- Email: [email protected]
Microwave spectrum of methyl vinyl sulfide
JOURNAL
%i, 2355213
OFMOLECULARSPECTROSCOPY
Microwave
Spectrum of Methyl Vinyl Sulfide*
R. E. PENN’j’ Department
(1967)
of Chemistry,
AND
R. F.
Rice University,
CURL,
JR.
Houston,
l’exas 77001
The microwave spectrum of methyl vinyl sulfide has been studied in the region 830 Gc/sec and b-type dipole transitions of the ground and two excited vibrational states have been assigned. The rotational constants of the ground state are consistent with the planar cis conformation. The Stark effects of four transitions in the ground state give average dipole moment components of pa2 = 0.0044 f 0.0003 (debye)2, mz = 1.286 f 0.010 (debye)2, and p = 1.14 f 0.010 debye. Internal rotation A-E splittings observed for the 2) = 1 methyl torsional state correspond to a barrier of 3230 f 100 cal mole-‘. INTRODUCTION
This investigation of the microwave spectrum of methyl vinyl sulfide was undert’aken with a primary interest, in determining the conformation about t,he C-S bond. Two conformations for t,he molecule were considered most probable-the one with the methyl group cis to t,he vinyl group and the one with the methyl group irans to the vinyl group, although nonplanar heavy atom configurations could not, be ruled out. A similar molecule, methyl vinyl ether, has been invest,igat,ed by Cahill, Gold, and Owen (1). They found only the planar cis form. The barrier to internal rotat,ion of the methyl group is of unusual interest, in this compound. Cahill, Gold, and Owen found the barrier t,o internal rotation of the met,hyl group in methyl vinyl ether t’o be unusually large (> 3500 cal,‘mole). The determination of the barrier in methyl vinyl sulfide allows some int’eresting comparisons to be made. EXPE 11MENTAL
A sample of methyl vinyl sulfide was obtained from Chemical Procurement. Laboratories Incorporated and was used without further purification. The microwave spectrum in the region S-26 Gc/sec was observed using :I. conventional lOO-kc/set Stark modulated spect#romet,er and transition fre-, quencies were measured to within ~0.3 Mc/sec. The spectrum of the 18-40 * This work was supported by grant GP 6305X of the Kational part by grant C-071 of the Robert A. Welch Foundation. t National Defense Education Act Fellow. P35
Science Foundation
and in
236
PENN
AND
Gc/sec region \vas recorded on a Tracerlab eter model 4001.
CURL
lOO-kcisec
&ark
modulated
spectrorn-
ABSI(:NMENT
The general pattern of the low J spectrum was predicted for the cis and the k’ans conformations from first approximation structural parameters. By comparison of lines with resolved St#ark effects of the observed spectrum with t#he predicted spectrum t#hc transitions for the cis isomer 000 --+ 111 , 20~ ---f 211 , and X12-+ 311 were assigned. Ot#her transitions belonging to this 211 -+ 22.0, assignment were measured (see Table I) t’o yield the refined rot’ational constants of the ground state (0, 0) given in Table II. So a type lines were observed at the predicted frequencies, indicating that the a component of t,he dipole moment was very small. No internal rot’ation splitNtings were observed for the vibrational ground stat’e indicating t)hat the barrier must be great’er than 1800 Cal/mole. VIB1MTIONAL
SilTELLITE
SPECTRUM
In order to determine the methyl barrier the vibrational satellite lines were studied with a view to observing internal rotation splittings in an escit’ed methyl torsional &ate. The 211 -+ 2zo transition for the ground state occurring at 17 695.S Mc,,sec had a very characteristic relatively fast (but, second order) Stark shift to lon frequency due t#o t#he interaction of the 22” level with the 313 level 240.3 Rlc/sec above it. A somewhat weaker absorption line with similar St’ark effect, was observed at 17 723.7 Mcjsec. This was assumed to be t’he corresponding 211 + 2zo transition of an excited state. A Q-branch assignment for this &ate was obtained by plott,ing several possible transition frequencies observed for each of ‘Lo2--f 211 and 313 -+ 332 along wi-ith the frequency 17 723.7 for the 211 + 220 as function of >;(A - CT)and Ray’s asymmet,ry parameter K. The partial assignment given by t#he intersection of t’hree lines at, !.$(A - c) = 3615.5 Mc/sec and K = -0.61361 was verified by t,he correct prediction of several ot#her AJ = 0 transitions from these parameters. The assignment was completed by the identification of Ooo-+ 111 for an absorption at 13 966.5 hlc/sec. The measurement of more t)ransition frequencies for this stat,e (see Table I) led to t,he refined rot8ationnl constjants in Table II for the st#ate (0, 1). The relative int,ensitjy 10l/l,,o for ticvera transitions of the (0, 0) and (0, 1) of 0.5 f 0.1 at room t,emperature (and less at’ dry ice t#emperature) precluded the possibility of t#he t\\-o states being an A-E: int,ernal rot’ation doublet. This relntive int,ensity corresponded t#o an energy difference for the two states of roughly 140 cm-‘. Taking the (0, 1) state t#o be t#he first excited state of a torsional harmonic oscillator implied a barrier t,o int8ernal rot,at,ion TTgof about 1000 Cal.1 mole. This 1~:~sinconsistent, however, with t,he lack of observable A-E splittings in tsansitions involving .I 5 6 for this state. It was t#hus assumed t,hat the (0, 1)
MICROWAVESPECTRUMOF CHkGHa Table
I.
