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Internal rotation in propyl mercaptan by microwave spectroscopy
JOURNAL
OF
MOLECULAR
SPECTROSCOPY
Internal
85,
Rotation in Propyl Mercaptan Microwave Spectroscopy
JUN NAKAGAWA Department
327-340 (1981)
AND
MICHIRO
by
HAYASHI
of Chemistry, Faculty of Science, Hiroshima Higashisenda-machi, Hiroshima 730, Japan
University,
The microwave spectra of propyl mercaptan and its six deuterated species were assigned for two rotational isomers, the trans-gauche and trans-trans forms, where the first refers to the isomerism around the central C-C bond, and the second, to the one around C-S bond. The double minimum splittings of the gauche isomers were directly observed for the species having the symmetry plane in the frame. The rotational constants and the torsional splitting of the gauche isomers of the normal species were determined to be A = 23 907.47 2 0.09, B = 2345.597 + 0.006, C = 2250.338 f 0.009, and Av = 1613.01 * 0.04 MHz. From the torsional splittings of the normal and SD species and the energy difference of two isomers, the Fourier coefficients of the thiol internal rotation potential function were determined to be Vz = -353 2 6, and V, = 1310 ? 9 cal/mol on the assumption that VI was the same as that of ethyl mercaptan. The dipole moments and their components were also obtained from the Stark effect measurements of the two isomers of the normal and SD species. The directions of the dipole moments were discussed. INTRODUCTION
One of the interesting subjects in microwave investigations is the direct observation of the torsional splitting in a molecule containing two asymmetric tops connected by an internal rotation axis. When both tops have a plane of symmetry, the possibility of the existence of two equivalent configurations occurs. The tunnelling effect through the potential barrier between the equivalent configurations makes an energy level split into symmetric and antisymmetric states. Since this splitting is very sensitive to the height of the potential barrier, this is useful information to determine the potential function. As far as we know, the torsional splittings of the SH internal rotation have been observed for three molecules; ethyl mercaptan (I, 2), isopropyl mercaptan (3), and propargyl mercaptan (4). With the aid of the torsional frequencies in the far infrared region (5), the potential functions of these molecules have been determined from the observed splittings. However, these potential functions are not able to be compared, because the types of the frames are not similar to each other. Propargyl mercaptan has a triple bond, which strongly affects the potential of the SH internal rotation through the electronic density of the adjacent carbon atom. Actually the fruns isomer does not exist in propargyl mercaptan, while the two other molecules have truns and gauche isomers. The rrans form of isopropyl mercaptan is much different from that of ethyl mercaptan, because the gauche positions with respect to the SH group contain the methyl groups in isopropyl mercaptan, whereas those in ethyl mercaptan are hydrogens. 327
0022-2852/81/020327-14$02.00/O Copyright All rights
Q 1981 by Academic of reproduction
Ress,
Inc.
in any form reserved.
NAKAGAWA
328
AND HAYASHI
The present paper concerns the thiol internal rotation potential of normal propyl mercaptan by microwave spectroscopy. This molecule has five possible rotational isomers, tram-tram, gauche-tram, tram-gauche, gauche-gauche, and gauchegauche’, where the first symbol refers to the isomerism around the central C-C bond and the second to the one around the C-S bond. Since the tunnelling effect of the thiol internal rotation does not occur for the gauche isomers with respect to the C-C bond, we are only interested in the isomers whose skeletal parts are tram forms. The trans-trans and trans-gauche isomers are hereafter referred to as trans and gauche, respectively, for abbreviation. Though Ohashi et al. (6) have assigned a-type transitions of the gauche isomer whose frequencies can be fitted by an ordinary rigid rotor expression, they have not given any comments on the assignment of c-type transitions, nor the splittings in some of the a-type transitions. They only showed that the assigned spectrum was due to the gauche isomer. Microwave spectra of a- and c-type transitions were measured for the normal and six deuterated species of the gauche isomer. Spectra due to the trans isomer were assigned for a- and b-type transitions of the normal and SD species. The potential function for the SH internal rotation was determined from the double minimum splittings and the energy difference between trans and gauche isomers. One of the interesting points in this investigation is to examine the transferability of the potential functions between ethyl and propyl mercaptans. EXPERIMENTAL
DETAILS
The commercially available sample of the normal species was used without further purification. The CH,CH,CH,SD species was prepared from sodium propanethiolate and D20 (7). The other five deuterated species, CD3CH2CH2SH, CH3CD2CH2SH, CH3CH2CD2SH, CH&HDCH$H, and CH,CH2CHDSH were prepared from the corresponding deuterated S-propyl isothiourea sulfate and aqueous sodium hydroxide solution. The S-propyl isothiourea sulfates were prepared from the corresponding deuterated propyl bromide with thiourea (8). The first three deuterated propyl bromides were purchased from Merck Sharp & Dohme (98% D), the sample of CH,CH,CHDBr was prepared by the reduction of propionaldehyde with LiAlD, and brominating the resultant alcohol with PBr, (9), and CH,CHDCH,Br was produced by addition of DBr to propylene in the presence of benzoylperoxide (10). The spectrometer used was a conventional IOO-kHz square- and sine-wave Stark modulation type with a phase sensitive detection. Measurements of the rotational spectra were usually made on an oscilloscope in the frequency region from 8 to 35 GHz at dry ice temperature. The c-type R branch transitions of the gauche isomer around 40 and 45 GHz for the normal species were also measured. The relative intensity measurement was made at dry ice and room temperatures on the strip chart recorder. MICROWAVE
SPECTRA
AND ANALYSIS
OF THE gauche ISOMERS
The seven isotopically substituted species of propyl mercaptan of interest can be classified into three groups by means of the splittings in a- and c-type transitions
MICROWAVE
SPECTRA
OF PROPYL
TABLE
MERCAPTAN
I
Observed Splittings in the n-Type Transitions of gauche CH3CH2CH2SH 404 414 413 505 515 514 606 616 615 707 717 716 a)
-
303 313 312 404 414 413 505 515 514 607 616 615
__ __ __ __ -1.88 1.73 -4.08 -4.21 __ -0.73 0.42
CH3CH2CH2SD
Propyl Mercaptana (in MHz)
CH3CD2CH2SH
__ __ ___ ----__ -__ __
329
CH3CH2CD2SH
-----
CDjCH2CH2SH
--__-
-3.06 3.04 __ 3.57 -3.71 __ -_ __
-10.17 10.03 -11.58 -11.65 -__ -_
--___ __ ---1.39 1.25 -1.59 -1.82
. The splitting is defined by Vrigid-vpsrturhedr where “rigid mdxates the transition frequency which canbe fitted by anordinary rigid rotor expression. The -- shows non splitting within the resolution limit (%0.3 MHz).
