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Microwave spectrum and selenol internal rotation of 2-propaneselenol
JOURNAL
OF MOLECULAR
SPECTROSCOPY
99,4 15-430 ( 1983)
Microwave Spectrum and Selenol Internal Rotation of 2-Propaneselenol JUN NAKAGAWA, AKIHIRO NAGAYAMA, AND MICHIRO HAYASHI Department of Chemistry, Faculty of Science. Hiroshima University, Higashi-sendamachi, Hiroshima, 730. Japan
Microwave spectra of 2-propaneselenol and its deuterated specieswere measured and assigned for the gauche and trans isomers. The double minimum splittings of the gauche isomers were directly observed from b-type transitions, which were assigned with the aid of a double resonance technique. Rotational constants and torsional splitting of the gauche isomer of the parent species were determined to be A = 7802.50 f 0.75, B = 2847.68 k 0.04, C = 2242.03 f 0.03, AA = -2.52 + 0.74, AB = 0.02 k 0.05, AC = -0.34 f 0.03, and Av = 368.91 f 0.94 MHz, where AA, and AB, and AC are the differences of the rotational constants between the (+) and (-) states. From the torsional splittings and the energy differences of the two isomers of the parent and SeD species, Fourier coefficients of the selenol internal rotation potential function were determined to be V2 = -88 f 15, Vj = 1543 + 29 Cal/mole on the assumption of I’, = 0. Dipole moments and their components were also obtained for the two isomers.
INTRODUCTION
This investigation of microwave spectra of 2-propaneselenol is the fourth of a series of studies on the internal rotation of molecules having an asymmetric internal rotor (I-5). Throughout these investigations, we have been interested in a direct observation of torsional splittings which occur when two equivalent configurations are present. Using these splittings, it is possible to estimate the potential function which governs the internal rotation of the top. A comparison of molecular structures of rotational isomers is also an interesting subject. In the preceeding papers on microwave spectra of ethanethiol (1, 2) and ethaneselenol(5) we reported r, structures of the gauche and tram isomers and found several similarities of structures between these two molecules. The most remarkable point is that the bond angles around carbon atoms adjacent to the mercapto and selenol groups exhibit a large difference between the gauche and tram isomers (5). The C-C-X angle of the gauche isomer is about 5” larger than that of the tram isomer, where X denotes S and Se corresponding to ethanethiol and ethaneselenol, respectively. In the present paper, we deal with the microwave spectra of the gauche and trans isomers of 2-propaneselenol and three of its deuterated species, (CH&CHSeD, (CH&CDSeH, and (CD&CHSeH. The samples contain several isotopic nuclei of selenium in natural isotopic abundance and two predominant isotopic species, 80Se (49.8%) and ‘*Se (23.5%) were treated in the measurement. As shown in Fig. 1, the gauche and tram forms are defined by relative positions of the hydrogen atom in the 415
0022-2852/83 $3.00 Copyright All nghts
0
1983 by Academic
of reproduction
Press,
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Inc reserved
416
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
TRANS
Two EQUIVALENT GAUCHE
FIG. 1. Molecular conformations
of the gauche and trans isomers of 2-propaneselenol.
selenol group with respect to the hydrogen atom in the isopropyl group and the truns isomer possesses a plane of symmetry. Direct transitions between the internal rotation sublevels in the gauche isomer have been detected with the aid of a double resonance technique. From the observed torsional splitting and the energy difference of the two isomers, the selenol internal rotation potential function has been estimated. Also, similarity of the structural parameters to ethaneselenol(5) has been examined, paying particular attention to the C-C-Se angles. EXPERIMENTAL
DETAILS
Using 2-bromopropane instead of bromoethane, the procedure for preparation of samples is essentially identical with that of ethaneselenol (5). Deuterated 2-bromopropanes were prepared by the following methods. (CH&CDBr was synthesized by reducing acetone with LiAlD4 and brominating the resultant alcohol with PBr3. (CD&CHBr was produced by the same reaction with acetone-& and LiA1H4. The spectrometer used was a conventional lOO-kHz square- and sine-wave Stark modulation type with a phase sensitive detection. Measurement of the rotational spectra was made in the frequency region from 8 to 35 GHz at dry ice temperature. The double resonance experiment and intensity measurement were carried out using a HP-8690B spectrometer at National Chemical Laboratory for Industry in Tsukuba. MICROWAVE
SPECTRA
OF THE frans ISOMER
Since the tram isomer has a plane of symmetry, the b-dipole component is identically equal to zero. The c-type transitions, however, were actually so weak that about 30 u-type transitions alone were assigned for all the isotopic species. Centrifugal distortion constants of dJ and dJK were included in a modified rigid rotor expression to fit the observed frequencies using a least squares method. The observed frequencies are shown in Table I together with the differences between the observed and calculated frequencies. The constants thus obtained are also given in Table I. MICROWAVE
SPECTRA
AND
ANALYSIS
OF THE gauche ISOMER
Since the gauche isomer has two equivalent configurations with respect to the SeC-H plane, the internal rotation level of the selenol group splits into symmetric (+) and antisymmetric (-) states. The Hamiltonian used in the analysis is the same as described previously (1, 6-8):
418
NAKAGAWA,
(+lHj+)
NAGAYAMA,
AND
HAYASHI
= A(+)PZ -I- B(+)Ps + C(+)Ps + centrifugal terms
(- IHI-) = A( -)Ps + B( -)P$ + C( -)P$ + Au + centrifugal terms (+[HI--)
= D(P,p, + P,P,) + E(P,p, + P,p,) + NP, + QP,
(la) ( 1b) (lc)
where Au indicates the pure internal rotation energy difference of the selenol group, and the x axis is chosen so as to be perpendicular to the symmetry plane of the frame. Since the a-, b-, and c-inertial axes are nearly coincident with the z, X, and y axes, respectively, the a- and c-dipole transitions take place within the same symmetry sublevels, whereas the b-type tmnsitions occur between the different sublevels. The c-type transitions, however, were so weak that we did not intend to make their assignments. The b-type transitions, which are possibly weaker than the c types judging from the dipole moment components determined in a later section, were assigned with the aid of a double resonance technique for the normal and SeD species, because splittings of the b-type transitions include contribution from the pure internal rotation energy difference more explicitly than those of the a types. For the SeH species, most of the u-type transitions have doublet structures typically with a few MHz spacings. However, some of them show larger splittings at certain transitions such as 423-322, 422-321, 735-634, 734-633, 808-707, and 8i8-7i7. This fact shows that the interaction terms in ( lc) play a significant role for accidentally degenerate levels. The b-type transitions for the normal species show the spacing of about 700 MHz. To determine parameters in Hamiltonian (I) for the six SeH species, a least squares analysis was carried out to fit all the observed transition frequencies having Jquantum numbers up to 8. Since almost all the a-type transitions exhibit doublet structures, the rotational constants of the two states should be determined independently. The centrifugal distortion constants dJ and dJK were also included in the analysis. As for the parameters in Eq. (lc), we could not determine the four parameters independently, so two of the parameters, N and Q, were adjusted for the following reasons. The interaction diagram due to the cross terms between the (+) and (-) states is given schematically in Fig. 2. Although the asymmetry parameter II of the isomer is -0.78, it is possible to show that the P, term connects two levels with the same K-i, while the (P,P, + PyPx) term connects two levels whose K-r values differ by two from each other except K-r = I levels. Furthermore, like the Py term, the (P,pZ + PzPx) terms unites two levels in a manner similar to the usual c-type connection within the same J value. Thus, the levels whose K_, values are larger than one interact mainly through the Pz terms, since it is impossible for the two levels with different K_ I values to approach in near degeneracy. In the actual spectrum, 4 23-322, 422-321, 735-634, and 734-633 transitions exhibit splittings of 19, 18, 39, and 38 MHz, respectively. Such large splittings can only be interpreted by the interaction term of P,, because 32: and 3, levels and 6z3 and 6~4 levels are accidentally degenerate, where 32+,indicates the 321rotational levels in the (+) states, and so on. Thus, we should include the P, term in the least squares analysis.
MICROWAVE SPECTRUM OF 2-PROPANESELENOL
‘\ i \ \ /
419
: ,
!'PXPY
422 423
413
414 4cn
FIG. 2. Interaction scheme between the (+) and (-) states. Solid and dotted lines indicate the levels connected by the interactions; the levels connected by the solid lines are in near degeneracy. The p, shows the angular momentum of the selenol internal rotation.
For the levels with K_, = 0 and K_, = 1, the Py and (PXpZ+ PzPX) terms connect JIJ and J& levels, which fall in accidental degeneracy with each other when J is near 9. Actually, the observed splittings of J + 10,J+,- Jo,Jand J + 1l,J++l- JIJ transitions become larger as the J value approaches 9. This fact indicates that we should include the P>,term and/or the (PxPz + P,P,) term in the analysis. Although there is no experimental evidence to judge which one is to be retained, the Py term was adjusted in the analysis because of analogy with the P, term. In Table II, the observed frequencies and the differences of the observed and calculated frequencies are shown with the parameters thus determined. For the parent and (CH&CH%eH species, the b-type transitions were also included in the analysis, For the SeD species, none of the u-type transitions show any doublet structures at all and the h types exhibit splitting of about 10 MHz. In the least squares analysis, the rotational constants of the two states were considered to be the same because of singlet structures of the a-type transitions. Also, all the parameters in the off-diagonal expression, Eq. ( lc), were neglected and two of the centrifugal distortion constants, dJ and dJK, were Included. The observed frequencies and the differences of the observed and calculated frequencies are shown in Table II. The parameters adjusted are also given in Table II.
420
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
++
523 -422
616
716 - 615
717-
707 - 60,
633-532
614 -533
++
6211-523
++ __ ++ __
++
l+
++
it
615 -51..