Assignment
of the Ground and Two Excited
23:
Vibrational
States
Methyl Vinyl Sulfide
Ground State (0,O)
*ohs.
V
ohs
-V
Transition
(Mc/sec)
2 02 - 211
8887.2
0.1
11735.8
-0.3
16039.7
0.3
21720.6
-0.2
16603.8
-0.2
17695.8
0
23980.8
-0.1
16757.5
-0.3
16122.2
-0.2
21778.5
0.2
13972.9
0.6
20705.2
0.8
10249.9
1 .o
303 -312 4
04 i
2
3
4
13
12 - 221
12 - 321
211 - 220 313 4322
%4 4
i 523
13 - 422
505 - 514 0
00 - I11
1 01-
2
12
1 11 - 202
talc
(Mc,/sec)
.o
for
I’nble I (continued)
Heavy Atom Out of Plane BendingFirst ExcitedState (0,l)
Ooo-)lll 101 + %2 2 12 + 221
211 + 220
202 -)211 313 j322 413 i422 404 -)413 414 --f 505
13966.5
0.1
20701.7
-0.1
21692.3
-0.7
17723.7
-0.5
8850.1
-0.0
23917.5
-0.4
16131.2
-0.0
15873.6
-0.3
35234.6
-0.3
Methyl TorsionFirst Excitqd State (1;O)
202 3211
8873.89
-0.02
211 -+220
17865.63
0.12
312 -+321
16770.63
0.02
413 -422
16245.00
-0.07
514 + 523
16784.32
-0.43
303 +312
11642.76
0.03
lo1 j212
20676.54
0.22
3 13 3322
24015.84
0.28
202 -)313
26777.12
0.03
606 + %5
28105.30
1.51
912P
MICKOWA\‘E 8PF;CTItUM OF CH,SC&
23!j
TABLE II ROTXPIOUL CONSTANTS, MOMENTS 0~ INERTIA,AND INERTL~L DEFECT FOR THE GROUND .4N1) 'h*o \'IBR.~TIONALLY EXCITED STATES OF METHYL VINYL SULFIDE State
(0, 0) (Ground) _____.
10 G06.33 4 784.00 3 366.03 47.649 105.639 150.140 0 .05,2
(0, lj(Bend) 10 598.7
4 764.7 3 367.i 47.683 106.067 150.066 -0.484
(1, 0) (Methyl torsion) 10 621.21 4 738.56 3 351.70 47.58 106.G5 150.78 -0.25
a Rotationa constants given in Mc/sec. b Itloments of inertia given in amu-iz from the constant 505 377 (arnu.izj (hlclsecj. e Inertial defect A is 1, f 1, - I,, - 16 and here I, is assumed to be 3.200 amu.b2.