of the gauche isomers. The species belonging to Group (I) consist of the normal, CD3CH2CH2SH, CH3CD,CH2SH, and CH3CH2CD2SH species which have a symmetry plane in the frame part. In addition to all the c-type transitions, some of the a-type transitions with K-, = 1 exhibit doublet structures due to the accidental degeneracy between symmetric and antisymmetric states as shown in Table I. The CH,CH,CH,SD species belongs to Group (II), whose u-type transitions do not show any splitting at all, while the c-type transitions exhibit doublet structures of about 110 MHz. The Group (III) consists of CH,CH,CHDSH and CH,CHDCH,SH, whose rotational spectra should be fitted by a rigid rotor expression because of the lack of a symmetry plane in the frame parts. Since the procedure for the analysis of the rotational spectra for the species belonging to Group (I) is essentially the same as described in previous papers (1, 1I), we deal with it briefly here. The tunnelling effect between two equivalent gauche minima in the thiol internal rotation potential produces two different energy states which are hereafter referred to as (+) and (-) states. The effective diagonal and off-diagonal Hamiltonians are written as (+ IH( +) = A+P$ + B+Pt + C+P: + centrifugal terms,
(la)
(- IHI -)
= A-P:
(lb)
(+ IH( -)
= D(P,P,
+ B-P”, + C-P:
+ Av + centrifugal terms,
+ PUP,) + E(P,P,
+ P,P,)
+ NP, + QPz,
(lc)
where the x axis is so chosen as to be perpendicular to the symmetry plane, z and y axes are in the plane, and Av denotes the pure internal rotational energy difference between (+) and (-) states. It is worthwhile to note that the a, b , and c axes for the moment of inertia are not exactly but nearly equal to z, y , and x axes in Eq. (1). According to the selection rules, a- and b-type transitions are allowed for (+) f, (+) and (-) .+ (-), whereas c-type transitions are allowed for (+) * (-). Taking account of the fact that all the u-type transitions except accidentally
330
NAKAGAWA
AND HAYASHI
TABLE II Observed Frequencies transition
202 2
12
2
313
- 212
14 - 313
413 5
- 312
05 - 404
5
15
- 414
14 - 413
606
- 505
16 - 515
615 707 7
- 202
04 - 303
4
6
- 111
I.2 - 211
4
5
101
11 - ll0
303
3
-
parity
- 514 -
606
17 - 616
716
- 615
808
- 707
818
- 717
817
- 716
212
- 202
3 4
13 - 303 14 - 404
515
- jo5
616
- 606
7
17 - 707
'18 9 10 11
19
- *08 - 909
110-10010 111-11011
12112-12012 13113-13013 14114-14014 15115-15015
211
-
101
312
- 202
413
- 303
++ -_ ++ _++ -_ ++ __ ++ __ ++ -++ __ ++ __ ++ _++ _++ -_ ++ __ ++ __ ++ -_ ++ -++ ++ _++ -_ ++ -++ -_ ++ --
of Group (I) and (II) Species of gauche Propyl MercaptaV
(MHz)
CH3CH2CH2SH
CH3CH2CH2SD
CHjCD2C
CH3CH2CD2SH
CD3CH2CH2SH
9191.19(
9048.75(
1)
9033.13
9099.77(
8226.64(
8960.21(
6)
8931.27
-7) , -21
20;
9096.37(
( 21) -9) ( -31 13786.14( 61 ( 131 13644.10( 71 ( 4) 13930.26( -2) ( 2) 18379.95( 1) ( 10) 18191.78( 15) ( -5) 18573.18( 1) (-10) 22972.53( 1) ( 13) 22738.96( 6) 22740.84( 5) 23214.17( -3) 23215.90( -7) 27563.50( 1) ( 15) 27285.72( -3) 27231.64( 4) 27862.41( -7) 27858.20( -41 32152.44( -9) 7) 31832.05( -5) 31832.78( -2) 32499.42( -3) 32499.84(-14) 9286.93(
(
9137.871-10)
9135.96
13572.40(
6)
13548.53
13440.12(
9)
13396.55
13706.72(
-4)
13703.61
18095.01(
-3)
18062.67
17919.87(
20)
17861.47
18275.20(-U) 22616.60(
81
22574.89 22325.94 22329.00 22834.71 22837.75 27084.98
22399.081
10)
22843.44(
-9)
27136.58(
11)
26877.73(-18) 27411.32(
-2; 3) -61 4) 41 5) 11) -2) -3)
18270.94
-3)
31654.61(
0)
31356.361
-1)
31978.54(-14)
9) 26789.83 1) 26786.26 0) -3) 27407.68 27403.97 -5) 31592.28(-111 3) 31253.02( -4) -7) 31969.53(-15)
( I
( -3)
4) ( 8) 8993.051 2) c 21 9207.48i 3j ( 8) 13648.48( 13) ( 19) 13489.36( 14) ( 111 13810.84( 2) ( 5) 18195.47( 0) ( 7) 17985.15( 13) ( -7) 18413.77( 9) ( -3) 22740.50( -2) I 7) 22480.31( 0) 22490.481 -II 23006.1?( -ij 23016.20( -8) 27283.16( -8) 41 26975.04( 7) 26963.46( -6) 27629.63( -4) 27617.98(-12) 31%?2.79(-11)
(
I 2) 31468.91(
3) (-17) 32219.03( -3) (-10)
(
8151.34( ( 8302.64( 12339.611
4)
(
16451.54(
20562.65( 20377.13(
2) 5)
-3) ( 51 21 ( 91
16301.99; 16604.72i
9)
( 9) 15) ( 21)
12226.761 12453.72(
-51 -21 8)
121 2i
c 12) -1) ( 8)
2) 3) -5) ( 6) 24672.59( -41 71 24451.79( -3) 24453.18(-12) 24904.37( 101 24905.621 -2) 28781.15( -5)
i
20755.34(
(
( 8) 28526.17( -2) 28524.58( 10) 29057.37( -1) 29055.55( -1) 32888.04(-11) 326OO.O~j -ii -5) 33205.26(-10)
( ( 9)
+-+ +-+ +-+ f-+ +-+ +-+ +-+ +-+ t-+ +-+ +-+ +-+ +-+ +-+
19854.09( 9) 23079.93 ( -1) 19712.13 m( 12) 22937.87 '( -5) 19523.92 1;; 22749.84 19290.35 22518.15 71 19012.68 22236.23 ( 0) 18692.34 -7) 21916.49 -2) 18331.05 -9) 21555.12 -3) 17930.65 -12) 21154.52 -4) 17493.31 -13) 20716.92 -4) 17021.37 -15) 20244.63 -9) 16517.43 (- -16) 19740.48 51 15984.31 .13) 19206.99 81 15424.99 -7) 18647.28 15) 14842.58 -2) 18064.59 34)
+-+ +-+ +_+
34070.6OJ
;
( 6) ( ( 01
i-
i
_- b)
_-
4)
“I
38857.87( 42083.78(
15) 19)
19629.29( -6) 19746.24( 29) 19496.90(-13) 19613.721 8) 19321.72( 6) 19438.49( 23) 19104.22( 10) 19220.85( 12) 18845.42(-14) 18962.27( 11)
18210.77(-20) 18327.52( -5) 17838.11( -2) 17954.84( -9) 17431.19(-21) 17547.901-11) 16992.25(-16) 17108.92(-12) 16523.73(-14) 16640.33(-14) 16028.30( -5) 16144.83(-121 15508.69( 1, 15625.18(-10) 14968.03( 18) 15084.51( 6)
29061.83( 33602.96( 337111.80(
40) 11) 35)
15676.421 9) 18933.29( 7) 15517.33( 7) 18774.35( 24) 15306.88( 0) 18563.59(-26) 15046.69( 1) 18313.741 -1) 14738.441 -91 17994.00( -3) 14384.51(-13) 17640.18( -4) 13987.48(-14) 17243.00( -7) 13550.30(-14) 16803.62( -6) 13076.22(-18) 16331.35( -4) 12568.96(-18) 15823.85( -2) 12032.44(-16) 15287.09( 4) 11470.901-111 14725.24( 9) 10888.731 -5) 14142.04( 24) 10290.54( 2) 13544.37( 36)
