Reaction products from a discharge of N2 and H2S: The microwave spectrum of two conformers of sulfur diimide (HNSNH)

JOURNAL

OF MOLECULAR

SPECTROSCOPY

112, 482-493 (1985)

Reaction Products from a Discharge of N2 and HpS: The Microwave Spectrum of Two Conformers of Sulfur Diimide (HNSNH) R. D. SUENRAM, F. J. LOVAS, AND W. J. STEVENS Molecular Spectroscop.v Division, National Bureau of Standards. Gaithershurg. Maryland 20899

The rotational spectra of two conformations of sulfur diimide (HNSNH) are reported. The HNSNH species are produced in a low-pressure microwave discharge of N2 and H2S. The microwave spectrum of the normal isolopic form, HNSNH. and dideutro form, DNSND. of the cis,trans and cis.cis forms have been observed. The electric dipole moment components of both forms have been determined. The molecular structures were determined from the experimental rotational constants and from geometry optimized ab initio calculations with 4-31G Gaussian basis sets and CEP-31G basis sets including polarization. The experimentally and theoretically derived molecular properties are found to be in good agreement. Q 1985 Academic Press. Inc.

I. INTRODUCTION

Sulfur diimides, compounds of the form X-N=S=N-X, have been shown to have unusual electrochemical and photochemical properties (1, 2). Typically polymeric compounds of the type (SN), behave as organic semiconductors. In the simple diimide form there is the possibility of three geometrical isomers, as shown in Fig. 1. For the several substituted sulfur diimides already studied, the cis,truns conformer (1) appears to be the most stable (3-.5), although in the case of dimethyl and di-t-butyl sulfur diimide, the presence of ciwis (2) or truns,truns (3) has been detected (6, 7). From CND0/2 calculations trans,truns appears to be lower in energy than cis,cis (8). More recent ab initio calculations (STO-3G and STO-3G* which include d polarization), however, indicate that for the parent compound (X=H) the cis,cis form is the most stable, with cis,truns only slightly higher in energy and the trans,trans 5.2 kcal/mol above the c&tram form (9). During the course of our experimental work on the reaction of H2S with N2 in a 2450-MHz discharge (IO, II), we have identified two conformers of sulfur diimide as reaction products from the mixture. In this paper we report the first observation of the parent sulfur diimide and describe its synthesis, structure, and conformational stability. II. EXPERIMENTAL

DETAILS

All measurements were performed with an 80-kHz Stark modulated spectrometer of conventional design. Some of the early measurements were carried out with freerunning reflex klystrons. Most of the measurements reported here were carried out with klystrons phase-locked and frequency-swept via computer control (12). 0022-2852185 $3.00 Copyright 0

1985 by Academic Press, Inc.

All rights of reproduction in any form reserved.

482

MICROWAVE SPECTRUM

OF HNSNH CONFORMERS

,,/cs\_ -\

I

“1

,Y\

\’

N+

Y-L‘N
,A % ,

N

‘N /j/H

us,

trans

CIS,

CIS

trans.

483

trans

(31

FIG. 1. Three possible conformations of the parent sulfur diimide molecule.

Dipole moment measurements were performed with a precise dc electric field and superimposed 80-kHz square-wave field. Only a small square-wave field was used (for line modulation) when compared to the dc field. Sulfur diimide is produced best by first running a 2450-MHz discharge of N2 and optimizing the conditions for production of thiohydroxylamine with added H2S (II). The polymer that is produced downstream from the discharge under these conditions is important for the production of sulfur diimide under conditions described below. After maintaining the initial reaction conditions for l-2 hr, the flow conditions and the N2/H2S ratio were changed to give sulfur diimide. Optimum conditions were obtained under extremely slow flow with only 2-3 mTorr (1 mTorr = 0.1333 Pa) of HzS and - 15 mTorr of N2 through the discharge. Under these conditions the strongest signals of both conformers can be observed for several hours. Many other reaction variations were tried including addition of NH3, HZ, and N2H4, but none provided signals equal to the H2S + N2 mixture with the slow flow condition. The deuterated isotopic species were synthesized by substituting D2S for H$. III. SPECTRAL IDENTIFICATION