corresponded to t,he first excited &ate of the heavy atom out-of-plane vibrational mot,ion. The great decrease in the inertial defect (-1, -Ib + I, + 1,; irt this state provides further evidence for the correct.neus of this vibrational. :w~ignment~. Similar changes have been observed for 2-fluoroprene (2) and vinyl format e (3). A Q-branch assignment’ for another excited state was made by the (,4-C)/:! 1-s. K plotting method for several t’ransitions (as before). Hobvever, a search of the ground stat,e 000 -+ 111 region revealed no satellite for this state which \vas consistentJ wit#hthe rest, of the spectrum. An investigation of the Stark effect of the ‘I1 - 220 for this st,at,e revealed that the line shift becomes proportional to the field strength at field strengths of about ‘LOOvolts/cm. It n-as assumed t#hat this behavior was due to the near degeneracy of the 220 and 313 levels. F’or t’n-o .wch St,ark-coupled levels the perklrbed energy levels are state
where yql,, and ysL3are t’he unperturbed energies, X is t’he field strength, and til? in t,he dipole coupling coefficient, for t,he tivo levels. It is clear that \vhen [I ~(vai:, - vZLO) 1” is much less than S’~~2 the Stark shift is linear with field strength. The average of the two unperturbed energy levels can be det’ermined by extrapolating the linear part of the Rark shift vs. field strength plot to zero field. The average t,hus oht’ained was roughly 12 Mc/sec above the 220 level and so the 313 level \vas roughly 2-l MC set shove the 2-Olevel. This information was sufficient to complete the assignment. The frequencies of more t,ransitions and the refined rotational constants are included for this the (1, 0) state in Tables I and II, reaptctively. The 000 -+ 111 t.ransition \vas not observed for this stat.e because it
PEKN An’11 CURL
"40
occurred Lvithin 0.1 AIc/sec of the ground state 000 -+ 111 and the t#wo were effectively superimposed. The relative intensity 110/1~~ for several t’ransitions of the (1, 0) and (0, 0) st,at,es was 0.3 =I=0.1 at room temperature. This corresponded to an energy difference of the k-o stat’es of roughly 260 cm-l which implied a barrier to internal rot,ation Tia of from 2500 to 3500 Cal/mole for a torsional harmonic oscillat’or. ;!-A’ type splittings for four transit’ions (of J 5 9 were observed and led to the identification of (1, 0) as t’he first excited methyl torsional state. STARK EFFECT
The Stark coeflicients for five 1 Jf 1 values transitions were dc%ermined. A direct current cell and was measured t’o within kO.3%, with field for applied voltages was calibrat,ed by t,he is summarized in Table III.
of four ground vibrational state voltage was applied to the St’ark a digit’al voltmeter. The electric Stark effect of OCS (4). The data
STRUCTURE So isotopic substitmion studies were made. In order to get some informat8ion about the structure, the methyl and vinyl group parameters were assumed md from a consideration of the methyl and vin)-1 halides th,e: vinyl C-S dist,ance was set equal to the methyl C-S distance minus 0.046 h. The met#hyl C-S mcl < CSC were varied until the rotational constants of the ground state were fit. The approximate struct8ure, thus deduced, is given in Fig. 1. BARRIER
TO INTERNAL
ROTATIOX
In the high barrier approximation both the A- and the E-type levels have rigid rotor spectra lvith slightly different rotat,ional constants. Since the metshy TABLE STARK
EFFECT
AND
DIPOLI? GROUND
//,I
+
111 21, 224 322 322
a Stark coefirients
III OF
AIETHYL
\'IISR.~TICJN.\I, ST.\TE
Sumher of measurements
Transition
0 202 --t 211 4 31.: ---f 31%i
?vIOhiEST
\-ISYL
Stark
coefficients
~-~ -~ -~ ~~ Observed
0 9 1.11 f-2 7 1.97 *2 7 -1.12 12 1 -0.67 f3 1 0.33 p,’ = 0.0044 i 0.0003 (debye)? ph2 = 1.286 Z!L 0.010 (debye)z P = 1.140 k 0.010 debye ill (Mc/sec)
(volts/cm)2.
aULFIDE
Is
THE
(0. 0) $
(X lo-“):’
~~~.~ ~~
Calculated 1.12 l.Oli -1.12 -0.G8 0.30
MICROWAVE
SPECTR.T!Al OF (:H,Wd,
b
l
PENN AND
24:!