16030.24( 1) 19162.30( 3) 15878.23( 3) 19010.22( 5) 15677.02( -3) 18808.98( 1) 15428.06( -5) 18563.031 6) 15132.94( -9) 18264.16( -61 14793.74(-11) 17924.86( -6) 14412.76(-10) 17543.67( -6) 13992.62(-131 17123.29( -9) 13536.381-113 16666.71(-14) 13047.26( -9) 16177.32(-10) 12523.78(-12) 15658.64( -1) 11984.92( -?I 15114.42( ;j 11419.561 4) 14548.69( 9) 10836.99( 17) l3965.S2( 28)
17111.07( 6) 20318.86(-U) 16998.33( -21 20206.23( 0) 16849.011 8) 20056.85( 14) 16663.391 -5) 19871.17( 7) 16442.68( -7) 19651.67f-11) 16187.83; -41 19395.10( 4) -- ') 19106.971 -5) 15580.5Ol -41 18787.231 -7) 15230.91( -II)) 18437.48( -0) 14853."6( -6) 18059.35( -2) 14448.66( -9) 17654.71( 0) 1401Y.ATi -71 17225.64( 8) 13568.81( i) 16774.28( 17) 13097.77( 81 16302.90( 23)
25097.73( 28354.70( 29808.72( 33065.67( 34574.26(
25370.79( 28) 28502.84( 31) 30041.42( 32) 33173.17( 17) 34763.13(-44) 10123.95( 2)
25564.91( 23) 28772.19(-42) 29791.85( 9) 32999.19(-39)
8) 181 -7) 9) 9)
MICROWAVE
SPECTRA TABLE
transition
parity
514
- 404
-';
707
- 615
-';
'08
- '16
;;
909
- 817
-';
"OlO-
'18
-';
~2012-1~110
-';
13013-12111
-';
514 616 515 615 3) b)
transitions - 414 - 413 514 - 515
due
to
the
331
II-Continued
CH3CH2CH2SD
CH3CH2CD2SH
cH3CD2CH2SH
15017.20(-11) 15133.94( 3) 19156.01(-111 19272.67( -6) 23240.00( 25) 23356.25(-101 27266.03( 6) 27382.55( -2) 31232.50( 13) 31348.74(-24) 35136.59( 18) 35252.94( -7)
CD3CH2CH2SH
11576.82( 2) 14831.73( -3) 15716.84( 8) 18971.48(-16) 19789.01( 12) 23043.28(-19) 23790.05( 15) 27043.841-25) 27716.38( 15) 30969.88C -91 31564.31i 24j 34817.22( -9)
11177.87( 14308.39( 15304.96( 18435.28( 19368.09( 22498.10( 2336::3;;
1, -8) 1) -1) 1) 8) 6)
27290.34( 6) 30419.12(-16) 31142.73( 6) 34270.96(-19) 34918.01( 19)
11374.84( 14581.38( 15163.00( 18369.11( 18904.99( 22110.?1( 22599.30( 25804.62( 26244.18( 29448.86( 29837.44( 33041.63(
-4) -4) 5) -3) -3) -8) -5) -5) 6) -9) 10)
-1)
interaction 22449.39( 23047.311 27004.56( 27588.55(
'; -+ +-
Figures in parentheses to the last digit. Overlapped with other
MERCAPTAN
11) 49)
11133.58( -1) 12150.02( -1) 15373.26( -4) 16332.47( -1) 19555.37(-10) 20455.97( 7) 23678.32(-12) 24517.99( 8) 27739.78(-15) 28516.10( 12) 31737.21(-20) 32447.58( 191
';
~1011-1019
Extra
CH3CH2CH2SH 43692.17C 4692O.llC
OF PROPYL
indicate
differences
between
the
0) 2) 7) 1) observed
22292.77( 7) 22871.03(-15) 26822.42(-10) 27370.82( 3) and
calculated
frequencies
lines.
degenerate ones do not show any splittings within the resolution limit, we can safely assume that the differences of the rotational constants between (+) and (-) states are negligibly small. Furthermore, the splitting in the c-type transitions mainly arises from the pure internal rotational energy difference. To determine the parameters in the Hamiltonian (l), a least-squares analysis was carried out so as to fit all the observed transition frequencies having quantum number./ up to 15. In this calculation, the centrifugal distortion terms of d., and dJK were alone taken into account, because of low K transitions. Furthermore, we were not able to determine four parameters in Eq. (lc) independently, since D and E are highly correlated with Q and N, respectively. Thus, we neglected Q and N. Since the simultaneous adjustment of A, B, C, Av, dJ, dJK, D, and E by a leastsquares method led to divergence of the fitting, the E value was fixed to a certain value so as to obtain the smallest standard deviation, that was about 4.4 times smaller than the standard deviation for E = 0. In Table II, the observed and calculated frequencies are given for the species belonging to Groups (I) and (II). The spectroscopic constants thus obtained are shown in Table III. In addition to the allowed a- and c-type transitions, some of the forbidden transitions were observed due to an interaction between (+) and (-) states through the off-diagonal Hamiltonian (1~). In the case of CH,CD,CH,SH and CH&H&D$H, the 5t4 and 5, levels are in extremely near degeneracy (32 and 22 MHz for the unperturbed levels, respectively), where we label 5,, in the (+) state as 5:,, and so on. Thus, since the wavefunctions of 5:, and 5, levels are strongly mixed together by D and/or Q terms in (lc), four transitions, 5:,-4,, 5,-4&, 6,-5:,, and 6&,-5,, were able to be observed with relatively strong intensities. Furthermore, the intensities of the perturbed transitions appeared to be weaker compared to the transitions which did not suffer the influence. For example, the 5t4-4&
NAKAGAWA
332
AND HAYASHI
TABLE
III
Obtained Constants of Propyl Mercaptans (MHz) A
B
c
dj*103
Gi3CX@$H cTIQl$kI~SD CIl$+i&TI$H ~Q2~2= C1)@l&TH2SH aI~cH*cx!BH-1 Ui~cH$XcSEZ Q1$XEH2SH-1 cHQiEH2SH-2
23907.48(9) 2345.597(6) 2250.338(9) 0.22( 8) -5.7(151 22082.94(10) 2306.7X( 7) 2217.804(11) 0.21(10) -6.8(19) 19740.00(7) 2328.654(6) 2221.559(9) 0.22( 9) -4.9(14) 20007.63(71 2309.58416) 2207.319(9) 0.23( 7) -3.5(14) 20884.85(6) 2094.557(4) 2019.009(6) 0.21( 4) -2.3( 9) 21587.27(29) 2336.990(U) 2235.399(10) 0.19(12) -6.5(24) 21647.72(30) 2339.929(U) 2233.443(10) 0.21(13) -5.9(25) 21769.30(27) 2329.602(11) 2225.685(9) 0.24(10) -5.0(24) 21784.11(30)2327.638(11) 2228.657(10) 0.21(12) -5.6(24)
aQ$X$H CH&!H&!H$D
23845.24(22)2393.469(12) 2269.621(X) 0.51(14) 22769.40(15)2335.348(8) 2207.905(10) 0.47( 9)
a) b) C) d)
Aub'
dmx103
DC'
$'
1613.01(4) 1.326(24) 57.9 58.30(5) --X28.47(4) 1.193(7) 43.0 1566.09(4) 0.682(11) 47.0 1604.02(31 0.241(6) 45.5 ------------
Figures in parentheses indicate 99% reliability interval attached to last significant figures. Energy difference between the (+) and (-) states. see text. Coefficient Of (PbP,+P,Pb) term. see text. Coefficient of (PaPc+P,Pa) term. Fixed value. see text.