- 515
++ _++
6*5-5zy
616
606 - 505
++
++
5zv -423
533 - 432
++
514 -411
532 - 431
++
515 - 414
++
++
50,j - 43#+
transition
24503.361-28) 24500.33<-25) 23733.88( 20) 23730.97( 19) 26703.01(-13) 26701.83(-17) 25532.87( 4) 25532.87( 5) 26273.41( 25) 26274.71( 19) 25601.03( 12) 25600.27( -1) 25656.34( 3) 25655.74(-10) ?9024.99(-22) 29020.73(-19) 28376.401 37) 28372.46( 34) 31855.80(-39) 31854.07(-41) 30304.23( -2) 30302.26( 4) 318?0.98( 17) 31810.98( 24) 30754.16( 5) 30762.14( -7) 30893.5o( 24) 30901.90( 6) 33473.27(-22) 33466.99(-24) 32981.61( 52) 32975.80( 45) 36884.66(-63) 36882.18(-58)
(CH3)2CHSeH
23886.54(-13) 26898.88( 6) 26897.W 0) 25512.32( -2) 25512.32( 15) 26480.331 5) 26481.52( 6) 25788.75( 16) 25788.05( 5) 25846.86( 3) 25846.30( -9) 29204.72( -8) 29200.34( -8) X560.44( -5) 28556.46( -1) 32084.19( 4) 32082.37( -8) 30516.66( 0) 30514.59( 2) 32063.31( -3) 32063.31( 12) 30979.91( 15) 30989.24( 4) 31125.22( 1) 31134.98( -6) 33677.95(-11) 33671.44(-10) 33193.53( 1) 33187.51( 6) 37140.91( -5) 37138.38( -8)
24656.22(-13) 23889.481-17)
24659.34(-111
(CH3)2CH7%eH 24241.47( -9) 24238.58(-14) 23510.51( -4) 23507.72( -4) 26270.48( -6) 26269.57( 5) 24993.88( 24) 24993.88( -9) 25845.34( -7) 25846.94( 1) 25235.97( 5) 25235.19( 2) 25284.36( -9) 25283.75( -8) 28741.08( 01 28737.06( -3) 28117.60(-10) 28113.98( 1) 31356.601 -1) 31355.20( 12) 29906.53( 3) 29904.65( 3) 31277.15( -7) 31277.15i 7) 30313.90( 5) 30319.41( -7) 30437.05( -3) 30442.89(-13) 33169.38( 5) 33163.57( 6) 32690.481 lj 32685.18( 14)
(CH3)2CCSeH
32898.32(
3290x95(
31532.31(
31555.42(
-1)
11)
-1)
33042.73(
sj 32581.57(
0)
1)
-5)
33237.48( 32783.30(
-1)
30849.58(
5)
31798.35(-10)
30188.17(
-5)
1)
28213.87( 31750.24(
-2)
6)
16)
0)
-6)
-2)
2)
-7)
28827.61(
25600.79(
25532.90(
26259.51(
9) 30672.30(
-9)
-4)
30455.91(
10)
-7)
-3)
28038.40(
-6)
19)
-8)
-4)
29985.19(
26631.66(
23604.83(
24351.47(
(CH3)2CH78SeDh
3) 25242.76(
-3)
-5)
-7)
28656.76(
25417.46(
25352.85(
26060.26(
X071.04(
X444.19(
23456.27(
24203.29(
(CH3)2CIiSeDb
30625.07(
25472.53( -3) 28922.35( 5) 28918.36( ioj 28302.84( -7) 28299.08( 2) 31583.58( -1) 31582.07( 4j 30118.22( -6) 30116.29( -3) 31525.96(-13) 31525.96( 16) 30538.09( 21) 30544.37( -6) 30666.60( -6) 30673.50( -1) 33375.83( 2) 33369.83( 2)
24398.39( -7) 24395.50( -9) 23666.88(-11) 23664.10( -2) 26464.79( -5) 26463.80( 3) 25172.55( 12) 25172.551 -7j 26049.40( -4) 26050.76( -1) 25422.19( -5) 25421.24(-24)
(CH3)2CD7%eH
TABLE II-Continued --_--__.-.--
4)
izj
-7) -7) -2) -1) 0) 4) 0) 5) 2)
I
22098.62( 9) 22186.56( -3) 22186.15(-11) 24638.87(-10) 24631.35(-11) 24209.85( 7) 24202.64( 1) 27384.60( 1) 27382.26( 4) 26026.66( -2) 26023.84( -3) 27685.23(-11) 27684.81( 5) 26547.10( 4) 26503.00( 0 26819.73( -4 26776.34( -1) 28398.75( -3) 18374.29( 3) 28127.94( 2) 28103.84( 5)
22861.14( 22099.23(
(CU3)ptSeH 20842.44( 20838.19( 20268.94( 20264.89( 23020.62( 23019.30( 21783.77( 21782.16( 22861.14(
(CD3)2CH7%eH
1)
%:::%1 26995.00( 28578.27( 28549.79( 28318.43( 28290.51( 31813.07( 31809.55(
-:I 8) 6) 2) -3) 8) 2) 2)
20977.44( -9) 20973.09( -9j 20407.08( 0) 20402.95( 2) 23196.66( -7) 23195.13( -6) 21945.66( 1) 21944.00( 1) 23054.57( 23054.57( -2) 22271.19( 4) 22270.90( 11) 22363.10( -8) 22363.10( 1) 24795.24( 5) 24787.16( -8) 24372.78( -2) 24365.22( -1) 27586.27( 2) 27583.85( -3) 26217.06( -3) 26214.39( 11) 27918.83( -1) 27913.33( -1) 26753.30(-16j
;: --
-tf +-; -+ t-
7,,
505
8,,-
51,
36)
366.12(
(
( 14)
2.0
8)
1) 1)
28) 25) ;I
2256.08( -0.35(
7799.41( -1.98( 287;:;9[
6.2
24) 13)
94)
3) 3)
75) 74) ;\
dJKxlo3
18.59( 7.52(
368.91(
2242.03( -0.34(
7802.50( -2.52( 284;:;;~
16989.00( 21) 21561.04(-56) 20866.48(-52)
12) 20)
14)
De dJ ~10~
Ne
A6 = U+)’ AC ’ Av d
A(+)=
lo,"-lo"."
*I,-80,
7,,-70,
61, -60,
-+ +-+ +-+
75) 11538.51( 68) 10818.30( 71) 60)
;I
8,,-7,,
11461.41( 10736.13( 14290.64( 13571.33(
;;
7 3'1-6,,
-
++
7 35 -63Lt
37785.14(
62)
;I __
35470.11( 1) 35467.28( 3) 37658.39( -1) 37657.97( -2) 36168.43( 1) 36128.98( 17) 36550.77(-22) 36512.17( -3) 38127.19( -6)
725-62,
35226.30( 0) 35223.51( 1) 37363.70( 15) 37363.27( 7) 35905.40( -2) 35866.50( 34) 36271,26(-l*) 36233.14( 11) 37895.84( -4)
(CH3)2CH7*SeH
++
(CH3)2CHSeH
7 26 -625
transition
67) 39) ;I
-5) 3)
-8) -1)
2228.63( 2) -0.33( 1) 347.96(273) 18.07( 7) 7.56( 24)
7'36.80( -1.62( 279;:;;[
35390.85( 35352.06(
34774.65( 34772.09(
(CH3)2CDSeH
;;
281;:X;I
2242.80( 2) -0.34( 1) 347.42(411) 18.44( 21) 7.69( 25)
71) 41)
7536.53( -l.lO(
35018.02(-11) 35015.39( -3)
(CH3)2CD7*SeH 35079.96(
37) 11072.38( 10) 11062.10( 34)
6)
1) 12)
5.05(
1)
29)
2214.14(
2824.69(
7565.50(
47) 21)
( 22)
( 19)
3.8
17)
5.01(
5.8
2)
2)
34)
2227.49(
2846.46(
7566.12(
(CD3)2CHSeH
356.42( 13.74(
1908:04( -0.41(
247;.;;;
6017.26( -1.33(
79) 5)
1) 1)
;;
27) 12)
7) 30211.90( -3) 30208.32( 3) 32473.76( -8) 32472.87( 0) 30983.51(-10) 31010.48( 2) 31447.S4( -2) 31475.90( -9) 32159.56( 11) 32216.99 -2) 31918.09 t 0) 31976.08( 9)
(CH3)2CH7*SeDb
17309.59( 27) 17489.23( 61) 17299.43( 21) 17478.82( 21) 21182.55( 8) 21172.40( 4) 21395.78(-70) 29505.18(-70) 29786.57( -3)
10988.97( 10978.60( 13864.53(
34847.10(
(CH3)2CHSeDb
TABLE II-Continued ~-
0.8
76) 5)
1';
;;
31) 14)
( 10)
356:49( 13.95(
"';:;;I
249;.;;[
6017.51( -1.35(
30429.11( 5) 30425.45( 4) 32742.63( 5) 32741.66( -3) 31223.28( 6) 31244.12( -6) 31717.91(-13) 31740.33( -2) 32363.69( 7) 32424.03(-11) 32125.49(-11) 32186.64( 17)
(CD3)2CH7*SeH
41760.99( -88) 41757.51( -77 40093.50( 4on90.07( 1) 42871.50( -7) 42870.50( -36)
42040.07( -13
(CH3)2CHSeH and (CH3)@178seH ~ -___I___(CH3)2CHSeH (CH3)2CH78SeH (CH3)2CHSeH (CH3)2CH78SeH
-~~-_ (CH3)2CHSeH (CH3)2CH78SeH
836-73s ++ 41043.00( -42) 41342.67( -6) 919-81, ++ 42134.26( 36) ~2407.01( 8) -- 41048.84( -19) 41347.22( -10) -- 42089.95( 98) 42355.99( 22) 83s-7ji,++ 4X64.27( 46) 4?993.52( 1) 91a-817 ++ 46469.67(-102) 46767.54( -5) 40363.71( 7) -- 41671.20( 26) 41999.54( -6) -- 46465.03( -82) 46762.77( I) 43204.24( -11) 9o9-8oo ++ 42320.81( 21) 42580.17( 9) 43203.42( -16) -- 422X.09( 88) 42528.75( 22) _.-_e a) Figures in the Parentheses for thetransitionsindicate the differences attached to the last significant figures between the observed and calculated frequencies. Those for the molecular constants denote the uncertainties calculated from 2.5 times the standard deviations. b) Since the a-type transitions of the SeD species exhibit no splittings at all, the frequencies are written in row of (++) parity. c) The A(+), B(+), and C(+) are the rotational constants of the (t) state and AA. AB, AC are the differences of the rotational constants between the (-) and (+) states difined by oA=A(-)-A(+), and so on. d) The selenol pure internal rotation energy difference. e) The coefficients of Pa and PC terms in Eq. (1~). These constants are actually imaginary. f) These transitions are confirmed by double resonance experiments.