CURL
TABLE IV INTERNAL RoTaTIoN SPLITTINGSOF FOUR LINES OF THE METHYL TORSIONAL FIRST EXCITED STlTE (1,0),PaRaMETERS FOR BARRIER CALCULATION, .LND THE BARRIER TO INTERNAL ROTATION Transition
Av,I,~ (Mc/sec)
Av,t,s - A~ca~c (MC/SK)
-0.53 -0.67 -0.69
+0.02 -0.02
-0.92
+0.01 -0.00
1, = 3.200arnu.;i? kb
=
VI = 8 Bssumed. b From structural
0.9291b
3230f 100 Cal/mole
approximation. DISCUSSION
Since by far most of the strong lines in the 8 + 26 Gq’sec region were attributable to one of t,he nsigned stat’es of the cis form, it appears that eit,her other forms are present in much lower concentration or their dipole moments are much smaller than t#hat of t(he cis form. The dipole moments of CHSSH, (‘7), (CH,).?S (8), and cis CH3SC2H3 of 1.26, 1.50, and 1.14 debye respectively, support the former conclusion. The barriers t#o internal rotation of the series CH#H (7), CH,SCHs (8),and cis CH$C&H, of 705, 2110, and 3230 cal~mole, respectively, are in line with the barriers of the series CHSOH (9), CHZOCOH (IO), CHS-0--CH:, (II), and cis CHaOCrH3 (1) of 1070, 1190, 2720, and >3500 Cal/mole, respectively. Probably the increase in Va for each series is in large part due to nonhonding int)eraction between hydrogen atoms. In cis methyl vinyl sulfide the nearest methyl and vinyl hydrogens in the staggered form have an internuclear distance of about 2.3 x. (roughly two Van der Waals radii) but in the eclipsed form the nearest,-hydrogen internuclear distance is about 1.6 x. The relatively large negative inert,ial defect of t’he excited methyl tsorsional statme (1, 0) about midway between the ground (0, 0) and out-of-plane bending (0, 1) st,at’e values suggest’s considerable coupling between t’he methyl torsion and out-of-plane bending. Such a coupling could arise from a strong nonbonded repulsion in t’he eclipsed form. The corresponding decrease in inertial defect, for the following molecules, having no such low-lying vib&ion, CH&X1CH2 (I$), CHL’l~CH, (13), and CH,CH,F (1.q) are 0.032, 0.060, and 0.113 amu.An, respectively. &\-olctrtldcd in proof: The final cnt,ry in Table I shordd be -1.51
not. +1.51.
ACK~VOWLEDGMENTS TOP authors are grateful lo the Manned Spacecraft Center, National Aeronalltics and Space Administration for making available the Tracerlab Spectrometer and to Mr. James
H. Iileutz for help in recordings of the li-i0 Cc region and measurement of -1-E internal rotation splittings. All cslcttlations were carried out. with t,he Rice Universit,y computer, constructed under Ir.S. Atomic Energy Commission contract No. AT-(10-l)-1825. RWEIVED:
April 24, l!Hi’i REFERENCES
1. P. CAHILL.L. P. GOLD, AND 5. L. OVEN, “The Microwave Spectrum of Methyl \.i~lyl Ether,” 1T.S. Dept. Corn. Office Tech. Serv. ill) 619 912 (1965). 2. I). I:. LlDE,J. Chem. &,/S.37,207-l (1962). 5. V. M. 11~0 AND R. F. CURL, JR.,J. Chem. Phys. 40, 3688 (1964)’ 4. S. A. MARSHALL AND J. WEBEII,Phys. Rev. 106, 1502 (1957). 5. C. C. LIN AND J. I>. RIVALEN,Ref'. 21fod. Phys. 31, 841 (1959). Ilepartment of 6. 1). R. HERSCHBACH, “Tables for the Internal Rotation Problem,” Chemistry, Harvard University, 1957. 7. R. W. KILB, J. Chem. Phys. 23, 1736 (1955). 8. L. PIERCEANDM.HAYASHI,J.C~~VI. Phys.36,-t79 (1961). 9. J. I>. SALEN, J. Chem. Phys. 23, 1739 (1955). 10. R. F. CURL, JR.,J. Chew Phys. 30, 1529 (1959). 11. P. H. KA~AI AND I<.J.MYERS, ,J. Chm. Phys. 30, 1696 (1959). 1%. I). R. LIDE,JR.,J. Cheru. Phys. 30, 37 (1959). 1~. M. L. IJNLAND,\-.WEISS,AND W. H. FLYGARE, J. Chene. Phys. 42, 2138 (1965). 14. L. PIERCE AND J.M. O'IIEILLY,J. Uol. Spectry. 3, 536 (19%).