was much weaker than the 5,-4, transitions for these two species, while the same transitions showed approximately equal intensities for the normal and CD,CH,CH,SH species. For the SD species, there is no source from which to determine the parameters in the off-diagonal Hamiltonian, because of the lack of an accidental degeneracy. Actually, when we tried to determine the E constant, the smallest standard deviation was obtained for E = 95 MHz, that was only 1.1 times smaller than that for E = 0 MHz. Thus, we neglected all the off-diagonal parameters and the leastsquares analysis was carried out so as to fit all the observed frequencies. The differences between the observed and calculated frequencies are shown in Table II. The spectroscopic constants are given in Table III. The microwave spectra of species belonging to Group (III) can be well fitted by an ordinary rigid rotor expression with appropriate centrifugal distortion corrections because of the lack of equivalent configurations. Thus, the rotational and two centrifugal distortion constants given in Table III were obtained by a leastsquares analysis from the observed frequencies which are shown in Table IV. MICROWAVE
SPECTRA
OF THE tram ISOMER
Since the tram isomer is a slightly asymmetric prolate top molecule, the assignment of the a-type transitions would be easy if there were no overlapping in the region. Unfortunately, however, the strong u-type transitions due to the gauche isomer (in both the ground and excited states) mask some of the a-type transitions of the tram isomer. This situation makes the assignment more difficult. A rough observation shows that the intensity of the u-type transitions of the tram isomer is about 10 times weaker than that of the gauche isomer at dry ice temperature. As for the CH,CH,CH,SD species, the assignment became further difficult from the fact that a trace of the normal species was included in the sample as an impurity because of an exchange of the deuterium atom with the hydrogen atom in the waveguide cell.
MICROWAVE
SPECTRA
OF PROPYL
TABLE
Observed Frequencies transition
MERCAFTAN
333
IV
of Group (III) Species of ga&ze
Propyl Mercaptan” (MHz)
CH3CH2CHDSH-1
CHJCH2CHDSH-2
CH3CBDCH2SH-1
CH3CHDCH2SH-2
413 414 312 313 505 4404 515 I 14 514 - 413 %6 606 - 505 515 615 - 514 707 - 606 717 - 616 716 - 615
9144.42( 3043.24( 9246.43( 13715.61( 13564.64( 13863.30( 18285.52( 18085.59( 18431.91( 22853.95( 22606.23( 23114.03( 27420.17( 27126.23( 27735.74( 31383.39( 31645.74( 32356.72(
9146.311 9040.36( 9253.17( 13718.39( 13560.23( 13873.57( 18283.12( 18073.68( 18505.611 22858.16( 22533.631 23131.05( 27424.63( 27117.16( 27755.93( 31388.70( 31634.35( 32330.17(
11 9) -7) 6) 11) -1) 8) 5) 4) 13) -6) -5) 1) -2) -5) -4) -5) -8)
13664.26 t 13509.02 13821.46 18216.30 ( 18012.45 18428.06 22768.49 ( 22514.73 23034.20 27317.01 27016.31 27640.01 31863.34 31517.65 ( 32245.03 (
13667.40 4) 13520.26 7) 13817.17 4) 18221.48 12) 18026.52 6) 18422.41 3) 22773.37 22532.22 -1:; 23027.15 -31 27324.41 -1) 27037.61 -121 27631.58 1) 31872.72 1) 31542.43 -11) 32235.37 8)
212 313 414 6515 -
13143.33( 20) 18938.15( I) 18738.32( 10) 18550.63( 18256.68( -61 5) 17318.40(-10) 17537.34( -3) 17117.49( -3) 16659.31(-11) 16168.16(-12) 15645.45(-14) 15095.37( -8) 14521.57( 0) 13328.00( 13)
19201.92 19043.66 18834.23 18574.35 18267.31 17313.56 17515.92 17076.34 (16593.63 16087.34 15543.43 14371.97 14376.79 13762.11
14) 9) 7) 7) -71 -8) -6) 10) -9) 12) 12) -7) 1) 7)
11533.91( 15782.20( 13908.16( 23968.90( 27961.81( 31883.21(
11485.18 15654.71 19759.13 23732.80 27755.40 31642.23
-7) 2) -7) -2) 6) 3)
212 202 211 303 313 404 312
-
101 111 110 202 212 303 211
202 303 404 205
716 17 - 706 07 818 - 808 913 - 903 10 110-10010
5) 3) 4) 7) 9) -21 2)
1)
-3) 12) 4) -4) 3) 01 -5) -3) -4) -7)
-5) -5) 1) -5) 12) 2)
9) 9)
-2) I -3) -1) I -7) 8) -3) I-13) 3) I -1) 4) I -9) 0) 9)
13ial.a4 ( 6) 18977.31 -1) -2) 18724.0z -- ) 18077.32 8) 17688.96 -6) 17259.33 ( -2) 16791.80 (- 14) 16233.75 -3) 15756.35 14)
0)
13210.90( 17) 19015.88( 5) 18774.34( 1) 18487.66( 2) 18157.43( -4) 17785.81( 0) 17374.80(-12) 16927.23(-12) 16445.81( -8) 15333.57( -4) 15393.781 -1) 14823.34( 2) 14245.87( 18)
23548.33( -2) 27510.41( 4) 31338.67( -2)
23672.00( 5) 27665.83( -4) 31531.23( 2)
I
I
I
14004.311
a)
Figures in parentheses indicate the differences between the observed and calculated
b)
Overlapped with other lines.
frequencies to the last digid.
The 11 u-type and 15 b-type transitions were assigned for the normal species with J 5 10 and K_, 5 1. For the SD species, 11 u-type and 13 b-types were assigned. In spite of the weakness of the transitions, b-type Q-branch series showed a typical Q-branch type Stark pattern and the assignment could be done without any ambiguity. As the influence of the centrifugal distortion effect seemed to be small, the rotational constants were determined by a least-squares method from all the observed transition frequencies given in Table V with a modified rigid rotor expression which included the dJ term. The rotational constants obtained are shown in Table III. MOLECULAR
STRUCTURE
OF galrche AND zrans ISOMER
As the numbers of the isotopically substituted species sufficient to determine the rs structures for two isomers, were estimated.
measured were not plausible structures
334
NAKAGAWA
AND HAYASHI
TABLE V Observed Frequencies transition 202 212 211 303 313
-
of tram Propyl Mercaptana (MHz) CH3CH2CH2SH
CH3CH2CH2SD
932_5;7;,( 10)
9085.94(
312
-
404 414
-
101 111 110 202 212 211 303 313
505 413 515 514
-
404 312 414 413
23304.49( 18898.76( -1) -9) -- b) 23622.43( -3)
-18426.6~/ -- b) 23032.06(
616 606 615 707 717 716
-
515 505 514 606 616 615
2795_9:3;,( -9) 28345.37( 4) 3261_0;9;,( -6)
27238.3~! -27636.62( 31768.88( 31348.40( 32240.26(
9449.92( -9) 13981;1;,( 10) 3) 1417_4y3/ -- b)
33067.37(
8)
4)
13627.3&( -11 -13820.57( 7) 1816!;9;)( -4)
2, 4) l) -2) -1) 2) 01
211 312 413 514 615 716 817 g18 1019
- 202 303 404 505 606 707 808 909 - 10010
21700.01( 1) 21887.52( -6) 22139.60( 5) 22457.60( 10) 22843.42( -2) 23299.73( 2) 23829.14( 8) 24434.59( 1) 25119.57(-12)
-- b) 20882.75( 5) 21142.36( 3) 21470.23( 3) 21868.56( 4) 22339.92( 3) 22887.41( 4) 23514.34( -3) 24224.60( -6)
212 313 707 808 909 10010
-
30654.06( 35131.52( 12368.30( 17426.56( 22531.23( 27678.29(
-- b) 33745.25(-12) 12576.54( 1) 17523.42( 2) 22516.58( -4) 27552.09( 1)
a) b)
101 202 616 717 818 919
-2) -4) 2) -3) 6) -2)
Figures in parentheses indicate differences between the observed and,calculated frequencies to the last digid. Overlapped mth theotherstrong lines.