8,~-7]~ ++ -&,-7.x ++ -&s-725 ++ --
transition
Additional transitionsfor
TABLE II-Conrintted
R w
424
NAKAGAWA,
NAGAYAMA,
MOLECULAR
AND
HAYASHI
STRUCTURE
A unique set of the structural parameters by solving Kraitchman equation for both the isomers cannot be obtained because of the insufficient number of data available. Meaningful information on the structure, however, may be derived from the observed rotational constants of the eight isotopic species. Since the moments of inertia calculated from a well-determined rs structure are usually shgbtly smaller than the observed ones and the differences are nearly equal among all the isotopic substituted species (see, for example, (1, 5)), the observed moments of inertia should be corrected for small amounts when one intends to obtain the r,-like structure rather than the v. structure. It is, however, too difficult to evaluate these differences from the theoretical background, and the values of 0.1, 0.62, and 0.54 amu 9A2, which are the averages of these differences for the trclns and g~~c~~ ethaneselenol(5), were assumed for the differences of the a, tr, and c inertial moments, respectively. Assuming tetrahedral angles and C-H bond lengths of 1.091 A in the two methyl groups and 1.100 A for the C-H bond length connecting the central carbon atom, skeletal parameters were adjusted by a least squares method to reproduce the corrected moments of inertia. Two carbon atoms in the methyl groups were designated by C, and Ct, where subscripts g and t indicate the methyl carbon situates at the gauche and tram positions, respectively, with regard to the hydrogen atom in the selenol group. For the truns isomer, r(C-C), r(C-Se), r(Se-H), ar(CCC), tw(C,CSe), and c$CSeH) were determined. In the fitting, the highest correlation among the parameters was found between r(C-C) and c&XX) with the value of 0.988 and the absolute values of the other elements are less than 0.9. Thus, the parameters in the tram isomer can be dete~ined fairly well, as long as the assumptions made are acceptable. For the ~~~c~e isomer, dihedral angle, T(HCSeH), and n(C&ZSe) were added in the fitting, since the symmetry plane no longer exists. Two C-f: bond Lengths were assumed to be the same. The a&CC) was fixed to the corresponding value in the truns isomer because the least squares analysis including this angle as a parameter caused divergence of the fit. Nevertheless, the correlations of the parameters are very bad. Especially the element of the correlation matrix between cu(C&Se) and a(C,CSe) is 0.9989. This situation makes the uncertainties of the parameters determined in the fit fairly large. The obtained parameters together with the uncertainties are shown in Table III and the observed moments of inertia and the differences between the observed and calculated moments of inertia are given in Table IV for both the isomers. It has been found that two parameters, r(C-Se) and r(Se-H), differ by 0.009 and 0.012 A between these two isomers. The angles around the central carbon atom have a big discrepancy, although their uncertainties of the gaucht isomer are very large. Although cy(C,CSe)‘s of both the isomers are nearly equal, cw(C,CSe)of the gaz&e isomer is 4”30’ smaller than cw(C,CSe). These results are in line with the r, determination of the related molecules such as ethaneselenol (5) and ethanethiol (I, 9). For example, in ethaneselenol, cr(C,CSe) is 4”48’ smaller than Lu(CgCSe). This difference of the angle can be explained by tilt phenomenon and the isopropyl group is tilted 3” towards the lone pair electrons of the selenol atom.
MI~OWAVE
425
SPECTRUM OF 2-PROPANESELENOL TABLE III Structural Parameters Adjustedab trans
gauche
1.546
(0.003)
1.542
r(C-se)&
1.930
(0.003)
1.939
(0.003)
r(Se-HI&
1.458
(0.015)
1.446
(0.021)
r(C-C)
G,
(0.003)
a(CCC)
llO"53
(15')
llo053'd
u(CgCSelC
113004' -
(10')
112'=58' (3"OS')
93O24'
161')
ufCtCSe)c a(CSeH1 7(HCSeH) a)
b)
C)
d)
108*29'
1180°)
(3*22'f
93042'
i
61047'
(1045')
61’)
Figures in the garentheses indicate uncertainties calculated from one standard deviations. 0 Tetrahedral angles and 1.091 A for C-H bond lengths in the two methyl groups and 1.100 & for C-H bond length connecting the hydrogen atom and the central carbon Remaining angles around atom are assumed. the central carbon atom are as following; for the trans isomer; a(HCSe)=107"04', u(HCC )=106'19' for til e gauche isomer; a(HCSe)=109“24' cr(HCCg)=10?'03' and u(HCCt)=107°51'. Subscripts g and t indicate that the carbon atom in the methyl group situates the gauche and trans positions, respectively, with respect to the hydrogen atom in the selenol group. This value is transferred from the corresponding value of the trans form.
TABLE IV Observed and Calculated Moments of Inertia of 2-Propaneselenol (amu. AZ) 1a
f
ram
ICH,l ZcHSeH (CHn)*cH'~SeH (CHa)zCDSeH (CH3)2CD'*SeH (CH,)2CHSeD (CH,)zCH'%eD (CDs)zCHSeH (CD3)2~~78~eH gauche
isomerb
(CHI (CH3 (CHI (CHI (CHI (CH, (CD3 (CD3
zCHSeH nCH "SeH zCDSeH zCD 7aSeH zCHSeD zCH 'BSeD 2CHSeH ZCH "SeD
v
a
64.5710 64.5122 66.7665 66.7670 66.6800 66.6799 83.7704 83.7665
(0.0613) (0.0625) (0.1066) (0.1071)
Ib
(AIb)=
64.7815 64.8049 67.0617 67.0617 66.8001 66.7946 83.9969 83.9937
4
(0.6948)
(0:1305) ~~.~~~~~
182.3277 180.8803 185.7481 184.2696 184.7675 209.5161 183.3384
(0.1267)
207.7454
(0.6887) (0.60631 (0.6015) (0.6401) (0.6348) (0.5572) (0.5512)
(0.0362) (0.0613) I:%;:;
177.4688 176.0708 181.0761 179.6426 178.9138 177.5454 204.0489 202.3366
(0.6745) (0.67211 (0.6391) (0.6350) (0.6032) (0.6007) (0.5868) (0.5813)
(0:1046) (0.1034) (0.1522) (0.1506)
)obs-(Ig)calce -here in Tab e m. Observed moment of inertia of the (+) and (-) states.
2%) AIg=(I b)
(AIa)
xc
(AIc)e
isomer
(Iglcalc
is calculated
is calculated
227.5321 226.0836 228.7628 227.2034 227.8829 226.4544 267.6468 265.8968
(0.5913) (0.5842) (0.4634) (0.4576) (0.5290) 10.5238) (0.58441 (0.5791)
225.4281 224.0241 226.7826 225.3491 228.2493 226.8814 264.8957 263.1851
(0.5423) (0.5325) (0.4775) (0.4722) (0.5597) 10.5568) (0.3763) (0.5711)
from the structure
from averaged
rotational
given
constant
426
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
It is of some interest to discuss the dependence of the structural parameters on the assumptions used here. If the C-H bond length at the central carbon atom was assumed to be 1.09 f A instead of I.100 & the values of all the parameters for the trans isomer change by only a slight amount (less than 0.003 w and lU), while for the gauche isomer, some variations’occur at r(Se-H) (0.007 8, longer), a(C,CSe) (1 o smaller), and &&Se) (45’ larger). If the assumption of certain differences of the moments of inertia was removed and the parameters were adjusted to fit the observed moments of inertia themsefves, the parameters change to some extent for both the isomers. The r(C-Se) of the trans isomer becomes 0.010 A larger, and r(C-Se), r(Se-H), cr(C,CSe), and &&Se) of the gauche isomer become 0.007 A longer, 0.005 8, longer, 40’ larger and 50’ smaller, respectively. The above results show that only the r(C-Se) parameter depends on the assumptions for the truns isomer. On the other hand, for the gable isomer, four parameters r(C-Se), r(Se-H), a(C,CSe), and cw(C,CSe) are rather sensitive to the assumptions made in the present determination of the structure. DIPOLE
MOMENT
AND
STARK
EFFECT
The dipole moments and their components were determined from the Stark effect measurements of several transitions with J = 3 - 2 for both the isomers of the (CH&CHSeH and (CH&CHSeD species. The electric field was calibrated using OCS as a standard (IO).