%i, 2355213
OFMOLECULARSPECTROSCOPY
Microwave
Spectrum of Methyl Vinyl Sulfide*
R. E. PENN’j’ Department
(1967)
of Chemistry,
AND
R. F.
Rice University,
CURL,
JR.
Houston,
l’exas 77001
The microwave spectrum of methyl vinyl sulfide has been studied in the region 830 Gc/sec and b-type dipole transitions of the ground and two excited vibrational states have been assigned. The rotational constants of the ground state are consistent with the planar cis conformation. The Stark effects of four transitions in the ground state give average dipole moment components of pa2 = 0.0044 f 0.0003 (debye)2, mz = 1.286 f 0.010 (debye)2, and p = 1.14 f 0.010 debye. Internal rotation A-E splittings observed for the 2) = 1 methyl torsional state correspond to a barrier of 3230 f 100 cal mole-‘. INTRODUCTION
This investigation of the microwave spectrum of methyl vinyl sulfide was undert’aken with a primary interest, in determining the conformation about t,he C-S bond. Two conformations for t,he molecule were considered most probable-the one with the methyl group cis to t,he vinyl group and the one with the methyl group irans to the vinyl group, although nonplanar heavy atom configurations could not, be ruled out. A similar molecule, methyl vinyl ether, has been invest,igat,ed by Cahill, Gold, and Owen (1). They found only the planar cis form. The barrier to internal rotat,ion of the methyl group is of unusual interest, in this compound. Cahill, Gold, and Owen found the barrier t,o internal rotation of the met,hyl group in methyl vinyl ether t’o be unusually large (> 3500 cal,‘mole). The determination of the barrier in methyl vinyl sulfide allows some int’eresting comparisons to be made. EXPE 11MENTAL
A sample of methyl vinyl sulfide was obtained from Chemical Procurement. Laboratories Incorporated and was used without further purification. The microwave spectrum in the region S-26 Gc/sec was observed using :I. conventional lOO-kc/set Stark modulated spect#romet,er and transition fre-, quencies were measured to within ~0.3 Mc/sec. The spectrum of the 18-40 * This work was supported by grant GP 6305X of the Kational part by grant C-071 of the Robert A. Welch Foundation. t National Defense Education Act Fellow. P35
Science Foundation
and in
236
PENN
AND
Gc/sec region \vas recorded on a Tracerlab eter model 4001.
CURL
lOO-kcisec
&ark
modulated
spectrorn-
ABSI(:NMENT
The general pattern of the low J spectrum was predicted for the cis and the k’ans conformations from first approximation structural parameters. By comparison of lines with resolved St#ark effects of the observed spectrum with t#he predicted spectrum t#hc transitions for the cis isomer 000 --+ 111 , 20~ ---f 211 , and X12-+ 311 were assigned. Ot#her transitions belonging to this 211 -+ 22.0, assignment were measured (see Table I) t’o yield the refined rot’ational constants of the ground state (0, 0) given in Table II. So a type lines were observed at the predicted frequencies, indicating that the a component of t,he dipole moment was very small. No internal rot’ation splitNtings were observed for the vibrational ground stat’e indicating t)hat the barrier must be great’er than 1800 Cal/mole. VIB1MTIONAL
SilTELLITE
SPECTRUM
In order to determine the methyl barrier the vibrational satellite lines were studied with a view to observing internal rotation splittings in an escit’ed methyl torsional &ate. The 211 -+ 2zo transition for the ground state occurring at 17 695.S Mc,,sec had a very characteristic relatively fast (but, second order) Stark shift to lon frequency due t#o t#he interaction of the 22” level with the 313 level 240.3 Rlc/sec above it. A somewhat weaker absorption line with similar St’ark effect, was observed at 17 723.7 Mcjsec. This was assumed to be t’he corresponding 211 + 2zo transition of an excited state. A Q-branch assignment for this &ate was obtained by plott,ing several possible transition frequencies observed for each of ‘Lo2--f 211 and 313 -+ 332 along wi-ith the frequency 17 723.7 for the 211 + 220 as function of >;(A - CT)and Ray’s asymmet,ry parameter K. The partial assignment given by t#he intersection of t’hree lines at, !.$(A - c) = 3615.5 Mc/sec and K = -0.61361 was verified by t,he correct prediction of several ot#her AJ = 0 transitions from these parameters. The assignment was completed by the identification of Ooo-+ 111 for an absorption at 13 966.5 hlc/sec. The measurement of more t)ransition frequencies for this stat,e (see Table I) led to t,he refined rot8ationnl constjants in Table II for the st#ate (0, 1). The relative int,ensitjy 10l/l,,o for ticvera transitions of the (0, 0) and (0, 1) of 0.5 f 0.1 at room t,emperature (and less at’ dry ice t#emperature) precluded the possibility of t#he t\\-o states being an A-E: int,ernal rot’ation doublet. This relntive int,ensity corresponded t#o an energy difference for the two states of roughly 140 cm-‘. Taking the (0, 1) state t#o be t#he first excited state of a torsional harmonic oscillator implied a barrier t,o int8ernal rot,at,ion TTgof about 1000 Cal.1 mole. This 1~:~sinconsistent, however, with t,he lack of observable A-E splittings in tsansitions involving .I 5 6 for this state. It was t#hus assumed t,hat the (0, 1)
MICROWAVESPECTRUMOF CHkGHa Table
I.