gauche Isomer Since the microwave spectra of seven isotopic species were assigned for the isomer, a fairly reliable structure could be estimated from the observed moment of inertia. As the first trial, the rs structural parameters of gauche ethyl mercaptan (I) and propane (12) were transferred. Usually, when the rs structure is well determined, the differences between the observed and calculated moments of inertia are small positive values and approximately equal for all the measured species. Then, taking this fact into account, some of the parameters in the skeletal part were so adjusted as to reproduce the observed moments of inertia. The fitted and transferred parameters are shown in Table VI, and the observed and calculated moments of inertia are given in Table VII. As seen from the Tables VI and VII, it appeared that only two adjusted parameters were sufficient to reproduce the moments of inertia and almost all the parameters could be fixed to the corresponding rs structural parameters of gauche ethyl mercaptan and propane. The adjusted parameters are the C-C bond length gauche
MICROWAVE
SPECTRA
OF PROPYL
TABLE Structure
Parameters
of gauche
CH3C
r(C-C) r(C-H) a(~-C-H) a(H-C-C)
CCH2C
r(C-CH2) r(C-H) ci(C-C-C) a(H3C-C-S) a(H-C-CH2) CL(H-C-H) ‘I
C-CH2-S e,
(C-S)
r(C-H) (u(C-C-S) a(H1-C-S) u(H2-C-S) a(H1-C-C) a(H2-C-C 'u(Hl-C-H 1 ) a(C-S-H)
Dihedral Angle r(CCSH) a) b) C) d) e) f)
335
VI
and trans Isomers
{r(S-HI
SH
MERCAPTAN
of Propyl Mercaptana
gauche
trans
,'I;;; :; 107056'C) lll~oo'c)
;:;;4" f; 107'56") lll~OO'c)
1.528
1.529 1.092
:i~Z~qvb) 108°22'd' 110037' 106'55'
;;;:$
,, .
110035' 108'07'.
1.814 1.089 113037' 104053' 109'16' 1 110'42' llla19' 1 106'35'
1.820 1.090 108'34'
1.336 96O 0'
1.336 f, 96'13'
109'26' 110014' 108054'
61445'
180'
Parameters without any notice were taken from the rs structures of corresponding isomers of ethyl mercaptan. Parameters fitted to the moments of inertia obtained. Taken from the rs structure of propane with an assumption of cylindrical symmetry along the C-C bond. Calculated from u(C-C-C), a(H-C-CH2) and cr(H-C-H). The symbols Hl and H2, denote the hydrogen atoms which situate the same and opposite direction to the hydrogen atom of the thiol group with respect to the C-C-C-S plane. Taken from the structure of the gauche isomer.
in the CH,-CH, part and the C-C-C angle, which are 0.01 A longer and 40’ smaller than the corresponding Y, parameters of propane, respectively. Pans Isomer
Because of the weakness of the spectra of trans isomer, we tried to assign two species, the normal and SD species. Thus, since the r,, structure of this isomer is less reliable than that of the gauche isomer, the structural parameters were transferred from the rS structure of trans ethyl mercaptan (13) and the corresponding parameters of the gauche isomer. Then, the observed and calculated moments of inertia were compared. The resultant parameters are shown in Table VI. The calculated moments of inertia agree with the observed ones as shown in Table VII. DIPOLE
MOMENT
AND STARK
EFFECT
The dipole moments and their components were determined from the Stark effect measurements of several transitions with J S 3 for the gauche CH3CH2CH2SH, CHJZHJH.$D, and trans CH,CH,CH,SH species. The electric field was calibrated using the OCS molecule as a standard (14).
336
NAKAGAWA
AND HAYASHI
TABLE VII Observed and Calculated Moments of Inertia of Propyl Mercaptan (amu. A’)
(AI,) a1
1, gauche
CH3CH2CH2SH CH,CH.,CH?SD CH;CH;CD;SH CH3CD2CH2SH CD3CH2CH2SH CH3CHZCHDSH-1 CH3CH2CHDSH-2 CH3CHDCH2SH-1 CH$HDCH*SH-2 tram
(AI,)=)
1,
21.1388 22.8854 25.6016 25.2592 24.1982 23.4108 23.3455 23.2151 23.1993
21.1940 22.1954
(0.0503) (0.0380) (0.0368) (0.0546) (0.0435) (0.0631) (0.0225) (0.0690) (0.0332)
215.4573 219.0890 217.0249 218.8169 '241.2806 216.2508 215.9792 216.9366 217.1197
(0.4862) (0.5637) (0.4807) (0.3234) (0.4127) (0.5019) (0.4541) (0.4965) (0.2967)
224.5778 227.8723 227.4871 228.9547 250.3089 226.0787 226.2766 227.0654 226.7626
0.3742) 0.4524) 0.31301 0.2190) 0.2886) 0.3481) 0.3691) 0.3150) 0.3127)
(0.0425) (0.0515)
211.1479 216.4029
(0.3401) (0.2744)
226.6698 228.8939
(0.1810) (0.0921)
isomer
CH3CH2CH2SH CH3CH2CH2SD a)
(AIb)='
Ib
isomer
AIg=(Iglobs -(Ig&lc
I
where
(Ig)calc
is calculated
from the structure
in TableVI.
Though Ohashi e? al. (6) have determined the dipole moment of the gauche isomer, their calculation was based on the assumed a rotational constant, which was about 500 MHz smaller than ours, and without any considerations of the torsional splitting. These facts make a fairly large discrepancy, especially for the c-dipole component. Since no electronic plane of symmetry exists in the gauche isomer, a-, b-, and c-dipole components are expected. The connection through a- and b-dipole components is within the same state, i.e., (+)-(+), or (-)-(-), whereas the c-dipole connects (+) and (-) states. Taking this fact into account, the Stark coefficients were calculated by second-order perturbation theory from the observed energy levels and the transition probabilities which were assumed to be the same as those of the rigid rotor model. For the normal species, the 3& level is nearly degenerate with 4, level (1905 MHz apart), while the separation between the 3, and 40’4levels is 5130 MHz. Thus, the c-components of the Stark coefficients concerned with 3,2 levels are not the same for the (+) and (-) states. For example, that of 3:,-2& (M = 0) is 0.860 (in lop5 MHz~D-~~V-~~~~~), which is 4 times bigger than that of 3,2, (M = 0). The other coefficients which are not concerned with the 3r2 levels are approximately equal within the experimental error limit. Actually, the Stark lobes of the 3,,-2,, transition for each M quantum number separate into two components due to the (+) and (-) states at high voltage, while the other transitions do not show any splittings. The dipole moments given in Table VIII were determined by a weighted least-squares fit, in which the weights were taken as to be proportional to the square inverses of the standard deviations of the Stark effect measurements. For the tram isomer, the c-dipole component is identically equal to zero because of the symmetry. The obtained dipole moment and components are; /& = 1.179 +- 0.018, ,.‘,, = 1.078 -e 0.075 and ptotal = 1.598 + 0.054 D. From the dipole components obtained, the direction of the dipole moment in the molecule was determined. Since the deuterated substitution rotates the inertia
MICROWAVE
SPECTRA
OF PROPYL
TABLE
MERCAPTAN
337
VIII
Dipole Moments and Stark Coefficients of Propyl Mercaptan= gaucheiscxnx cwx!~2~ Sta& coefficient(Av/E')~) absd. c&XI. lMtparity ____calcd~ 303 - 203 2 ti 0.659( 5) 0.634 1 -H -0.126( 4) -0.127 0 -H -0.391( 5) -0.390 312 - 211 0 ++ 0.193( 2) 0.195 -_ -0.024( 1) -0.029I 313 - 212 0 t+ -0.091( 2) -0.091 211 - 110 0 -H 1.692( 4) 1.680 202 - lo1 1 3 1.737( 5) 1.733 0 ti -2.085( 7) -2.119 212 - 111 0 ++ 1.723( 8) 1.720 313 - 303 3 -+ -6.581(66) -6.570 + -6.248(164)-6.520
transi%CWW"
Ctl3CV2M2SD c&d. 0.661( 2) 0.661 -0.132( 1) -0.139 -0.409( 5) -0.405 0.192
0.189
-0.091 -0.092 1.715 1.697 1.738( 91 1.742 -2.126(12) -2.095 1.752(23) 1.727 -6.895(145) -6.928 -6.895020) -6.928
IMl 303 - 202 0 1 2 312 - 211 1
312 - 303 3 2
cbsd._ 0.391( 7) -0.160( 6) -0.353(27) -2.299(24)
calal.