AND ANALYSIS

The cis,truns conformer was first assigned while experiments on thiohydroxylamine were in progress. An u-type R-branch pattern which is typical for a prolate asymmetric rotor species was found near 104 GHz. Closer inspection of this pattern of lines indicated that the transitions were rotational transitions from .I = 6-5. Verification was easily obtained by observing the J = 5-4 transitions near 86.7 GHz. Additional transitions were then assigned after fitting these observations and predicting further transitions. Once these predictions were available we were able to assign a number of previously observed transitions that we had identified as originating from low-J levels but were unable to assign. These were b-type transitions which had a less recognizable pattern compared to the a-type transitions. Once both a- and b-type transitions were assigned, there still was a series of observed transitions which were not consistent with the assigned set. These transitions exhibited

484

SUENRAM, LOVAS, AND STEVENS

characteristics similar to the b-type series of the first spectrum assigned. Eventually we were able to attribute the second spectrum to another conformer of sulfur diimide. (a) Species Identification It was necessary to employ both chemical and structural arguments to identify the molecular sources of the assigned spectra described above. By restricting the chemicals used in the discharge we could limit the composition of the new species to H, N, and S atoms, and could eliminate oxygen. A comparison of the derived rotational constants with those known for various H,N,S, species (or isoelectronic species of these) indicated a composition of two N atoms, one S atom, and several H atoms. For example, the rotational constants of the isoelectronic species, HNSO, are very similar to those of the new species. Based on the derived inertial defect A = +O. 12 pA2 (A = 1, - I, - Zb) it was clear that the molecular carrier must be planar. Several candidate species were considered with the composition H2N2S. These are listed in Table I with their estimated rotational constants. Of the species shown we felt that NH2SN might be eliminated since one would expect it to have a nonplanar geometry, i.e., a pyramidal amine configuration. On the other hand a planar NH2SN, like the linear isoelectronic species FSN, would exhibit only an a-type spectrum while both a- and b-type transitions were observed. As will be seen in the description of the assigned spectra given below, the NH2NS species has a very poor match in rotational constants with those observed and, thus, also seemed an unlikely source. As a result we were lead to consider the various conformations of sulfur diimide most seriously as sources of the spectra. The c&tram form would exhibit both a- and b-type transitions while the trans,trans and cis,cis forms would have only b-type transitions. TABLE I Estimated Rotational Constants of Various Candidate Species for the Observed Microwave Spectra’ II

B

c

H*NSNb

speck¶

42160.

8577.

7144.

H2NNSC

65225.

6838.

6189.

cis.trans-HNSNHd

47464.

9513.

7924.

cis.cIs-HNSNd

36378.

10482.

8137.

tPB%3,tPBldiNSd

39880.

10136.

8082.

%otationa1 cspdea A

-

11 43558..

conSta”ts A. 42158.. S - 9240.

in and

F-MB. me observed Constants S - 9550. and c - 7771 and c - 7607.

bAs3”md <“NH-120°,

Seometry:
r(NH)-1.071,

c~.wmed <“NH-IZO’,

Seometry: rCNH)-1.011.
dAsmmed
@metry:

r(NH)-1.0161,

r&N-S)-1.71.4.