Assignment
of the Ground and Two Excited
23:
Vibrational
States
Methyl Vinyl Sulfide
Ground State (0,O)
*ohs.
V
ohs
-V
Transition
(Mc/sec)
2 02 - 211
8887.2
0.1
11735.8
-0.3
16039.7
0.3
21720.6
-0.2
16603.8
-0.2
17695.8
0
23980.8
-0.1
16757.5
-0.3
16122.2
-0.2
21778.5
0.2
13972.9
0.6
20705.2
0.8
10249.9
1 .o
303 -312 4
04 i
2
3
4
13
12 - 221
12 - 321
211 - 220 313 4322
%4 4
i 523
13 - 422
505 - 514 0
00 - I11
1 01-
2
12
1 11 - 202
talc
(Mc,/sec)
.o
for
I’nble I (continued)
Heavy Atom Out of Plane BendingFirst ExcitedState (0,l)
Ooo-)lll 101 + %2 2 12 + 221
211 + 220
202 -)211 313 j322 413 i422 404 -)413 414 --f 505
13966.5
0.1
20701.7
-0.1
21692.3
-0.7
17723.7
-0.5
8850.1
-0.0
23917.5
-0.4
16131.2
-0.0
15873.6
-0.3
35234.6
-0.3
Methyl TorsionFirst Excitqd State (1;O)
202 3211
8873.89
-0.02
211 -+220
17865.63
0.12
312 -+321
16770.63
0.02
413 -422
16245.00
-0.07
514 + 523
16784.32
-0.43
303 +312
11642.76
0.03
lo1 j212
20676.54
0.22
3 13 3322
24015.84
0.28
202 -)313
26777.12
0.03
606 + %5
28105.30
1.51
912P
MICKOWA\‘E 8PF;CTItUM OF CH,SC&
23!j
TABLE II ROTXPIOUL CONSTANTS, MOMENTS 0~ INERTIA,AND INERTL~L DEFECT FOR THE GROUND .4N1) 'h*o \'IBR.~TIONALLY EXCITED STATES OF METHYL VINYL SULFIDE State
(0, 0) (Ground) _____.
10 G06.33 4 784.00 3 366.03 47.649 105.639 150.140 0 .05,2
(0, lj(Bend) 10 598.7
4 764.7 3 367.i 47.683 106.067 150.066 -0.484
(1, 0) (Methyl torsion) 10 621.21 4 738.56 3 351.70 47.58 106.G5 150.78 -0.25
a Rotationa constants given in Mc/sec. b Itloments of inertia given in amu-iz from the constant 505 377 (arnu.izj (hlclsecj. e Inertial defect A is 1, f 1, - I,, - 16 and here I, is assumed to be 3.200 amu.b2.