4.066(81) 2.054(91)
3.943 1.960
0.386 -0.168 -0.352 -2.323
Dipolewnsnt K&ye)
u.3
1.575(7) 0.115(82) 0.580(9) 1.68300)
!'b UC IUT I a) b)
1.566(11) 0.067(40) 0.594(13) 1.6771ll)
1.179v3) 1.078(75) Y.598154)
Figures in parentheses indicate the uncertainties calculated from 2.5 times standard deviations attached to the last significant figures. in 10s5 MHz (V/cm12 unit.
axes considerably for the gauche isomer, the direction was uniquely determined from the present measurement. In our coordinate system of the molecule, the signs of ,..&b and & are the same as and opposite to that of pa component, respectively. Taking this fact into account, it is concluded that the dipole moment for the normal species makes angles of 24”03’ and 2422’ with the C-S bond and the bisector of C-S-H angle, and it is almost in the C-S-H plane (the angle between the dipole moment and the plane is 3”14’ + 2”25’). Since the dipole moment of the tram isomer was determined for only the parent species, the direction could not be uniquely determined from the experimental data alone. There are two possible directions from the experimental data, direction (A) makes an angle of 17”53’ with the C-S bond in the apex of C-S-H, and direction (B) consists of the angle of 67”Ol’ with the C-S bond out of the apex. However, comparison with related molecules makes it possible to determine the direction. Taking consideration of the fact that the angles between the dipole moments and C-S bonds are 29”45’ and 19”20’ for methyl mercaptan (15) and tram ethyl mercaptan (13), it appears that direction (A) is more reasonable than direction (B). The dipole moments and their directions are compared with those of other normal alkyl mercaptans in Table IX. The amounts of the dipole moment in the tram CH,SH, CH,CH,SH, and CH3CH2CH2SH series and the gauche CH,CH,SH, CH$H,CH$H series show a quite regular increase upon the substitution of the smaller alkyl group by a bigger one. The direction of the dipole moment becomes more parallel to the C-S bond as the alkyl group becomes bigger. ENERGY
SEPARATION
BETWEEN
gauche AND truns ISOMERS
The energy separation of the two isomers was estimated from the relative intensity measurement at dry ice (195 K) and room (298 K) temperatures. A pair
NAKAGAWA
338
AND HAYASHI
TABLE IX Comparison of Dipole Moments and Directions in n-Alkyl Mercaptans u(Debye)
trans
CH3SH d) CH3CH2SH e, CHJCH2CH2SiI f, CH$H2SH g) CH3CH2CH2SH f,
gauche
a) b)
C) d) e) f) 9)
The angle between The angle between The angle between Ref. (i4). Ref. (13). Present study. Ref. (2).
a(uXC-S)
moment moment moment
b)
a(uxbis)
29045' 19020' 17053' 25000' 24"03'
1.532 1.560 1.598 1.61 1.683
the dipole the dipole the diDok
a)
u(vxC-S-H) C)
18"30' 28"46' 30013' 23"27' 24"22'
0 0 0 2056 3014'
and the C-S bond. and the bisector of a(C-S-H). and the C-S-H olane.
of transitions of f-303-202 (13 987.18 MHz) and g-3&-2& (13 930.26 MHz) was chosen, because these lines exist in a nearly equal frequency region and are well isolated from other transitions. The ratios Z(g)/Z(t) obtained were 9.32 k 0.50 and 5.84 2 0.56, at dry ice and room temperatures, respectively. After suitable corrections for the dipole moments, the line strengths and the partition functions, the energy separation AE = E(trans) - E(gauche) was determined to be 146 and 126 cm-‘, at dry ice and room temperatures. Thus, we obtained the average of 136 k 40 cm-’ for the energy separation. This value is very similar to that of ethyl mercaptan reported by Schmidt and Quade (Z), 142 cm-‘. POTENTIAL
FUNCTION
OF THIOL INTERNAL
ROTATION
Assuming a one-dimensional Hamiltonian for the thiol internal rotation, the potential function was determined from the observed (+)-(-) splittings of the normal and SD species and the energy difference of the two isomers of the normal species. The one-dimensional Hamiltonian is written as a Fourier series of the TABLE X Result for the Thiol Internal Rotation
Barrier "1 "2
-207(67) -386(27) 1305(20)
normal 10.501 -0.021 0.016 diff.
go- - go+ to - go,
a) b) C)
-207(assumed) -353( 6) 1310( 9)
-
(cm-l)
V,,(O) L_(l) L.,(2) Energy
CH3CH2SH a)
(cal/mol)"
v3
Kinetic
CH3CH2CH2SR
Normal 10.870 -0.011 0.020
SD 5.857 -0.040 0.004
0.046 0.006
(cm-l)') 0.05380( 136_+40(
-1) 0)
0.00194(
12)
0.05851( -31 142(-21)
0.00234( 20) 138(-90)
Ref. (1). Figures in parentheses indicate the 2.5 times standard deviations. Figures in parentheses indicate the deviations of the observed values the calculated ones attached to the last significant figures.
from
MICROWAVE
-
-
SPECTRA OF PROPYL MERCAFTAN
339
CH$H&H2SH
---- CH$H$3i$SD O180”
SH Dihedral
90”
Argle
62”
FIG. 1. Potential curve, energy levels, and transition frequencies
of thiol internal rotation.
internal rotation angle (Yas Hint = Z:l*‘n’(pzCOS nzQI+ COS flap:)+ ZV,/2(1 - COS flcY),
(2)
where the symbols have their usual meanings. For a molecule having an asymmetric internal rotor, it is necessary to take at least the first three Fourier coefficients in the potential, in contrast with the internal rotation of the threefold symmetric rotor like CH3 and SiHS groups. Unfortunately, since there are not sufficient experimental data available, we could not determine the first three coefficients independently. Thus, a least-squares analysis was carried out taking the V, and V, as parameters with the V, value fixed to that in ethyl mercaptan (I). The first three coefficients of the kinetic part in Eq. (2) were calculated from the structure listed in Table VII taking consideration of the tilt angle against the C-S bond in a manner similar to that in the previous analysis (Z). The results are shown in Table X, and the potential function and the energy levels of the normal and SD species are shown in Fig. 1. It is of some interest to compare the potential functions of the thiol internal rotation of propyl and ethyl mercaptans, though the comparison of potential functions for asymmetric rotors is more ambiguous than for symmetric rotors. This is because the potential function and the structure cannot be determined near the top of the potential, but are defined at near the equilibrium position of the molecule. For a symmetric rotor, we do not have to compare the whole of the potential but near the bottom. For an asymmetric rotor, however, the comparison should be made as a whole potential function, because there are two or more nonequivalent potential minima. Nevertheless, it is worthwhile to examine the transferability of the potential functions. As shown in Table X, V3 of propyl mercaptan is 1310 2 9 calimol, which is nearly equal to that of ethyl mercaptan, 1305 -+ 20 cal/mol. Also the agreement of the V2 values is very good. From the present result, it is concluded that the thiol internal rotation has a good transferability between ethyl and propyl mercaptans. RECEIVED: March 10, 1980
340
NAKAGAWA
AND HAYASHI
REFERENCES J. NAKAGAWA, K. KUWADA, AND M. HAYASHI, Bull. Chem. Sot. Japan 49, 3420-3432 (1976). R. E. SCHMIDT AND C. R. QUADE, J. Chem. Phys. 62, 3864-3874 (1975). J. H. GRIFFITHS AND J. E. BOGGS, J. Mol. Spectrosc. 56, 257-269 (1975). K. BOLTON AND J. SHERIDAN, Spectrochim. Acru A 26, 1001-1006 (1970); F. SCAPPINI, H. MADER, AND J. SHERIDAN, Z. Nafurforsch. A 28, 77-81 (1973). 5. F. INAGAKI, I. HARADA, AND T. SHIMANOLJCHI,.I. Mol. Spectrosc. 46, 381-396 (1973). 6. 0. OHASHI, M. OHNISHI, A. TAGUI, T. SAKAIZUMI, AND 1. YAMAGUCHI. Bull. Chem. Sot. Jmpan 50, 1749- 1753 (1977). 7. A. MURRAY III AND D. L. WILLIAMS, “Organic Syntheses with Isotopes,” Part II, p. 1341, Interscience, New York, 1958.