rCNN)-,.45A,

r(W)-1.511,

are: (species

2)

r(SN)-1.451,

r(NS)-1.511


and

MICROWAVE

SPECTRUM

OF HNSNH

CONFORMERS

485

(b) Analysis The measured transitions of the two normal isotopic species are presented in Tables II and III. The transitions were fit to Watson’s centrifugal distortion Hamiltonian (23) with quadratic centrifugal distortion terms with a program written by Kirchhoff (14). The resulting rotational parameters are listed in Tables IV and V. The dideuterated species were assigned and fitted in a fashion similar to the normal species. The measured transitions of the corresponding deuterated species are included in Tables II and III and the rotational parameters are presented in Tables IV and V. Efforts to identify the monodeuterated forms were unsuccessful due to the weakness of the transitions and the complexity of the spectra when both HzS and D$ were mixed in the discharge. Although the N nuclear quadrupole hyperfme structure was not well resolved, several transitions of both observed spectra showed a partially split structure. This structure provides at least qualitative support for the presence of nitrogen atoms in the carrier molecules. The derived rotational constants in Table IV for cis,trans-HNSNH agreed fairly well with our estimates in Table I as did the selection rules. However, assigning the pure b-type spectrum of Table III to the cis,cis-HNSNH form rather than the tranqtrans form was achieved only after carrying out ab initio structure and dipole moment calculations described in Section IV. IV. STRUCTURAL

DETERMINATION

(a) Ab initio Calculations The comparison between the observed rotational constants and those obtained from the estimated structures were inadequate to allow us to discern the difference between the cis,cis and the trans,transconformers. In order to gain a better estimate of the structures of the various conformers and to obtain relative energies a set of ab initio calculations were carried out. Full geometry optimizations, using single-configuration self-consistent-field (SC-SCF) wavefunctions and energy gradient optimization methods, were carried out for the three HNSNH conformers using the HONDO computer program of Ring et al. (IS). Initial calculations were done with 4-31G Gaussian basis sets (16, 17) augmented with a single set of polarization functions on each atom. The exponents of the d polarization functions on nitrogen and sulfur were taken from Roos and Siegbahn (18), who recommended values of ad(N) = 0.95 and (u&j) = 0.54. A set of p functions was also included for each hydrogen with a* = 0.8. The calculated optimum sturctures, dipole moments, and relative energies of the cis,cis and cis,trans conformers are given in Tables VI and VII, respectively. The agreement between the experimentally deduced structures and the 4-3 1G plus polarization calculations is not as good as in previous comparisons for thiohydroxylamine (II). As expected, the internuclear distances determined at the SC-SCF level are consistently too small due to the neglect of electron correlation

486

SUENRAM, LOVAS, AND STEVENS TABLE II Measured Rotational Transitions of cis,trans-Sulfur Diimide

515-414

ala69.78(3)

-0.10

a26%7

83969.37(a)

0.03

505-404

85224.22(l)

-0.03

524-423

86452.42(6)

0.01

542-441

a6774.07(12)b*0

0.27

533-432

86837.94(6)

0.07

532-431

86814.66(6)

-0.12

624‘615

872'3.90('2)

0.04

918-909

87803.80(6)

0.0,

523-422

87835.29(4)

0.01

89072.90(6)

-0.03

523-514

89887.36(24)

-0.09

514-413

90725.au7)

9wa27

91070.31(24)

-0.02

'22,10-'21.1,

943a2.92(50jb

-0.54

"2

I9-1'1 010

0.00

'o1,9-'o0.10

10'987.19~~0~

-0.04

625-524

103611.85(10)

-0.0'

652-551

10412,.43(9)c

-0.07

643-542

,04,79.10(a)b*C

634-533

,04275.00(6)

-0.03

633-532

104373.03(S)

-0.02

624-523

,05973.06(7)

0.12

0.68

515-404

107347.83(15)

615-514

108646.55(4)

0.02 0.0,

-0.0,

142,,2-14,,,3

,,,3i9.35(8)

717-616

1,4,99.95(6)

0.0,

707-606

1,7604.60(S)

-0.02

616-505

120197.54(17)

726-625

120700.54(a)

761‘660

,21473.01(10)c

753-652

121517.44(5)C

744-643

,2,609.90(,4jb

743-642

121613.89(14)

-0.03

735-634

121735.76(7)

-0.08

734-633

121954.17(5)

-0.05

0.00 0.01 -0.07 0.04 0.33

414-313

58780.25(4)