There are three dipole components expected in the gauche isomer. The pa and gc components connect the rotational levels within the same internal rotation substate, i.e., (+)-(C) and f-)-(-), while the ph connects the substates of the different parity. Taking account of these connections, the Stark coefficients were calculated by a second-order ~~ur~tion treatment from the observed energy levels and the transition probabilities which were assumed to be the same as those of the rigid rotor expression. For the parent species, since all the transitions measured exhibit doublet structures at zero electric held, Stark coefficients of the (+) and (-) states are able to be determined indep~ndentIy. On the other hand, for the SeD species, all the &ype transitions are singlet and the Stark lobes do not split any more by the (+) and f-) states. Since the energy djfference of the two substates is negligibly small compared to the difference of the rotational energy levels, we can safely neglect the effect of the selenol internal rotation in calculating the Stark coefficients. The observed and calculated Stark coefficients and the dipole moments determined are given in Table V. The dipole moment of the parent species is i-474 I$ making angles of 21 0 lg’, 79” 53, and 7 l”28’ with the a-, b-, and c-inertial axes, respectively. To determine the direction of the dipole moment in the molecule, four possibilities of the signs of the components were examined by comparing direction of the parent species with that of the SeD species in the same molecular coordinate system. The best agreement is obtained by the direction that makes angles of 11Dt@ and 82”36’ with the C-Se and
b)
4
-0.602 -2.484
-0.115 -0.117 0.720 3.232
}
-0.131
0.819
-0.293
1.453(l.5) 0.376(72) 0.462(76) 1.5?0(33)
0.697(14) 0.689 3.201(X) 3.148 0.013( 2) 0.012 -0.626(13) -0.606 -2.434(26) -2.460
-0.128( 2)
-0*3oo 1 -0.295( 4) -0.297 ;:;;; ) 0.820( 7)
1.474( 7) 0.278(55) 0.503(34) 1.581(17)
-0.608( 4) -2.476(B)
__ __
(Dzbye)
-0.299( 3) -0.296( 2) 0.799( 7) 0.804111) -0.114i lj -0.1X( 2) 0.717( 7) 3.223(24)
++ __ * f_ Cc __ __ x
(UIa ) zUEeD
gaudx iscmr
312-211
313
303-202
2
;
2
'0 1 2
1.547(23) 0.125(36) 1.552(23)
-0.071( 1) -0.072 -0.842( 5) -0.828 -3.140(19) -3.098
-0.083( 1) -0.093 0.957( 4) 0.948 4.099(24) 4.070
-0.315( 3) -0.312 -0.085( li -0.087 0.591( 5) 0.589
zufseu
0.996
u) the
1.540(13) 0.119133) 1.544(13)
-0.077( 2) -0.077 -0.879(19) -0.1182 -3.247(E) -3.295
0.985(20)
-0.320( 7) -0.312 -0.088( 2) -0.087 0.588(14) 0.588
(ai,)
trans isaner (aI3 ) 2aseH
F'iquresin prentheses indicate the mmxt.ainties calculated from 2.5 tines the standard deviations attachd last significant figures. in 10e5 MfZ (V/cm,2 unit.
Ub UC i uT'I
Il.3
Dipole f%mnt
2
1 2 312-211 ;
313-212 Cl
2
303-202 0
(Gf3 ) 2-
Dipole Moments and Stark Coefficients of 2-Propaneselenol’
TABLE V
428
NAKAGAWA,NAGAYAMA,AND HAYASHI
Se-H bonds, respectively, though the experimental uncertainty is quite large. Furthermore, the dipole moment lies almost on the C-Se-H plane. The dipole moment of the parent species is slightly larger than that of the SeD species. Thus, the selenol group is considered to be situated at the side of the negative pole of the moment (II).
Since the tram isomer has a plane of symmetry, the b dipole component vanishes automatically. Using a usual second-order theory, the Stark coefficients and dipole moments were calculated for the parent and SeD species. The result is shown in Table V. The dipole moment of the parent species is 1.552 D, making angles of 4”38’ and 85”22’ with the a and c axes, respectively. Since the isotopic substitution rotates the inertia axis only by an amount of 19’, we could not select one definite direction for the dipole moment from two possible directions. One of these makes an angle of 6“45’ with the C-Se bond and the other, 16”02’. For the SeD species, the former and the latter make angles of 6”39’ and 15”28’, respectively. Thus, the agreement of the former is slightly better than the latter and we select tentatively the former as the direction in the molecule. Like the gauche isomer, since the dipole moment of the parent species is slightly larger than that of the SeD species, the selenol group is situated at the side of the negative pole. ENERGY SEPARATIONOF THE TWO ISOMERS The energy separation between the gauche and tram isomers was estimated from relative intensity measurements at dry ice temperature for the parent and SeD species. For the parent species, two pairs of the lines were chosen, 313-212(tram 14 127.02 MHz, and gauche, 14329.50 and 14328.0 MHz) and 301-202 (tract, 14810.04 MHz, and gauche, 1506 1.39 and 15062.70 MHz). After averaging the intensities of the two gauche lines, the ratios I(g)/Z(t) obtained were 1.28 + 0.15 and 0.88 4 0.10, respectively. These ratios, after correcting for the dipole moments, line strengths, and partition functions, lead to the energy separation AE = ~(tra~~)-~(ga~che~ of 49 rt 13 and -2 rt 13 cm-‘. Thus, we obtained the average of 23 It: 38 cm-’ for the energy separation of the parent species, and the gauche isomer is slightly more stable. For the SeD species, we chose two pairs of transitions, 313-2i2 (tram, 14058.68 MHz and gauche, 14167.44 MHz) and 3~-2~, (tram, 15609.36 MHz and gauche, 15995.97 MHz). The ratios I(g)/l(t) obtained were 2.26 & 0.10 and 1.78 -+ 0.12. It should be noted that since the a-type transitions exhibit no splitting, the ratios are approximately twice those of the parent species. After the corrections similar to the parent species were taken into account, AE = 34 f 8 and AE = 2 rt 9 cm-’ were obtained. The average of 18 z?z25 cm-’ was estimated to be the energy separation of the SeD species. POTENTIALFUNCTIONOF SELENOLINTERNALROTATION From the observed (+)-(-) splittings and the energy differences of the two isomers of the parent and SeD species, the potential function of the selenol internal rotation
Difference
-go’
-to
t1
(cm -1)
_-
& 0.01231 23 ___
talc 0.01231 20 179.5 179.0 172.7 --
--
& 0.00017 18
130.2
134.7
& 0.00016 21
-0.0189
SeD -0.0192 4.2436
160.8 160.1 142.8
--
123.3
110.5
123.3
& 0.00110
-0.0478 0.0058
& 4.9630
ohs 0.00127
-44(17) -260( 3! 1202(16) -43( 9)
& 0.03613 73 160.9 160.0 142.8
-0.0165 0.0030
8.9432
prent
QJ& 0.03613
-
Ref.(5). Observed data except go--go+ are taken from Ref.(12). Figures in parentheses indicate 2.5 times the standard deviations.
q1+ _. - CT,_.
g1-
to - 40
go--go+
Energy
-0.0553
lJ2
(cm -I) p?Irent -0:0328 8
Coefficient
Kinetic
(CHs) 2C:fSeH O(assumed) -88(151 1543(29) O(assumed)
$
(cal/m01)~)
Barrier VI V2 V3 V6
TABLE VI Result for the Selenol Internal Rotation
430 was determined angle (Y,
NAKAGAWA, NAGAYAMA, AND HAYASHI
assuming a one-dimensional
Hamiltonian
with internal rotation
12) Although it is desirable to retain at least the first three Fourier coefficients in the potential, we could not determine three constants inde~ndently because of insufficient experimental data. A ieast squares analysis was carried out taking I$ and V, as parameters with I’, fixed to zero, since the result of ethaneselenol (5) shows that the V, value is negligibly small compared to the I’, and I’, values. The first three coefficients of the kinetic part in Eq. (2) were calculated from the structure described in an earlier section including the tilt angle around the Ce-Se bond. The result is given in Table VI together with that of ethaneselenol. ACKNOWLEDGMENTS The authors express their thanks to Dr. C. Matsumura, Dr. H. Takeo, and Dr. M. Sugie at The National chemical Laboratory for Industry for permitting the authors to use their spectrometer. RECEIVED: December 28, 1982 REFERENCES 1. J. NAKAGAWA, K. KLJWADA,AND M. HAYASHI,Bull. Chem. Sot. Japan 49, 3420-3432 (1976). 2. M. HAYASHI,J. NAKAGAWA,AND K. KUWADA, Chem. Lett., 1267-1270 (1975). 3. J. NAKAGAWAAND M. HAYASHI,J. Mol. Spectrosc. 85, 327-340 (198 1). 4. J. NAKAGAWAAND M. HAYASHI,Chem. L&t., 1321-1322 (1979). 5. J. NAKAGAWA,H. OKUTANI,AND M. HAYASHI,J. Mol. Speetrosc. 94,410-425 (1982). 6. R. E. SCHMIDTAND C. R. QUADE,J. Chem. Phys. 62, 3864-3874 (1975). 7. E. HIROTA,J. Phys. Chem. 83, 1457-1465 (1979). 8. 9. 10. 11. 12.
J. H. GRIFRTHSAND J. E. I3ocxx, J. Mol. Spectrosc. 56, 257-269 (1975). M. HAYASHI,H. IMAISHI,AND K. KUWADA, Buil. Chem. Sot. Japan 47,2382-2388 J. S. MUENTER,J. Chem. Phys. 48,4544-4547 (1968). J. S. MUENTERAND V. W. LAURIE,J. Chem. Phys. 45, 855-858 (1966). J. R. DURIG AND W. E. BUCY, J, MoL Spectrosc. 64, 474-490 (1977).