1. 2. 3. 4.
8. Reference (7) p. 1340. 9. A. STREITWIESER, JR., J. Amer.
Chem. Sot. 75, 5014-5018
(1953).
10. M. S. KHARASCH, M. C. MCNAB, AND F. R. MAYO, J. Amer. Chem. Sot. 55, 2531-2533 II. 12. IS. 14. 15.
J. NAKAGAWA AND M. HAYASHI, Chem. Lett., 1321-1322 (1979). D. R. LIDE, JR., J. Chem. Phys. 33, 1514-1518 (1961). M. HAYASHI, M. IMAISHI, AND K. KUWADA, Bull. Chem. Sot. Japan, J. S. MUENTER, J. Chem. Phys. 48, 4544-4547 (1968). T. KOJIMA, J. Phys. Sot. Japan IS, 1284-1291 (1960).
2382-2388
(1933).
(1974).
OF
MOLECULAR
SPECTROSCOPY
Internal
85,
Rotation in Propyl Mercaptan Microwave Spectroscopy
JUN NAKAGAWA Department
327-340 (1981)
AND
MICHIRO
by
HAYASHI
of Chemistry, Faculty of Science, Hiroshima Higashisenda-machi, Hiroshima 730, Japan
University,
The microwave spectra of propyl mercaptan and its six deuterated species were assigned for two rotational isomers, the trans-gauche and trans-trans forms, where the first refers to the isomerism around the central C-C bond, and the second, to the one around C-S bond. The double minimum splittings of the gauche isomers were directly observed for the species having the symmetry plane in the frame. The rotational constants and the torsional splitting of the gauche isomers of the normal species were determined to be A = 23 907.47 2 0.09, B = 2345.597 + 0.006, C = 2250.338 f 0.009, and Av = 1613.01 * 0.04 MHz. From the torsional splittings of the normal and SD species and the energy difference of two isomers, the Fourier coefficients of the thiol internal rotation potential function were determined to be Vz = -353 2 6, and V, = 1310 ? 9 cal/mol on the assumption that VI was the same as that of ethyl mercaptan. The dipole moments and their components were also obtained from the Stark effect measurements of the two isomers of the normal and SD species. The directions of the dipole moments were discussed. INTRODUCTION
One of the interesting subjects in microwave investigations is the direct observation of the torsional splitting in a molecule containing two asymmetric tops connected by an internal rotation axis. When both tops have a plane of symmetry, the possibility of the existence of two equivalent configurations occurs. The tunnelling effect through the potential barrier between the equivalent configurations makes an energy level split into symmetric and antisymmetric states. Since this splitting is very sensitive to the height of the potential barrier, this is useful information to determine the potential function. As far as we know, the torsional splittings of the SH internal rotation have been observed for three molecules; ethyl mercaptan (I, 2), isopropyl mercaptan (3), and propargyl mercaptan (4). With the aid of the torsional frequencies in the far infrared region (5), the potential functions of these molecules have been determined from the observed splittings. However, these potential functions are not able to be compared, because the types of the frames are not similar to each other. Propargyl mercaptan has a triple bond, which strongly affects the potential of the SH internal rotation through the electronic density of the adjacent carbon atom. Actually the fruns isomer does not exist in propargyl mercaptan, while the two other molecules have truns and gauche isomers. The rrans form of isopropyl mercaptan is much different from that of ethyl mercaptan, because the gauche positions with respect to the SH group contain the methyl groups in isopropyl mercaptan, whereas those in ethyl mercaptan are hydrogens. 327
0022-2852/81/020327-14$02.00/O Copyright All rights
Q 1981 by Academic of reproduction
Ress,
Inc.
in any form reserved.
NAKAGAWA
328
AND HAYASHI
The present paper concerns the thiol internal rotation potential of normal propyl mercaptan by microwave spectroscopy. This molecule has five possible rotational isomers, tram-tram, gauche-tram, tram-gauche, gauche-gauche, and gauchegauche’, where the first symbol refers to the isomerism around the central C-C bond and the second to the one around the C-S bond. Since the tunnelling effect of the thiol internal rotation does not occur for the gauche isomers with respect to the C-C bond, we are only interested in the isomers whose skeletal parts are tram forms. The trans-trans and trans-gauche isomers are hereafter referred to as trans and gauche, respectively, for abbreviation. Though Ohashi et al. (6) have assigned a-type transitions of the gauche isomer whose frequencies can be fitted by an ordinary rigid rotor expression, they have not given any comments on the assignment of c-type transitions, nor the splittings in some of the a-type transitions. They only showed that the assigned spectrum was due to the gauche isomer. Microwave spectra of a- and c-type transitions were measured for the normal and six deuterated species of the gauche isomer. Spectra due to the trans isomer were assigned for a- and b-type transitions of the normal and SD species. The potential function for the SH internal rotation was determined from the double minimum splittings and the energy difference between trans and gauche isomers. One of the interesting points in this investigation is to examine the transferability of the potential functions between ethyl and propyl mercaptans. EXPERIMENTAL
DETAILS
The commercially available sample of the normal species was used without further purification. The CH,CH,CH,SD species was prepared from sodium propanethiolate and D20 (7). The other five deuterated species, CD3CH2CH2SH, CH3CD2CH2SH, CH3CH2CD2SH, CH&HDCH$H, and CH,CH2CHDSH were prepared from the corresponding deuterated S-propyl isothiourea sulfate and aqueous sodium hydroxide solution. The S-propyl isothiourea sulfates were prepared from the corresponding deuterated propyl bromide with thiourea (8). The first three deuterated propyl bromides were purchased from Merck Sharp & Dohme (98% D), the sample of CH,CH,CHDBr was prepared by the reduction of propionaldehyde with LiAlD, and brominating the resultant alcohol with PBr, (9), and CH,CHDCH,Br was produced by addition of DBr to propylene in the presence of benzoylperoxide (10). The spectrometer used was a conventional IOO-kHz square- and sine-wave Stark modulation type with a phase sensitive detection. Measurements of the rotational spectra were usually made on an oscilloscope in the frequency region from 8 to 35 GHz at dry ice temperature. The c-type R branch transitions of the gauche isomer around 40 and 45 GHz for the normal species were also measured. The relative intensity measurement was made at dry ice and room temperatures on the strip chart recorder. MICROWAVE
SPECTRA
AND ANALYSIS
OF THE gauche ISOMERS
The seven isotopically substituted species of propyl mercaptan of interest can be classified into three groups by means of the splittings in a- and c-type transitions
MICROWAVE
SPECTRA
OF PROPYL
TABLE
MERCAPTAN
I
Observed Splittings in the n-Type Transitions of gauche CH3CH2CH2SH 404 414 413 505 515 514 606 616 615 707 717 716 a)
-
303 313 312 404 414 413 505 515 514 607 616 615
__ __ __ __ -1.88 1.73 -4.08 -4.21 __ -0.73 0.42
CH3CH2CH2SD
Propyl Mercaptana (in MHz)
CH3CD2CH2SH
__ __ ___ ----__ -__ __
329
CH3CH2CD2SH
-----
CDjCH2CH2SH
--__-
-3.06 3.04 __ 3.57 -3.71 __ -_ __
-10.17 10.03 -11.58 -11.65 -__ -_
--___ __ ---1.39 1.25 -1.59 -1.82
. The splitting is defined by Vrigid-vpsrturhedr where “rigid mdxates the transition frequency which canbe fitted by anordinary rigid rotor expression. The -- shows non splitting within the resolution limit (%0.3 MHz).