0.02

423-322

62155.22(E)

-0.01

432-331

62346.9OC.3)

-0.06

431-330

62356.12(S)

-0.13

422-321

62852.15(11)

0.27

523-422

78975.96(10)

-0.04

514-413

81548.92(,0)

0.13

,22.10-'21.11

83969.3S(lSP

-0.67

606-505

90976.84(,2)

-0.06

625-524

92996.26(6)

0.02

652-551

93498.7O(lO)C

0.09

643-542

9355&04(50)b'C

634-533 633-532

Il.83

93651.48(6)

-0.0,

93756.06(6)

-0.10

7~6.~25

108317.18(9)

761-660

109077.5.3(9)=

0.00 -0.04

752-651

109124.65(9)=

744-643

109218.13(9)

0.02

743-642

109222.90(9)

-0.2,

735-634

109338.25(7)

-0.02

734-633

109572.,0(71

0.15

0.15

725-624

111892.57~:10)

-0.06

716-615

113578.52(10)

-0.02

MICROWAVE SPECTRUM

OF HNSNH CONFORMERS

487

TABLE 111 Measured Rotational Transitions of cis.cis-Sulfur Diimide

615-606 'OW"38

55242.25(l)

0.06

615-606

49339.28(6)

55251.35(7)

-0.01

725-7~6

63598.97(53

-0.04 -0.04

56570.34(10)

0.02

a26-a17

64020.87(5)

79509.59(6)

0.0,

624-615

64514.76(l)

313.*II2

80801.76(6)

0.02

927-918

66085.37(6)

-0.01

414-303

78867.75(20)

817-726 91

a-%7

'32,11-'23,lO

83433.46(6)

915-909

83819.47(3)

0.00

927'918

87986.06(7)

-0.03

102a-10~9

88591.72(6)

-0.03

725-716

90275.46(6)

-0.02

"2

90666.57(12)

I9."1

I,O

0.05

0.08 0.00 -0.09

98135.90(7)

0.01

101755.84(6)

-0.02

“3ad129

102859.20(6)

-0.01

'O37-'028

107131.68(6)

'53,12-'52,13 616-505

0.02

0.15

8oa-71

936-9z7

111669.49(5)

-0.02

'019-928

119630.22(6)

-0.01

7

109277.47(10)

-0.02

624-615

92624.04LlO)

0.00

707.%6

93861.42(6)

0.01

'72.10~'Zl,,,

94392.28(12)

-0.02

734-725

119743.98(9)

0.06

414-303

94501.98(6)

-0.01

322-211

120265.21(7)

-0.02

321-312

1ooa16.*7~6~

0.07

%a-707

124467.29(7)

'42,12-'41.13

107364.50~20)b

0.26

9d18

125195.31(a)

515-404

107597.58(6)

-0.02

221-212

107838.15(a)

-0.03

14 2.12~'33,ll

109279.83(a)

0.00

322-313

110321.92(a)

0.00

"1,10-"0.11

111005.20~5~

0.00

ao8-717

112801.04(8)

423-414

113652.58(a)

-0.06

524-515

117842.96(10)

-0.14

616.505

120244.10~5~

-0.02

0.09 -0.04

0.01

effects. However, the differences between experiment and theory for r(SN>) and LH,N,S in the ci.s,truns isomer and r(SN) and LNSN in the c&-is form are larger than one would like to see. In addition, the calculated dipole moment components for the cis,truns conformer are in serious disagreement with the experimental measurements. The predicted energy ordering at the 4-31G plus polarization level also disagrees with the experimentally deduced ordering, but the energy difference between the two conformers is too small for a definitive theoretical determination. Since it is known that structure calculations on molecules containing second-row atoms are sensitive to the inclusion of polarization functions (19-21), another series of geometry optimizations were carried out, this time with two sets of d polarization

488

SUENRAM, LOVAS, AND STEVENS TABLE IV Rotational and Centrifugal Distortion Constants for cis,trans-Sulfur Diimide

