be observed. All the remaining transitions from the lb1 orbital will be to Rydberg orbit:& and will give rise to lit!tIe change of shape. None of t’he transitions will have extensive vibrational structure. Transitions from t)he 2~ orbital may also he observed. The 2al clect,rons have :I much lolver energy in t,he nonplanar configuration. Hence excitation of :L L’n, electron .should give rise to planar excited state s irrespective of whether the upper orbital is an intravalence or a Rydberg orbital. Therefore all transitions from the ?a1 orbital will be associated lvith long progressions in vLx. Excitation of :I 1~ or a lb, electron would probably produce continuous ahsorptjion since these are strongly bonding electrons. The 2260 .t transition is the lowest energy transition and gives rise t#o a planar excited state. There can be little doubt that the WiO Li system is due to trawlsition i 1 1. . . . (2aJ(lb1)(3aJ
R, +- ( laI)“( 16~)~(,2aI)‘( Ml)” ‘A1
(1)
Seglccting for the t,ime being the 1‘31s 8 progression, all the remaining elcct~ronic transitions have little vibrational st’ructure associated with them. Therefore they must all he due to the excitation of a 161 electron. The term values of trimethylene sulfide and of argon ( the unit’ed atom of H,S ) I\-ere compared and found to agree quite closely. The comparison showed that the transitions near 1600 Ll \vere to As and 3d Rydberg orbitals. The transitions to 5s and St/ orbitals should he near to 1800 8. The strong transition at 174-I .I is probably t,hc 5s Rydherg transition. There should be four allowed transitions to 3rl orbitals which will probably show appreciable splitting since these are transitions to the lowest c/: orbit& These 3d Rydberg transitions probably account for many of the WIassigned bards in the 1SOO A region. The onlv transit’ion from a lb1 orbital yet to be assigned is the strong transition 0. at 2027 A. This is in t,he region where transitions to the 4p Rydhcrg orbit8als should be seen. There are three -1-7~ orbit,als but only transit,ions to the al and bl orbital!: are allowed. The 1O1orbital is almost a pure 3~ orbital and the transitions correspond to forbidden p-p transitions in the united atom. It would not be surprising if a11 transitions to 7, Rydberg orbitals \\-ere very weak. Indeed no transitions to 5p and higher (J Iiydberg orbit& have been identified. However one of the 41’ orbitals has the same symmetry as the 3~ orbital and has near1J the same energy. Thus the 4p( al ) orbit’al may be perturbed by the %L~orbital and hence give rise to a relatively intense transition. The S/J( al) and higher energ>- orbitals will not be sufficient Iv close in energy to the 3u, orbit,al to be
WHITESIDE
12 significantly transition
perturbed.
Whatever
AND WARSOP
the reason for there being only one strong
near to 2000 8 it is most probable that the 2027 8 transition can be
assigned to transition
(2).
* * . (2a,)“( 1&)(4p)
+
* . . (2a,(“( lb1)2 ‘Al.
(2)
The only transition yet to be assigned is the progression at 1818 8. It is most probable that this represents transition (3).
. . . (2~1)(&)~(3a1)
‘A, t
. . - (2~)~(lb1)~
‘Al.
(3)
This is the lowest energy transition in which a 2u1electron is excited. It is possible that the 181s i progression is due to transition (4).
- . - (2~1>~(lb1)(2b2) ‘A2 t It does not seem probable
. . . (2c~l)~(lbl)~ ‘A,.
since this is a forbidden
spectra it may be possible to differentiate
(4) since for (3) the selection rule Au18= 0, f2, selection rule will be Aqs = fl, Comparison
+3,
transition.
between transition
(4) With improved
(3 ) and transition
. . . will hold while for (4) the
...
of the term values of the Rydberg
the levels in argon shows that the two Rydberg
series with the term values of
series which have been identified
are s and d series. The two series are too close together for the positions of the bands to be used to differentiate
between s and d but it seems probable that the
stronger series is the s series. If this is so then the s series has a quantum defect of 2.14 and the d series a quantum defect of 0.06. RECEIVED: May
8, 196s REFERENCES
1. D. 0. HARRIS, H. W. HARRINGTON,A. C. LUNTZ, AND W. D. GWINN, J. Chew. Phys. 44, 3467 (1966). 2. T. It. BORGERSANDH. L. STRAUSS, J. Chem. Phys. 46, 947 (1966). 3. D. W. SCOTT,H. L. FINKE, W. N. HUBBARD,J. P. MCCULLOUGH,C. KATZ, M. E. CROSS, J. F. MESSERLY,R. E. PENNINGTON,AND G. W~DDINGTON,J. Am. Chenz. Sot. 76, 2795 (1953). 4. J. LEE, thesis, University of Leeds (1955). 5. L. B. CLARK AND W. T. SIMPSON,J. Chem. Phys. 43, 3666 (1965). 6. J. B. COON,N. W. NXJGLE, AND R. D. MCKENZIE,d. Mol. Spectry. 20, 107 (1966). 7. A. 1). WALSH, J. Ch,em. Sot. 1963, 2260.