(1974).
OF MOLECULAR
SPECTROSCOPY
99,4 15-430 ( 1983)
Microwave Spectrum and Selenol Internal Rotation of 2-Propaneselenol JUN NAKAGAWA, AKIHIRO NAGAYAMA, AND MICHIRO HAYASHI Department of Chemistry, Faculty of Science. Hiroshima University, Higashi-sendamachi, Hiroshima, 730. Japan
Microwave spectra of 2-propaneselenol and its deuterated specieswere measured and assigned for the gauche and trans isomers. The double minimum splittings of the gauche isomers were directly observed from b-type transitions, which were assigned with the aid of a double resonance technique. Rotational constants and torsional splitting of the gauche isomer of the parent species were determined to be A = 7802.50 f 0.75, B = 2847.68 k 0.04, C = 2242.03 f 0.03, AA = -2.52 + 0.74, AB = 0.02 k 0.05, AC = -0.34 f 0.03, and Av = 368.91 f 0.94 MHz, where AA, and AB, and AC are the differences of the rotational constants between the (+) and (-) states. From the torsional splittings and the energy differences of the two isomers of the parent and SeD species, Fourier coefficients of the selenol internal rotation potential function were determined to be V2 = -88 f 15, Vj = 1543 + 29 Cal/mole on the assumption of I’, = 0. Dipole moments and their components were also obtained for the two isomers.
INTRODUCTION
This investigation of microwave spectra of 2-propaneselenol is the fourth of a series of studies on the internal rotation of molecules having an asymmetric internal rotor (I-5). Throughout these investigations, we have been interested in a direct observation of torsional splittings which occur when two equivalent configurations are present. Using these splittings, it is possible to estimate the potential function which governs the internal rotation of the top. A comparison of molecular structures of rotational isomers is also an interesting subject. In the preceeding papers on microwave spectra of ethanethiol (1, 2) and ethaneselenol(5) we reported r, structures of the gauche and tram isomers and found several similarities of structures between these two molecules. The most remarkable point is that the bond angles around carbon atoms adjacent to the mercapto and selenol groups exhibit a large difference between the gauche and tram isomers (5). The C-C-X angle of the gauche isomer is about 5” larger than that of the tram isomer, where X denotes S and Se corresponding to ethanethiol and ethaneselenol, respectively. In the present paper, we deal with the microwave spectra of the gauche and trans isomers of 2-propaneselenol and three of its deuterated species, (CH&CHSeD, (CH&CDSeH, and (CD&CHSeH. The samples contain several isotopic nuclei of selenium in natural isotopic abundance and two predominant isotopic species, 80Se (49.8%) and ‘*Se (23.5%) were treated in the measurement. As shown in Fig. 1, the gauche and tram forms are defined by relative positions of the hydrogen atom in the 415
0022-2852/83 $3.00 Copyright All nghts
0
1983 by Academic
of reproduction
Press,
in any form
Inc reserved
416
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
TRANS
Two EQUIVALENT GAUCHE
FIG. 1. Molecular conformations
of the gauche and trans isomers of 2-propaneselenol.
selenol group with respect to the hydrogen atom in the isopropyl group and the truns isomer possesses a plane of symmetry. Direct transitions between the internal rotation sublevels in the gauche isomer have been detected with the aid of a double resonance technique. From the observed torsional splitting and the energy difference of the two isomers, the selenol internal rotation potential function has been estimated. Also, similarity of the structural parameters to ethaneselenol(5) has been examined, paying particular attention to the C-C-Se angles. EXPERIMENTAL
DETAILS
Using 2-bromopropane instead of bromoethane, the procedure for preparation of samples is essentially identical with that of ethaneselenol (5). Deuterated 2-bromopropanes were prepared by the following methods. (CH&CDBr was synthesized by reducing acetone with LiAlD4 and brominating the resultant alcohol with PBr3. (CD&CHBr was produced by the same reaction with acetone-& and LiA1H4. The spectrometer used was a conventional lOO-kHz square- and sine-wave Stark modulation type with a phase sensitive detection. Measurement of the rotational spectra was made in the frequency region from 8 to 35 GHz at dry ice temperature. The double resonance experiment and intensity measurement were carried out using a HP-8690B spectrometer at National Chemical Laboratory for Industry in Tsukuba. MICROWAVE
SPECTRA
OF THE frans ISOMER
Since the tram isomer has a plane of symmetry, the b-dipole component is identically equal to zero. The c-type transitions, however, were actually so weak that about 30 u-type transitions alone were assigned for all the isotopic species. Centrifugal distortion constants of dJ and dJK were included in a modified rigid rotor expression to fit the observed frequencies using a least squares method. The observed frequencies are shown in Table I together with the differences between the observed and calculated frequencies. The constants thus obtained are also given in Table I. MICROWAVE
SPECTRA
AND
ANALYSIS
OF THE gauche ISOMER
Since the gauche isomer has two equivalent configurations with respect to the SeC-H plane, the internal rotation level of the selenol group splits into symmetric (+) and antisymmetric (-) states. The Hamiltonian used in the analysis is the same as described previously (1, 6-8):
418
NAKAGAWA,
(+lHj+)
NAGAYAMA,
AND
HAYASHI
= A(+)PZ -I- B(+)Ps + C(+)Ps + centrifugal terms
(- IHI-) = A( -)Ps + B( -)P$ + C( -)P$ + Au + centrifugal terms (+[HI--)
= D(P,p, + P,P,) + E(P,p, + P,p,) + NP, + QP,
(la) ( 1b) (lc)
where Au indicates the pure internal rotation energy difference of the selenol group, and the x axis is chosen so as to be perpendicular to the symmetry plane of the frame. Since the a-, b-, and c-inertial axes are nearly coincident with the z, X, and y axes, respectively, the a- and c-dipole transitions take place within the same symmetry sublevels, whereas the b-type tmnsitions occur between the different sublevels. The c-type transitions, however, were so weak that we did not intend to make their assignments. The b-type transitions, which are possibly weaker than the c types judging from the dipole moment components determined in a later section, were assigned with the aid of a double resonance technique for the normal and SeD species, because splittings of the b-type transitions include contribution from the pure internal rotation energy difference more explicitly than those of the a types. For the SeH species, most of the u-type transitions have doublet structures typically with a few MHz spacings. However, some of them show larger splittings at certain transitions such as 423-322, 422-321, 735-634, 734-633, 808-707, and 8i8-7i7. This fact shows that the interaction terms in ( lc) play a significant role for accidentally degenerate levels. The b-type transitions for the normal species show the spacing of about 700 MHz. To determine parameters in Hamiltonian (I) for the six SeH species, a least squares analysis was carried out to fit all the observed transition frequencies having Jquantum numbers up to 8. Since almost all the a-type transitions exhibit doublet structures, the rotational constants of the two states should be determined independently. The centrifugal distortion constants dJ and dJK were also included in the analysis. As for the parameters in Eq. (lc), we could not determine the four parameters independently, so two of the parameters, N and Q, were adjusted for the following reasons. The interaction diagram due to the cross terms between the (+) and (-) states is given schematically in Fig. 2. Although the asymmetry parameter II of the isomer is -0.78, it is possible to show that the P, term connects two levels with the same K-i, while the (P,P, + PyPx) term connects two levels whose K-r values differ by two from each other except K-r = I levels. Furthermore, like the Py term, the (P,pZ + PzPx) terms unites two levels in a manner similar to the usual c-type connection within the same J value. Thus, the levels whose K_, values are larger than one interact mainly through the Pz terms, since it is impossible for the two levels with different K_ I values to approach in near degeneracy. In the actual spectrum, 4 23-322, 422-321, 735-634, and 734-633 transitions exhibit splittings of 19, 18, 39, and 38 MHz, respectively. Such large splittings can only be interpreted by the interaction term of P,, because 32: and 3, levels and 6z3 and 6~4 levels are accidentally degenerate, where 32+,indicates the 321rotational levels in the (+) states, and so on. Thus, we should include the P, term in the least squares analysis.
MICROWAVE SPECTRUM OF 2-PROPANESELENOL
‘\ i \ \ /
419
: ,
!'PXPY
422 423
413
414 4cn
FIG. 2. Interaction scheme between the (+) and (-) states. Solid and dotted lines indicate the levels connected by the interactions; the levels connected by the solid lines are in near degeneracy. The p, shows the angular momentum of the selenol internal rotation.
For the levels with K_, = 0 and K_, = 1, the Py and (PXpZ+ PzPX) terms connect JIJ and J& levels, which fall in accidental degeneracy with each other when J is near 9. Actually, the observed splittings of J + 10,J+,- Jo,Jand J + 1l,J++l- JIJ transitions become larger as the J value approaches 9. This fact indicates that we should include the P>,term and/or the (PxPz + P,P,) term in the analysis. Although there is no experimental evidence to judge which one is to be retained, the Py term was adjusted in the analysis because of analogy with the P, term. In Table II, the observed frequencies and the differences of the observed and calculated frequencies are shown with the parameters thus determined. For the parent and (CH&CH%eH species, the b-type transitions were also included in the analysis, For the SeD species, none of the u-type transitions show any doublet structures at all and the h types exhibit splitting of about 10 MHz. In the least squares analysis, the rotational constants of the two states were considered to be the same because of singlet structures of the a-type transitions. Also, all the parameters in the off-diagonal expression, Eq. ( lc), were neglected and two of the centrifugal distortion constants, dJ and dJK, were Included. The observed frequencies and the differences of the observed and calculated frequencies are shown in Table II. The parameters adjusted are also given in Table II.
420
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
++
523 -422
616
716 - 615
717-
707 - 60,
633-532
614 -533
++
6211-523
++ __ ++ __
++
l+
++
it
615 -51..