4215.9.016(92)a

36636.0(17P

9550.4131(31)

8599.682(29)

7111.5453(33)

6952.173(25)

152.81(52)

95.5(29)

10.58(16) 397.(V) -3.598(71)

4.6(15) 315.(85) -1.85(71)

-38.92(11)

-29.6(11)

-11.01(18)

-9.3(25)

0.12505(9)

0.1256(V)

functions on the sulfur and nitrogen atoms. The exponents of the additional functions were q,(N) = 0.2 and ad(S) = 0.1. To make these calculations more tractable, the 1s core electrons on nitrogen and the IS, 2s, and 2p core electrons on sulfur were replaced by compact effective potentials that have been developed

TABLE V Rotational and Centrifugal Distortion Constants for cis,cis-Sulfur Diimide PaPameteP

cis cls-HN-S-NH _I_

c&,c&-ON-S-HO

43558.346(10)'

33213.013(15)a

B”o(HZ)

9239.3952(32)

8693.1459(93)

C”(MHZ)

7607.5546(32)

6817.1174(69)

r,(kHz)

202.29(11T)

TZ(kHZ.)

15.43(25)

A’(MiZ)

r3(kHzJb l,,,,[WZ)

512.(Z) -3.1981(20)

Tbbbb(kHz)

-39.840(91)

'cccc(kHz)

-11.61(11)

* ("12)

0.13058(Z)

113.85(1.33) 1.506(431) 406.(5) -,.594(12) -42.21(26) -9.899(211) 0.1394(l)

489

MICROWAVE SPECTRUM OF HNSNH CONFORMERS TABLE VI Structure, Dipole Moment, and Relative Energy of cis,cis-Sulfur Diimide

,.O,l

pp&j

(A)

1.025(15)

1.024(15)

1.005

rNs

(A)

1.533(7)

1.534(7)

1.510

I.523

CHWS

C&g)

115.4C.3)'

115.5(a)

116.3

115.3


C&g)

124.0(9)0

123.9(E)

121.4

Pb

CD1

EC

(cm“)

1.7818(5)

D

51(50)

121.8

1.667

D

1.729

0.0

D

284

recently in our laboratory (22). The nodeless s and p atomic valence orbitals were represented by four Gaussian primitives contracted to two basis functions in a 3-l contraction (22) similar to the valence contractions in the 4-31G all-electron basis set. The combined effective potentials (CEP) and sp basis sets are denoted CEP3 1G. The results of the CEP-31G calculations with two sets of d polarization functions on nitrogen and sulfur and one set of p functions on hydrogen are given in Tables VI and VII for the cis,cis and c&tram conformers, respectively. The calculated equilibrium geometric parameters and dipole moments are seen to be consistently in better agreement with the experimentally determined values than are the 4-31G plus single polarization results. Also, the energy ordering of the two conformers predicted by the CEP-31G calculations agrees with the experimentally deduced order, but this is still not significant for such a small energy separation.

TABLE VII Structural Parameters, Dipole Moments, and Relative Energy of cis,trans-Sulfur Diimide

’

Fixed

a;E;l;:p wl.d'

PWWiletW=

EXpBPI.*rN.a1

r(N,H,)

1.025b

1.006

r($H2) rm,)

1.025b

1.004

I.012 1.OlO

1.517(3)

1.505

1.517

rw$)

1.550(3)

1.517

1.530


113.5c

113.5

113.0

+*s

106.3(5)

109.5

107.5


115.7(3)

115.2

115.5

pa

1.1924(131

Ub

0.6423(B)

h.

1.354(3)

E(cme')

0

at the -ab initio

value.