corresponded to t,he first excited &ate of the heavy atom out-of-plane vibrational mot,ion. The great decrease in the inertial defect (-1, -Ib + I, + 1,; irt this state provides further evidence for the correct.neus of this vibrational. :w~ignment~. Similar changes have been observed for 2-fluoroprene (2) and vinyl format e (3). A Q-branch assignment’ for another excited state was made by the (,4-C)/:! 1-s. K plotting method for several t’ransitions (as before). Hobvever, a search of the ground stat,e 000 -+ 111 region revealed no satellite for this state which \vas consistentJ wit#hthe rest, of the spectrum. An investigation of the Stark effect of the ‘I1 - 220 for this st,at,e revealed that the line shift becomes proportional to the field strength at field strengths of about ‘LOOvolts/cm. It n-as assumed t#hat this behavior was due to the near degeneracy of the 220 and 313 levels. F’or t’n-o .wch St,ark-coupled levels the perklrbed energy levels are state
where yql,, and ysL3are t’he unperturbed energies, X is t’he field strength, and til? in t,he dipole coupling coefficient, for t,he tivo levels. It is clear that \vhen [I ~(vai:, - vZLO) 1” is much less than S’~~2 the Stark shift is linear with field strength. The average of the two unperturbed energy levels can be det’ermined by extrapolating the linear part of the Rark shift vs. field strength plot to zero field. The average t,hus oht’ained was roughly 12 Mc/sec above the 220 level and so the 313 level \vas roughly 2-l MC set shove the 2-Olevel. This information was sufficient to complete the assignment. The frequencies of more t,ransitions and the refined rotational constants are included for this the (1, 0) state in Tables I and II, reaptctively. The 000 -+ 111 t.ransition \vas not observed for this stat.e because it
PEKN An’11 CURL
"40
occurred Lvithin 0.1 AIc/sec of the ground state 000 -+ 111 and the t#wo were effectively superimposed. The relative intensity 110/1~~ for several t’ransitions of the (1, 0) and (0, 0) st,at,es was 0.3 =I=0.1 at room temperature. This corresponded to an energy difference of the k-o stat’es of roughly 260 cm-l which implied a barrier to internal rot,ation Tia of from 2500 to 3500 Cal/mole for a torsional harmonic oscillat’or. ;!-A’ type splittings for four transit’ions (of J 5 9 were observed and led to the identification of (1, 0) as t’he first excited methyl torsional state. STARK EFFECT
The Stark coeflicients for five 1 Jf 1 values transitions were dc%ermined. A direct current cell and was measured t’o within kO.3%, with field for applied voltages was calibrat,ed by t,he is summarized in Table III.
of four ground vibrational state voltage was applied to the St’ark a digit’al voltmeter. The electric Stark effect of OCS (4). The data
STRUCTURE So isotopic substitmion studies were made. In order to get some informat8ion about the structure, the methyl and vinyl group parameters were assumed md from a consideration of the methyl and vin)-1 halides th,e: vinyl C-S dist,ance was set equal to the methyl C-S distance minus 0.046 h. The met#hyl C-S mcl < CSC were varied until the rotational constants of the ground state were fit. The approximate struct8ure, thus deduced, is given in Fig. 1. BARRIER
TO INTERNAL
ROTATIOX
In the high barrier approximation both the A- and the E-type levels have rigid rotor spectra lvith slightly different rotat,ional constants. Since the metshy TABLE STARK
EFFECT
AND
DIPOLI? GROUND
//,I
+
111 21, 224 322 322
a Stark coefirients
III OF
AIETHYL
\'IISR.~TICJN.\I, ST.\TE
Sumher of measurements
Transition
0 202 --t 211 4 31.: ---f 31%i
?vIOhiEST
\-ISYL
Stark
coefficients
~-~ -~ -~ ~~ Observed
0 9 1.11 f-2 7 1.97 *2 7 -1.12 12 1 -0.67 f3 1 0.33 p,’ = 0.0044 i 0.0003 (debye)? ph2 = 1.286 Z!L 0.010 (debye)z P = 1.140 k 0.010 debye ill (Mc/sec)
(volts/cm)2.
aULFIDE
Is
THE
(0. 0) $
(X lo-“):’
~~~.~ ~~
Calculated 1.12 l.Oli -1.12 -0.G8 0.30
MICROWAVE
SPECTR.T!Al OF (:H,Wd,
b
l
PENN AND
24:!