- 515
++ _++
6*5-5zy
616
606 - 505
++
++
5zv -423
533 - 432
++
514 -411
532 - 431
++
515 - 414
++
++
50,j - 43#+
transition
24503.361-28) 24500.33<-25) 23733.88( 20) 23730.97( 19) 26703.01(-13) 26701.83(-17) 25532.87( 4) 25532.87( 5) 26273.41( 25) 26274.71( 19) 25601.03( 12) 25600.27( -1) 25656.34( 3) 25655.74(-10) ?9024.99(-22) 29020.73(-19) 28376.401 37) 28372.46( 34) 31855.80(-39) 31854.07(-41) 30304.23( -2) 30302.26( 4) 318?0.98( 17) 31810.98( 24) 30754.16( 5) 30762.14( -7) 30893.5o( 24) 30901.90( 6) 33473.27(-22) 33466.99(-24) 32981.61( 52) 32975.80( 45) 36884.66(-63) 36882.18(-58)
(CH3)2CHSeH
23886.54(-13) 26898.88( 6) 26897.W 0) 25512.32( -2) 25512.32( 15) 26480.331 5) 26481.52( 6) 25788.75( 16) 25788.05( 5) 25846.86( 3) 25846.30( -9) 29204.72( -8) 29200.34( -8) X560.44( -5) 28556.46( -1) 32084.19( 4) 32082.37( -8) 30516.66( 0) 30514.59( 2) 32063.31( -3) 32063.31( 12) 30979.91( 15) 30989.24( 4) 31125.22( 1) 31134.98( -6) 33677.95(-11) 33671.44(-10) 33193.53( 1) 33187.51( 6) 37140.91( -5) 37138.38( -8)
24656.22(-13) 23889.481-17)
24659.34(-111
(CH3)2CH7%eH 24241.47( -9) 24238.58(-14) 23510.51( -4) 23507.72( -4) 26270.48( -6) 26269.57( 5) 24993.88( 24) 24993.88( -9) 25845.34( -7) 25846.94( 1) 25235.97( 5) 25235.19( 2) 25284.36( -9) 25283.75( -8) 28741.08( 01 28737.06( -3) 28117.60(-10) 28113.98( 1) 31356.601 -1) 31355.20( 12) 29906.53( 3) 29904.65( 3) 31277.15( -7) 31277.15i 7) 30313.90( 5) 30319.41( -7) 30437.05( -3) 30442.89(-13) 33169.38( 5) 33163.57( 6) 32690.481 lj 32685.18( 14)
(CH3)2CCSeH
32898.32(
3290x95(
31532.31(
31555.42(
-1)
11)
-1)
33042.73(
sj 32581.57(
0)
1)
-5)
33237.48( 32783.30(
-1)
30849.58(
5)
31798.35(-10)
30188.17(
-5)
1)
28213.87( 31750.24(
-2)
6)
16)
0)
-6)
-2)
2)
-7)
28827.61(
25600.79(
25532.90(
26259.51(
9) 30672.30(
-9)
-4)
30455.91(
10)
-7)
-3)
28038.40(
-6)
19)
-8)
-4)
29985.19(
26631.66(
23604.83(
24351.47(
(CH3)2CH78SeDh
3) 25242.76(
-3)
-5)
-7)
28656.76(
25417.46(
25352.85(
26060.26(
X071.04(
X444.19(
23456.27(
24203.29(
(CH3)2CIiSeDb
30625.07(
25472.53( -3) 28922.35( 5) 28918.36( ioj 28302.84( -7) 28299.08( 2) 31583.58( -1) 31582.07( 4j 30118.22( -6) 30116.29( -3) 31525.96(-13) 31525.96( 16) 30538.09( 21) 30544.37( -6) 30666.60( -6) 30673.50( -1) 33375.83( 2) 33369.83( 2)
24398.39( -7) 24395.50( -9) 23666.88(-11) 23664.10( -2) 26464.79( -5) 26463.80( 3) 25172.55( 12) 25172.551 -7j 26049.40( -4) 26050.76( -1) 25422.19( -5) 25421.24(-24)
(CH3)2CD7%eH
TABLE II-Continued --_--__.-.--
4)
izj
-7) -7) -2) -1) 0) 4) 0) 5) 2)
I
22098.62( 9) 22186.56( -3) 22186.15(-11) 24638.87(-10) 24631.35(-11) 24209.85( 7) 24202.64( 1) 27384.60( 1) 27382.26( 4) 26026.66( -2) 26023.84( -3) 27685.23(-11) 27684.81( 5) 26547.10( 4) 26503.00( 0 26819.73( -4 26776.34( -1) 28398.75( -3) 18374.29( 3) 28127.94( 2) 28103.84( 5)
22861.14( 22099.23(
(CU3)ptSeH 20842.44( 20838.19( 20268.94( 20264.89( 23020.62( 23019.30( 21783.77( 21782.16( 22861.14(
(CD3)2CH7%eH
1)
%:::%1 26995.00( 28578.27( 28549.79( 28318.43( 28290.51( 31813.07( 31809.55(
-:I 8) 6) 2) -3) 8) 2) 2)
20977.44( -9) 20973.09( -9j 20407.08( 0) 20402.95( 2) 23196.66( -7) 23195.13( -6) 21945.66( 1) 21944.00( 1) 23054.57( 23054.57( -2) 22271.19( 4) 22270.90( 11) 22363.10( -8) 22363.10( 1) 24795.24( 5) 24787.16( -8) 24372.78( -2) 24365.22( -1) 27586.27( 2) 27583.85( -3) 26217.06( -3) 26214.39( 11) 27918.83( -1) 27913.33( -1) 26753.30(-16j
;: --
-tf +-; -+ t-
7,,
505
8,,-
51,
36)
366.12(
(
( 14)
2.0
8)
1) 1)
28) 25) ;I
2256.08( -0.35(
7799.41( -1.98( 287;:;9[
6.2
24) 13)
94)
3) 3)
75) 74) ;\
dJKxlo3
18.59( 7.52(
368.91(
2242.03( -0.34(
7802.50( -2.52( 284;:;;~
16989.00( 21) 21561.04(-56) 20866.48(-52)
12) 20)
14)
De dJ ~10~
Ne
A6 = U+)’ AC ’ Av d
A(+)=
lo,"-lo"."
*I,-80,
7,,-70,
61, -60,
-+ +-+ +-+
75) 11538.51( 68) 10818.30( 71) 60)
;I
8,,-7,,
11461.41( 10736.13( 14290.64( 13571.33(
;;
7 3'1-6,,
-
++
7 35 -63Lt
37785.14(
62)
;I __
35470.11( 1) 35467.28( 3) 37658.39( -1) 37657.97( -2) 36168.43( 1) 36128.98( 17) 36550.77(-22) 36512.17( -3) 38127.19( -6)
725-62,
35226.30( 0) 35223.51( 1) 37363.70( 15) 37363.27( 7) 35905.40( -2) 35866.50( 34) 36271,26(-l*) 36233.14( 11) 37895.84( -4)
(CH3)2CH7*SeH
++
(CH3)2CHSeH
7 26 -625
transition
67) 39) ;I
-5) 3)
-8) -1)
2228.63( 2) -0.33( 1) 347.96(273) 18.07( 7) 7.56( 24)
7'36.80( -1.62( 279;:;;[
35390.85( 35352.06(
34774.65( 34772.09(
(CH3)2CDSeH
;;
281;:X;I
2242.80( 2) -0.34( 1) 347.42(411) 18.44( 21) 7.69( 25)
71) 41)
7536.53( -l.lO(
35018.02(-11) 35015.39( -3)
(CH3)2CD7*SeH 35079.96(
37) 11072.38( 10) 11062.10( 34)
6)
1) 12)
5.05(
1)
29)
2214.14(
2824.69(
7565.50(
47) 21)
( 22)
( 19)
3.8
17)
5.01(
5.8
2)
2)
34)
2227.49(
2846.46(
7566.12(
(CD3)2CHSeH
356.42( 13.74(
1908:04( -0.41(
247;.;;;
6017.26( -1.33(
79) 5)
1) 1)
;;
27) 12)
7) 30211.90( -3) 30208.32( 3) 32473.76( -8) 32472.87( 0) 30983.51(-10) 31010.48( 2) 31447.S4( -2) 31475.90( -9) 32159.56( 11) 32216.99 -2) 31918.09 t 0) 31976.08( 9)
(CH3)2CH7*SeDb
17309.59( 27) 17489.23( 61) 17299.43( 21) 17478.82( 21) 21182.55( 8) 21172.40( 4) 21395.78(-70) 29505.18(-70) 29786.57( -3)
10988.97( 10978.60( 13864.53(
34847.10(
(CH3)2CHSeDb
TABLE II-Continued ~-
0.8
76) 5)
1';
;;
31) 14)
( 10)
356:49( 13.95(
"';:;;I
249;.;;[
6017.51( -1.35(
30429.11( 5) 30425.45( 4) 32742.63( 5) 32741.66( -3) 31223.28( 6) 31244.12( -6) 31717.91(-13) 31740.33( -2) 32363.69( 7) 32424.03(-11) 32125.49(-11) 32186.64( 17)
(CD3)2CH7*SeH
41760.99( -88) 41757.51( -77 40093.50( 4on90.07( 1) 42871.50( -7) 42870.50( -36)
42040.07( -13
(CH3)2CHSeH and (CH3)@178seH ~ -___I___(CH3)2CHSeH (CH3)2CH78SeH (CH3)2CHSeH (CH3)2CH78SeH
-~~-_ (CH3)2CHSeH (CH3)2CH78SeH
836-73s ++ 41043.00( -42) 41342.67( -6) 919-81, ++ 42134.26( 36) ~2407.01( 8) -- 41048.84( -19) 41347.22( -10) -- 42089.95( 98) 42355.99( 22) 83s-7ji,++ 4X64.27( 46) 4?993.52( 1) 91a-817 ++ 46469.67(-102) 46767.54( -5) 40363.71( 7) -- 41671.20( 26) 41999.54( -6) -- 46465.03( -82) 46762.77( I) 43204.24( -11) 9o9-8oo ++ 42320.81( 21) 42580.17( 9) 43203.42( -16) -- 422X.09( 88) 42528.75( 22) _.-_e a) Figures in the Parentheses for thetransitionsindicate the differences attached to the last significant figures between the observed and calculated frequencies. Those for the molecular constants denote the uncertainties calculated from 2.5 times the standard deviations. b) Since the a-type transitions of the SeD species exhibit no splittings at all, the frequencies are written in row of (++) parity. c) The A(+), B(+), and C(+) are the rotational constants of the (t) state and AA. AB, AC are the differences of the rotational constants between the (-) and (+) states difined by oA=A(-)-A(+), and so on. d) The selenol pure internal rotation energy difference. e) The coefficients of Pa and PC terms in Eq. (1~). These constants are actually imaginary. f) These transitions are confirmed by double resonance experiments.