D D D

a%%tio +d

1.54 D

1.40 D

1.28 D

0.89

2.00

1.62 D

283

D

0

D

490

SUENRAM,

LOVAS, AND STEVENS

Test calculations with several other basis sets, including triple-zeta contractions of the atomic bases and added sets of diffuse s and p functions on nitrogen and sulfur, did not produce lower total energies for the conformers of HNSNH than those calculated with the CEP-3 1G plus two d polarization sets. This is an indication that the added d functions are really performing as polarization functions rather than simply accounting for a deficiency in the atomic sp basis sets. CEP-3 1G calculations on the tram, truns conformer, which was not seen experimentally, predict that it lies 965 cm-’ above the c&runs form and has a dipole moment of 3.45 Debye. The large dipole moment is a result of the positioning of the partially negatively charged nitrogen atoms on one side of the molecule and the partially positively charged hydrogens and sulfur on the other side. The predicted geometric parameters for the truns,truns conformers are r(SN) = 1.524 A, &NH) = 1.010 A, LHNS = 107.0”, and LNSN = 111.4”. It is interesting to note that all electron calculations with the 4-3 1G plus single polarization basis set predict the truns,truns conformer to lie 2000 cm-’ above the c&runs with a dipole moment of 4.13 Debye, which is remarkably different from the more accurate CEP-3 1G calculations. (b) Experimental Structures Since every atom in the two observed HNSNH conformers could not be substituted, it was not possible to determine the structure in the usual r, fashion. However, an r, type of fit was carried out. Here the six moments of inertia (from two isotopic species) can be least squares fit to the molecular parameters, either atomic coordinates, or bond lengths and angles. For the cis,cis conformer there are only four structural parameters, the HNS and NSN angles and the NH and NS bond lengths. From symmetry arguments one can assume the duplicate bond lengths and angles are identical. Since two isotopic species have been studied, six moments of inertia are available for use in structural determination. Due to the planarity of the molecule, there are only four independent moments which can be fitted to obtain an r, structure. This was done with a fitting programs STRFIT (23), which fits the atomic coordinates, and STRFTQ (23) which fits bonds and angles. The results of these fits are given in Table VI. The cis,truns conformer presented more difficulty since it lacks symmetry in the a, b plane. There are a total of three independent angles and four independent bond lengths to be determined in this case. With only two isotopic species assigned, there are only four independent moments of inertia. In order to obtain some structural information, at least three parameters must be fixed. The NH bond lengths were fixed at the values arrived at for the cis,cis conformer and the H,N,S angle was fixed at the (4-3 1G + d basis set) ub initio value. The remainder of the structural parameters could then be determined. The results are presented in Table VII. V. DIPOLE MOMENT

DETERMINATION

The molecular dipole moments for each species were measured by observation and analysis of the second order Stark effect. For the cis,truns conformer two components, pa and &, must be determined. Suitable transitions were located which

MICROWAVE

SPECTRUM

OF HNSNH

CONFORMERS

491

had sufficient dependence or both values of the dipole components. The M = 0, 1, and 2 Stark transitions of the && transition at 86452.455(61) MHz were used along with the M = 2, 3, and 4 Stark transitions of the 625-524at 103611.879(56) MHz. All measurements were simultaneously fit in a least-squares fashion to yield the values shown in Table VII. The cis,cis form was somewhat simpler to deal with since only the pb component exists. The three Stark transitions of the 313-202line at 80801.776(85) MHz were observed at a variety of electric field settings. The measurements were than fitted in a simultaneous least squares fashion to yield & = 1.78 18(5)D. The parallel plate spacing was calibrated in both cases by using either the .Z = 2-l or J = 3-2 OCS transition. The dipole moment of OCS was taken from Reinartz and Dymanus (pots = 0.7 15 19 D) (24). VI. RELATIVE