CURL
TABLE IV INTERNAL RoTaTIoN SPLITTINGSOF FOUR LINES OF THE METHYL TORSIONAL FIRST EXCITED STlTE (1,0),PaRaMETERS FOR BARRIER CALCULATION, .LND THE BARRIER TO INTERNAL ROTATION Transition
Av,I,~ (Mc/sec)
Av,t,s - A~ca~c (MC/SK)
-0.53 -0.67 -0.69
+0.02 -0.02
-0.92
+0.01 -0.00
1, = 3.200arnu.;i? kb
=
VI = 8 Bssumed. b From structural
0.9291b
3230f 100 Cal/mole
approximation. DISCUSSION
Since by far most of the strong lines in the 8 + 26 Gq’sec region were attributable to one of t,he nsigned stat’es of the cis form, it appears that eit,her other forms are present in much lower concentration or their dipole moments are much smaller than t#hat of t(he cis form. The dipole moments of CHSSH, (‘7), (CH,).?S (8), and cis CH3SC2H3 of 1.26, 1.50, and 1.14 debye respectively, support the former conclusion. The barriers t#o internal rotation of the series CH#H (7), CH,SCHs (8),and cis CH$C&H, of 705, 2110, and 3230 cal~mole, respectively, are in line with the barriers of the series CHSOH (9), CHZOCOH (IO), CHS-0--CH:, (II), and cis CHaOCrH3 (1) of 1070, 1190, 2720, and >3500 Cal/mole, respectively. Probably the increase in Va for each series is in large part due to nonhonding int)eraction between hydrogen atoms. In cis methyl vinyl sulfide the nearest methyl and vinyl hydrogens in the staggered form have an internuclear distance of about 2.3 x. (roughly two Van der Waals radii) but in the eclipsed form the nearest,-hydrogen internuclear distance is about 1.6 x. The relatively large negative inert,ial defect of t’he excited methyl tsorsional statme (1, 0) about midway between the ground (0, 0) and out-of-plane bending (0, 1) st,at’e values suggest’s considerable coupling between t’he methyl torsion and out-of-plane bending. Such a coupling could arise from a strong nonbonded repulsion in t’he eclipsed form. The corresponding decrease in inertial defect, for the following molecules, having no such low-lying vib&ion, CH&X1CH2 (I$), CHL’l~CH, (13), and CH,CH,F (1.q) are 0.032, 0.060, and 0.113 amu.An, respectively. &\-olctrtldcd in proof: The final cnt,ry in Table I shordd be -1.51
not. +1.51.
ACK~VOWLEDGMENTS TOP authors are grateful lo the Manned Spacecraft Center, National Aeronalltics and Space Administration for making available the Tracerlab Spectrometer and to Mr. James
H. Iileutz for help in recordings of the li-i0 Cc region and measurement of -1-E internal rotation splittings. All cslcttlations were carried out. with t,he Rice Universit,y computer, constructed under Ir.S. Atomic Energy Commission contract No. AT-(10-l)-1825. RWEIVED:
April 24, l!Hi’i REFERENCES
1. P. CAHILL.L. P. GOLD, AND 5. L. OVEN, “The Microwave Spectrum of Methyl \.i~lyl Ether,” 1T.S. Dept. Corn. Office Tech. Serv. ill) 619 912 (1965). 2. I). I:. LlDE,J. Chem. &,/S.37,207-l (1962). 5. V. M. 11~0 AND R. F. CURL, JR.,J. Chem. Phys. 40, 3688 (1964)’ 4. S. A. MARSHALL AND J. WEBEII,Phys. Rev. 106, 1502 (1957). 5. C. C. LIN AND J. I>. RIVALEN,Ref'. 21fod. Phys. 31, 841 (1959). Ilepartment of 6. 1). R. HERSCHBACH, “Tables for the Internal Rotation Problem,” Chemistry, Harvard University, 1957. 7. R. W. KILB, J. Chem. Phys. 23, 1736 (1955). 8. L. PIERCEANDM.HAYASHI,J.C~~VI. Phys.36,-t79 (1961). 9. J. I>. SALEN, J. Chem. Phys. 23, 1739 (1955). 10. R. F. CURL, JR.,J. Chew Phys. 30, 1529 (1959). 11. P. H. KA~AI AND I<.J.MYERS, ,J. Chm. Phys. 30, 1696 (1959). 1%. I). R. LIDE,JR.,J. Cheru. Phys. 30, 37 (1959). 1~. M. L. IJNLAND,\-.WEISS,AND W. H. FLYGARE, J. Chene. Phys. 42, 2138 (1965). 14. L. PIERCE AND J.M. O'IIEILLY,J. Uol. Spectry. 3, 536 (19%).