8,~-7]~ ++ -&,-7.x ++ -&s-725 ++ --
transition
Additional transitionsfor
TABLE II-Conrintted
R w
424
NAKAGAWA,
NAGAYAMA,
MOLECULAR
AND
HAYASHI
STRUCTURE
A unique set of the structural parameters by solving Kraitchman equation for both the isomers cannot be obtained because of the insufficient number of data available. Meaningful information on the structure, however, may be derived from the observed rotational constants of the eight isotopic species. Since the moments of inertia calculated from a well-determined rs structure are usually shgbtly smaller than the observed ones and the differences are nearly equal among all the isotopic substituted species (see, for example, (1, 5)), the observed moments of inertia should be corrected for small amounts when one intends to obtain the r,-like structure rather than the v. structure. It is, however, too difficult to evaluate these differences from the theoretical background, and the values of 0.1, 0.62, and 0.54 amu 9A2, which are the averages of these differences for the trclns and g~~c~~ ethaneselenol(5), were assumed for the differences of the a, tr, and c inertial moments, respectively. Assuming tetrahedral angles and C-H bond lengths of 1.091 A in the two methyl groups and 1.100 A for the C-H bond length connecting the central carbon atom, skeletal parameters were adjusted by a least squares method to reproduce the corrected moments of inertia. Two carbon atoms in the methyl groups were designated by C, and Ct, where subscripts g and t indicate the methyl carbon situates at the gauche and tram positions, respectively, with regard to the hydrogen atom in the selenol group. For the truns isomer, r(C-C), r(C-Se), r(Se-H), ar(CCC), tw(C,CSe), and c$CSeH) were determined. In the fitting, the highest correlation among the parameters was found between r(C-C) and c&XX) with the value of 0.988 and the absolute values of the other elements are less than 0.9. Thus, the parameters in the tram isomer can be dete~ined fairly well, as long as the assumptions made are acceptable. For the ~~~c~e isomer, dihedral angle, T(HCSeH), and n(C&ZSe) were added in the fitting, since the symmetry plane no longer exists. Two C-f: bond Lengths were assumed to be the same. The a&CC) was fixed to the corresponding value in the truns isomer because the least squares analysis including this angle as a parameter caused divergence of the fit. Nevertheless, the correlations of the parameters are very bad. Especially the element of the correlation matrix between cu(C&Se) and a(C,CSe) is 0.9989. This situation makes the uncertainties of the parameters determined in the fit fairly large. The obtained parameters together with the uncertainties are shown in Table III and the observed moments of inertia and the differences between the observed and calculated moments of inertia are given in Table IV for both the isomers. It has been found that two parameters, r(C-Se) and r(Se-H), differ by 0.009 and 0.012 A between these two isomers. The angles around the central carbon atom have a big discrepancy, although their uncertainties of the gaucht isomer are very large. Although cy(C,CSe)‘s of both the isomers are nearly equal, cw(C,CSe)of the gaz&e isomer is 4”30’ smaller than cw(C,CSe). These results are in line with the r, determination of the related molecules such as ethaneselenol (5) and ethanethiol (I, 9). For example, in ethaneselenol, cr(C,CSe) is 4”48’ smaller than Lu(CgCSe). This difference of the angle can be explained by tilt phenomenon and the isopropyl group is tilted 3” towards the lone pair electrons of the selenol atom.
MI~OWAVE
425
SPECTRUM OF 2-PROPANESELENOL TABLE III Structural Parameters Adjustedab trans
gauche
1.546
(0.003)
1.542
r(C-se)&
1.930
(0.003)
1.939
(0.003)
r(Se-HI&
1.458
(0.015)
1.446
(0.021)
r(C-C)
G,
(0.003)
a(CCC)
llO"53
(15')
llo053'd
u(CgCSelC
113004' -
(10')
112'=58' (3"OS')
93O24'
161')
ufCtCSe)c a(CSeH1 7(HCSeH) a)
b)
C)
d)
108*29'
1180°)
(3*22'f
93042'
i
61047'
(1045')
61’)
Figures in the garentheses indicate uncertainties calculated from one standard deviations. 0 Tetrahedral angles and 1.091 A for C-H bond lengths in the two methyl groups and 1.100 & for C-H bond length connecting the hydrogen atom and the central carbon Remaining angles around atom are assumed. the central carbon atom are as following; for the trans isomer; a(HCSe)=107"04', u(HCC )=106'19' for til e gauche isomer; a(HCSe)=109“24' cr(HCCg)=10?'03' and u(HCCt)=107°51'. Subscripts g and t indicate that the carbon atom in the methyl group situates the gauche and trans positions, respectively, with respect to the hydrogen atom in the selenol group. This value is transferred from the corresponding value of the trans form.
TABLE IV Observed and Calculated Moments of Inertia of 2-Propaneselenol (amu. AZ) 1a
f
ram
ICH,l ZcHSeH (CHn)*cH'~SeH (CHa)zCDSeH (CH3)2CD'*SeH (CH,)2CHSeD (CH,)zCH'%eD (CDs)zCHSeH (CD3)2~~78~eH gauche
isomerb
(CHI (CH3 (CHI (CHI (CHI (CH, (CD3 (CD3
zCHSeH nCH "SeH zCDSeH zCD 7aSeH zCHSeD zCH 'BSeD 2CHSeH ZCH "SeD
v
a
64.5710 64.5122 66.7665 66.7670 66.6800 66.6799 83.7704 83.7665
(0.0613) (0.0625) (0.1066) (0.1071)
Ib
(AIb)=
64.7815 64.8049 67.0617 67.0617 66.8001 66.7946 83.9969 83.9937
4
(0.6948)
(0:1305) ~~.~~~~~
182.3277 180.8803 185.7481 184.2696 184.7675 209.5161 183.3384
(0.1267)
207.7454
(0.6887) (0.60631 (0.6015) (0.6401) (0.6348) (0.5572) (0.5512)
(0.0362) (0.0613) I:%;:;
177.4688 176.0708 181.0761 179.6426 178.9138 177.5454 204.0489 202.3366
(0.6745) (0.67211 (0.6391) (0.6350) (0.6032) (0.6007) (0.5868) (0.5813)
(0:1046) (0.1034) (0.1522) (0.1506)
)obs-(Ig)calce -here in Tab e m. Observed moment of inertia of the (+) and (-) states.
2%) AIg=(I b)
(AIa)
xc
(AIc)e
isomer
(Iglcalc
is calculated
is calculated
227.5321 226.0836 228.7628 227.2034 227.8829 226.4544 267.6468 265.8968
(0.5913) (0.5842) (0.4634) (0.4576) (0.5290) 10.5238) (0.58441 (0.5791)
225.4281 224.0241 226.7826 225.3491 228.2493 226.8814 264.8957 263.1851
(0.5423) (0.5325) (0.4775) (0.4722) (0.5597) 10.5568) (0.3763) (0.5711)
from the structure
from averaged
rotational
given
constant
426
NAKAGAWA,
NAGAYAMA,
AND
HAYASHI
It is of some interest to discuss the dependence of the structural parameters on the assumptions used here. If the C-H bond length at the central carbon atom was assumed to be 1.09 f A instead of I.100 & the values of all the parameters for the trans isomer change by only a slight amount (less than 0.003 w and lU), while for the gauche isomer, some variations’occur at r(Se-H) (0.007 8, longer), a(C,CSe) (1 o smaller), and &&Se) (45’ larger). If the assumption of certain differences of the moments of inertia was removed and the parameters were adjusted to fit the observed moments of inertia themsefves, the parameters change to some extent for both the isomers. The r(C-Se) of the trans isomer becomes 0.010 A larger, and r(C-Se), r(Se-H), cr(C,CSe), and &&Se) of the gauche isomer become 0.007 A longer, 0.005 8, longer, 40’ larger and 50’ smaller, respectively. The above results show that only the r(C-Se) parameter depends on the assumptions for the truns isomer. On the other hand, for the gable isomer, four parameters r(C-Se), r(Se-H), a(C,CSe), and cw(C,CSe) are rather sensitive to the assumptions made in the present determination of the structure. DIPOLE
MOMENT
AND
STARK
EFFECT
The dipole moments and their components were determined from the Stark effect measurements of several transitions with J = 3 - 2 for both the isomers of the (CH&CHSeH and (CH&CHSeD species. The electric field was calibrated using OCS as a standard (IO).