INTENSITY

MEASUREMENTS

Several factors complicated the relative intensity measurements which are necessary to determine which of the two conformers is the lowest in energy. Since the spectrum of each conformer is fairly sparse, it was difficult to find transitions of each conformer adjacent to one another which can be used for intensity comparisons. This is usually done to eliminate any spectrometer variations which may affect intensity comparisons that are made over large frequency ranges. An additional complication in the analysis resulted from the fact that for one conformer b-type transitions must be measured and for the other u-type transitions were preferred due to their larger intensities. Careful examination of the calculated spectra indicated that the region of 100 GHz was the best region to use. In this vicinity, the 62,5-52,4 and the 63,4-53,3u-type transitions of the cis,truns form occur at 103611.87(5) and 104275.24(7) MHz, respectively, while the 101,9-92,8and the 32,1-31,2b-type transitions of the cis,cis form occur at 102752.73(5) and 100816.19(7) MHz, respectively. A final complicating factor that must also be taken into account is that the cis,cis form exhibits nuclear spin statistics so that transitions which have ee-oo K quantum numbers have spin weights of 15 and those having eo-oe K quantum numbers have weights of 21 (25). Thus the measured intensity of the 32,1-31,2must be corrected by multiplying by 18/21 and the 101,9-92,8must be multiplied by 18/15 before being used in the intensity calculations. The intensities were employed in obtaining the energy difference using

where all the quantities have their usual meanings (12) and g is the nuclear spin statistical weight factor discussed above. Since the dipole moments of the two conformers differ, the Au terms were combined with the intensity values to give YfC* &s 7 where the integrated intensity, y = ZAv is now being ratioed. Each of the 6-5 transitions of the cis,trans conformer was compared with the two transitions of the cis,cis form to given an average value of EC, = EC, + 5 1 cm-’ k 50 cm-‘.

(2)

492

SUENRAM, LOVAS, AND STEVENS VII. DISCUSSION

One of the more difficult aspects of this study was determinating whether the cis,cis or trans,transconformer was the one that was observed. The initially assumed structures of either conformer could be modified slightly to reproduce the observed rotational constants. However, the dipole moment values provided additional evidence which we felt favored assignment to the ciwis conformer. In the past we have used ab initio calculations to aid us in this type of species identification (II, 12). In the case of sulfur diimide the ab initio calculations were quite helpful in a number of ways. The calculations indicated that there was a sizable energy difference between the cis,cis and the tramtrans forms, with the trans,trans form being much higher in energy. Furthermore, the caculated dipole moment for the cis,cis form was in good agreement with the observed value. From this information we were certain that the observed conformer with the pure b-type spectrum is indeed the cis,cis conformer. Table VI shows the structural comparisons of the microwave versus the ab initio calculated structures. For all parameters the agreement is within 5% which is quite good. The energy difference between the cis,trans and the ciwis conformers is quite small and, therefore, the ab initio calculations cannot be used to determine which conformer is the lower in energy. However, we can easily discern the spectral difference between these two conformers since one has both an a- and b-type spectrum and the other has a pure b-type spectrum. Here the ab initio calculations help in another way. For the cis,trans conformer there are too many parameters to be determined compared with the limited amount of isotopic data that is available. We have therefore used the ab initio calculations (4-31G + d basis set) to fix one of the parameters in the molecule (the H,N,S angle). The NH bond lengths were fixed at the values obtained for the cis,cis conformer structural fit. Once this had been done, the remainder of the parameters were determined from the microwave data. The experimental structure is shown in Table VII along with the ab initio structure. It is interesting to note that the two NS bond lengths differ significantly in the cis,trans form but that their average (1.534 A) is within 0.002 A of the value for the cis,cis form. There remains some uncertainty about which conformer is lower in energy. The experimental relative intensities indicate that the cis,trans form is lowest in energy with the cis,cis at 51 + 50 cm-’ above the cis,trans form. The ab initio calculations place the cis,cis conformer lower than the cis,trans by 283 cm-’ at the double-zeta plus one polarization function level, and the cis,trans lower by 244 cm-’ when two polarization functions are used on N and S. When the energy difference becomes this small it is difficult to tell which conformer is lower in energy by either method but here the experimentally determined value is to be preferred since the ab initio result is expected to be accurate to no more than 350-700 cm-‘. ACKNOWLEDGMENTS We thank Dr. Walter J. Lafferty and Dr. Jon Hougen for many helpful discussions throughout the course of this work. RECEIVED:

March 26, 1985

MICROWAVE

SPECTRUM OF HNSNH CONFORMERS

493

REFERENCES 1. M. L. KAPLAN,R. C. HADWN, K. RAGHAVACHARI, S. MENEZES,F. C. S~HILLING,J. J. HAUSER, AND J. H. MARSHALL,Mol. Cryst. Liq. Cryst. 80, 51-66 (1982). 2. J. R. GRUNWELLAND H. S. BAKER,J. Chem. Sot., Perkin Trans. 2, 1542-1544 (1973). 3. V. BUSETQ Acta Crystallo@ Sect. B 38,665-667 (1982). 4. G. LEANDRI,V. BUSETTI,G. VALLE,AND M. MAMMI, J. Chem. Sot. D, 413-414 (1970). 5. R. T. KOPS, E. VAN AKEN, AND H. S~HENK,Acta Crystallogr. Sect. B 29, 9 13-9 14 ( 1973). 6. J. R. GRUNWELL,C. F. HOYNG, AND J. A. RIECK,Tetrahedron Lett. 26, 2421-2423 (1973). 7. J. KUYPEN,P. H. ISSELMANN, F. C. MIJLHOFF, A. SPELBOS, AND G. RENES,J. Mol. Strut. 29,247-

252 (1975). J. R. GRUNWELLAND W. C. DANISON,JR., Tetrahedron 27, 5315-5321 (1971). K. RAGHAVACHRI AND R. C. HADDON,J. Phys. Chem. 87, 1308-l 3 I2 (I 983). F. J. IAXAS AND R. D. SUENRAM,J. Mol. Spectrosc. 93,416-422 (1982). F. J. L~VAS, R. D. SUENRAM,AND W. J. STEVENS,J. Mol. Spectrosc. 100, 316-331 (1983). R. D. SUENRAMAND F. J. L~VAS, J. Amer. Chem. Sot. 102,7180-7 184 (1980). 13. J. K. G. WATSON, Chem. Phys. 46, 1935-1949 (1967). 14. W. H. KIRCHHOFF, J. Mol. Spectrosc. 41, 333-380 (1972). 15. M. DUPUISAND H. F. KING, Int. J. Quant. Chem. 11, 613-625 (1977); J. Chem. Phys. 68, 3998-

8. 9. 10. Il. 12.

4004 (1978). 16. 17. 18. 19. 20. 21. 22. 23.

R. DITCHRELD,W. J. HEHRE,AND J. A. POPLE,J. Chem. Phys. 54,724-728 (1971). W. J. HEHREAND W. A, LATHAN,J. Chem. Phys. Ss, 5255-5257 (1972). B. ROOSAND P. SIEGBAHN,Theor. Chim. Acta (Berlin) 17, 199-208 (1970). J. B. COLLINS,P. S~HLEYER,J. S. BINKLEY,AND J. A. POPLE,J. Chem. Phys. 64,5 142-5 151 ( 1976). P. G. MEZEYAND E.-C. HAAS, J. Chem. Phys. 77.870-876 (1982). S. S~HEINER,J. Chem. Phys. 78, 599-600 (1983). W. J. STEVENS,H. BASCH,AND M. KRAUSS,J. Chem. Phys. 81,6026-6033 (1984). R. H. S~HWENDEMAN in “Critical Evaluationof Chemical and StructuralInformation” (D. R. Lide Jr. and M. A. Paul, Eds.), pp. 94-l 15, National Academy of Sciences, National Research Council,

Washington, D. C., 1974. 24. J. M. L. J. REINARTZAND A. DYMANUS,Chem. Phys. Lett. 24, 346-35 I (1974). 25. C. H. TOWNESAND A. L. SCHAWLOW,“Microwave Spectroscopy,”p. 101, McGraw-Hi& New York, 1965.