There are three dipole components expected in the gauche isomer. The pa and gc components connect the rotational levels within the same internal rotation substate, i.e., (+)-(C) and f-)-(-), while the ph connects the substates of the different parity. Taking account of these connections, the Stark coefficients were calculated by a second-order ~~ur~tion treatment from the observed energy levels and the transition probabilities which were assumed to be the same as those of the rigid rotor expression. For the parent species, since all the transitions measured exhibit doublet structures at zero electric held, Stark coefficients of the (+) and (-) states are able to be determined indep~ndentIy. On the other hand, for the SeD species, all the &ype transitions are singlet and the Stark lobes do not split any more by the (+) and f-) states. Since the energy djfference of the two substates is negligibly small compared to the difference of the rotational energy levels, we can safely neglect the effect of the selenol internal rotation in calculating the Stark coefficients. The observed and calculated Stark coefficients and the dipole moments determined are given in Table V. The dipole moment of the parent species is i-474 I$ making angles of 21 0 lg’, 79” 53, and 7 l”28’ with the a-, b-, and c-inertial axes, respectively. To determine the direction of the dipole moment in the molecule, four possibilities of the signs of the components were examined by comparing direction of the parent species with that of the SeD species in the same molecular coordinate system. The best agreement is obtained by the direction that makes angles of 11Dt@ and 82”36’ with the C-Se and
b)
4
-0.602 -2.484
-0.115 -0.117 0.720 3.232
}
-0.131
0.819
-0.293
1.453(l.5) 0.376(72) 0.462(76) 1.5?0(33)
0.697(14) 0.689 3.201(X) 3.148 0.013( 2) 0.012 -0.626(13) -0.606 -2.434(26) -2.460
-0.128( 2)
-0*3oo 1 -0.295( 4) -0.297 ;:;;; ) 0.820( 7)
1.474( 7) 0.278(55) 0.503(34) 1.581(17)
-0.608( 4) -2.476(B)
__ __
(Dzbye)
-0.299( 3) -0.296( 2) 0.799( 7) 0.804111) -0.114i lj -0.1X( 2) 0.717( 7) 3.223(24)
++ __ * f_ Cc __ __ x
(UIa ) zUEeD
gaudx iscmr
312-211
313
303-202
2
;
2
'0 1 2
1.547(23) 0.125(36) 1.552(23)
-0.071( 1) -0.072 -0.842( 5) -0.828 -3.140(19) -3.098
-0.083( 1) -0.093 0.957( 4) 0.948 4.099(24) 4.070
-0.315( 3) -0.312 -0.085( li -0.087 0.591( 5) 0.589
zufseu
0.996
u) the
1.540(13) 0.119133) 1.544(13)
-0.077( 2) -0.077 -0.879(19) -0.1182 -3.247(E) -3.295
0.985(20)
-0.320( 7) -0.312 -0.088( 2) -0.087 0.588(14) 0.588
(ai,)
trans isaner (aI3 ) 2aseH
F'iquresin prentheses indicate the mmxt.ainties calculated from 2.5 tines the standard deviations attachd last significant figures. in 10e5 MfZ (V/cm,2 unit.
Ub UC i uT'I
Il.3
Dipole f%mnt
2
1 2 312-211 ;
313-212 Cl
2
303-202 0
(Gf3 ) 2-
Dipole Moments and Stark Coefficients of 2-Propaneselenol’
TABLE V
428
NAKAGAWA,NAGAYAMA,AND HAYASHI
Se-H bonds, respectively, though the experimental uncertainty is quite large. Furthermore, the dipole moment lies almost on the C-Se-H plane. The dipole moment of the parent species is slightly larger than that of the SeD species. Thus, the selenol group is considered to be situated at the side of the negative pole of the moment (II).
Since the tram isomer has a plane of symmetry, the b dipole component vanishes automatically. Using a usual second-order theory, the Stark coefficients and dipole moments were calculated for the parent and SeD species. The result is shown in Table V. The dipole moment of the parent species is 1.552 D, making angles of 4”38’ and 85”22’ with the a and c axes, respectively. Since the isotopic substitution rotates the inertia axis only by an amount of 19’, we could not select one definite direction for the dipole moment from two possible directions. One of these makes an angle of 6“45’ with the C-Se bond and the other, 16”02’. For the SeD species, the former and the latter make angles of 6”39’ and 15”28’, respectively. Thus, the agreement of the former is slightly better than the latter and we select tentatively the former as the direction in the molecule. Like the gauche isomer, since the dipole moment of the parent species is slightly larger than that of the SeD species, the selenol group is situated at the side of the negative pole. ENERGY SEPARATIONOF THE TWO ISOMERS The energy separation between the gauche and tram isomers was estimated from relative intensity measurements at dry ice temperature for the parent and SeD species. For the parent species, two pairs of the lines were chosen, 313-212(tram 14 127.02 MHz, and gauche, 14329.50 and 14328.0 MHz) and 301-202 (tract, 14810.04 MHz, and gauche, 1506 1.39 and 15062.70 MHz). After averaging the intensities of the two gauche lines, the ratios I(g)/Z(t) obtained were 1.28 + 0.15 and 0.88 4 0.10, respectively. These ratios, after correcting for the dipole moments, line strengths, and partition functions, lead to the energy separation AE = ~(tra~~)-~(ga~che~ of 49 rt 13 and -2 rt 13 cm-‘. Thus, we obtained the average of 23 It: 38 cm-’ for the energy separation of the parent species, and the gauche isomer is slightly more stable. For the SeD species, we chose two pairs of transitions, 313-2i2 (tram, 14058.68 MHz and gauche, 14167.44 MHz) and 3~-2~, (tram, 15609.36 MHz and gauche, 15995.97 MHz). The ratios I(g)/l(t) obtained were 2.26 & 0.10 and 1.78 -+ 0.12. It should be noted that since the a-type transitions exhibit no splitting, the ratios are approximately twice those of the parent species. After the corrections similar to the parent species were taken into account, AE = 34 f 8 and AE = 2 rt 9 cm-’ were obtained. The average of 18 z?z25 cm-’ was estimated to be the energy separation of the SeD species. POTENTIALFUNCTIONOF SELENOLINTERNALROTATION From the observed (+)-(-) splittings and the energy differences of the two isomers of the parent and SeD species, the potential function of the selenol internal rotation
Difference
-go’
-to
t1
(cm -1)
_-
& 0.01231 23 ___
talc 0.01231 20 179.5 179.0 172.7 --
--
& 0.00017 18
130.2
134.7
& 0.00016 21
-0.0189
SeD -0.0192 4.2436
160.8 160.1 142.8
--
123.3
110.5
123.3
& 0.00110
-0.0478 0.0058
& 4.9630
ohs 0.00127
-44(17) -260( 3! 1202(16) -43( 9)
& 0.03613 73 160.9 160.0 142.8
-0.0165 0.0030
8.9432
prent
QJ& 0.03613
-
Ref.(5). Observed data except go--go+ are taken from Ref.(12). Figures in parentheses indicate 2.5 times the standard deviations.
q1+ _. - CT,_.
g1-
to - 40
go--go+
Energy
-0.0553
lJ2
(cm -I) p?Irent -0:0328 8
Coefficient
Kinetic
(CHs) 2C:fSeH O(assumed) -88(151 1543(29) O(assumed)
$
(cal/m01)~)
Barrier VI V2 V3 V6
TABLE VI Result for the Selenol Internal Rotation
430 was determined angle (Y,
NAKAGAWA, NAGAYAMA, AND HAYASHI
assuming a one-dimensional
Hamiltonian
with internal rotation
12) Although it is desirable to retain at least the first three Fourier coefficients in the potential, we could not determine three constants inde~ndently because of insufficient experimental data. A ieast squares analysis was carried out taking I$ and V, as parameters with I’, fixed to zero, since the result of ethaneselenol (5) shows that the V, value is negligibly small compared to the I’, and I’, values. The first three coefficients of the kinetic part in Eq. (2) were calculated from the structure described in an earlier section including the tilt angle around the Ce-Se bond. The result is given in Table VI together with that of ethaneselenol. ACKNOWLEDGMENTS The authors express their thanks to Dr. C. Matsumura, Dr. H. Takeo, and Dr. M. Sugie at The National chemical Laboratory for Industry for permitting the authors to use their spectrometer. RECEIVED: December 28, 1982 REFERENCES 1. J. NAKAGAWA, K. KLJWADA,AND M. HAYASHI,Bull. Chem. Sot. Japan 49, 3420-3432 (1976). 2. M. HAYASHI,J. NAKAGAWA,AND K. KUWADA, Chem. Lett., 1267-1270 (1975). 3. J. NAKAGAWAAND M. HAYASHI,J. Mol. Spectrosc. 85, 327-340 (198 1). 4. J. NAKAGAWAAND M. HAYASHI,Chem. L&t., 1321-1322 (1979). 5. J. NAKAGAWA,H. OKUTANI,AND M. HAYASHI,J. Mol. Speetrosc. 94,410-425 (1982). 6. R. E. SCHMIDTAND C. R. QUADE,J. Chem. Phys. 62, 3864-3874 (1975). 7. E. HIROTA,J. Phys. Chem. 83, 1457-1465 (1979). 8. 9. 10. 11. 12.
J. H. GRIFRTHSAND J. E. I3ocxx, J. Mol. Spectrosc. 56, 257-269 (1975). M. HAYASHI,H. IMAISHI,AND K. KUWADA, Buil. Chem. Sot. Japan 47,2382-2388 J. S. MUENTER,J. Chem. Phys. 48,4544-4547 (1968). J. S. MUENTERAND V. W. LAURIE,J. Chem. Phys. 45, 855-858 (1966). J. R. DURIG AND W. E. BUCY, J, MoL Spectrosc. 64, 474-490 (1977).
(1974).