of the gauche isomers. The species belonging to Group (I) consist of the normal, CD3CH2CH2SH, CH3CD,CH2SH, and CH3CH2CD2SH species which have a symmetry plane in the frame part. In addition to all the c-type transitions, some of the a-type transitions with K-, = 1 exhibit doublet structures due to the accidental degeneracy between symmetric and antisymmetric states as shown in Table I. The CH,CH,CH,SD species belongs to Group (II), whose u-type transitions do not show any splitting at all, while the c-type transitions exhibit doublet structures of about 110 MHz. The Group (III) consists of CH,CH,CHDSH and CH,CHDCH,SH, whose rotational spectra should be fitted by a rigid rotor expression because of the lack of a symmetry plane in the frame parts. Since the procedure for the analysis of the rotational spectra for the species belonging to Group (I) is essentially the same as described in previous papers (1, 1I), we deal with it briefly here. The tunnelling effect between two equivalent gauche minima in the thiol internal rotation potential produces two different energy states which are hereafter referred to as (+) and (-) states. The effective diagonal and off-diagonal Hamiltonians are written as (+ IH( +) = A+P$ + B+Pt + C+P: + centrifugal terms,
(la)
(- IHI -)
= A-P:
(lb)
(+ IH( -)
= D(P,P,
+ B-P”, + C-P:
+ Av + centrifugal terms,
+ PUP,) + E(P,P,
+ P,P,)
+ NP, + QPz,
(lc)
where the x axis is so chosen as to be perpendicular to the symmetry plane, z and y axes are in the plane, and Av denotes the pure internal rotational energy difference between (+) and (-) states. It is worthwhile to note that the a, b , and c axes for the moment of inertia are not exactly but nearly equal to z, y , and x axes in Eq. (1). According to the selection rules, a- and b-type transitions are allowed for (+) f, (+) and (-) .+ (-), whereas c-type transitions are allowed for (+) * (-). Taking account of the fact that all the u-type transitions except accidentally
330
NAKAGAWA
AND HAYASHI
TABLE II Observed Frequencies transition
202 2
12
2
313
- 212
14 - 313
413 5
- 312
05 - 404
5
15
- 414
14 - 413
606
- 505
16 - 515
615 707 7
- 202
04 - 303
4
6
- 111
I.2 - 211
4
5
101
11 - ll0
303
3
-
parity
- 514 -
606
17 - 616
716
- 615
808
- 707
818
- 717
817
- 716
212
- 202
3 4
13 - 303 14 - 404
515
- jo5
616
- 606
7
17 - 707
'18 9 10 11
19
- *08 - 909
110-10010 111-11011
12112-12012 13113-13013 14114-14014 15115-15015
211
-
101
312
- 202
413
- 303
++ -_ ++ _++ -_ ++ __ ++ __ ++ -++ __ ++ __ ++ _++ _++ -_ ++ __ ++ __ ++ -_ ++ -++ ++ _++ -_ ++ -++ -_ ++ --
of Group (I) and (II) Species of gauche Propyl MercaptaV
(MHz)
CH3CH2CH2SH
CH3CH2CH2SD
CHjCD2C
CH3CH2CD2SH
CD3CH2CH2SH
9191.19(
9048.75(
1)
9033.13
9099.77(
8226.64(
8960.21(
6)
8931.27
-7) , -21
20;
9096.37(
( 21) -9) ( -31 13786.14( 61 ( 131 13644.10( 71 ( 4) 13930.26( -2) ( 2) 18379.95( 1) ( 10) 18191.78( 15) ( -5) 18573.18( 1) (-10) 22972.53( 1) ( 13) 22738.96( 6) 22740.84( 5) 23214.17( -3) 23215.90( -7) 27563.50( 1) ( 15) 27285.72( -3) 27231.64( 4) 27862.41( -7) 27858.20( -41 32152.44( -9) 7) 31832.05( -5) 31832.78( -2) 32499.42( -3) 32499.84(-14) 9286.93(
(
9137.871-10)
9135.96
13572.40(
6)
13548.53
13440.12(
9)
13396.55
13706.72(
-4)
13703.61
18095.01(
-3)
18062.67
17919.87(
20)
17861.47
18275.20(-U) 22616.60(
81
22574.89 22325.94 22329.00 22834.71 22837.75 27084.98
22399.081
10)
22843.44(
-9)
27136.58(
11)
26877.73(-18) 27411.32(
-2; 3) -61 4) 41 5) 11) -2) -3)
18270.94
-3)
31654.61(
0)
31356.361
-1)
31978.54(-14)
9) 26789.83 1) 26786.26 0) -3) 27407.68 27403.97 -5) 31592.28(-111 3) 31253.02( -4) -7) 31969.53(-15)
( I
( -3)
4) ( 8) 8993.051 2) c 21 9207.48i 3j ( 8) 13648.48( 13) ( 19) 13489.36( 14) ( 111 13810.84( 2) ( 5) 18195.47( 0) ( 7) 17985.15( 13) ( -7) 18413.77( 9) ( -3) 22740.50( -2) I 7) 22480.31( 0) 22490.481 -II 23006.1?( -ij 23016.20( -8) 27283.16( -8) 41 26975.04( 7) 26963.46( -6) 27629.63( -4) 27617.98(-12) 31%?2.79(-11)
(
I 2) 31468.91(
3) (-17) 32219.03( -3) (-10)
(
8151.34( ( 8302.64( 12339.611
4)
(
16451.54(
20562.65( 20377.13(
2) 5)
-3) ( 51 21 ( 91
16301.99; 16604.72i
9)
( 9) 15) ( 21)
12226.761 12453.72(
-51 -21 8)
121 2i
c 12) -1) ( 8)
2) 3) -5) ( 6) 24672.59( -41 71 24451.79( -3) 24453.18(-12) 24904.37( 101 24905.621 -2) 28781.15( -5)
i
20755.34(
(
( 8) 28526.17( -2) 28524.58( 10) 29057.37( -1) 29055.55( -1) 32888.04(-11) 326OO.O~j -ii -5) 33205.26(-10)
( ( 9)
+-+ +-+ +-+ f-+ +-+ +-+ +-+ +-+ t-+ +-+ +-+ +-+ +-+ +-+
19854.09( 9) 23079.93 ( -1) 19712.13 m( 12) 22937.87 '( -5) 19523.92 1;; 22749.84 19290.35 22518.15 71 19012.68 22236.23 ( 0) 18692.34 -7) 21916.49 -2) 18331.05 -9) 21555.12 -3) 17930.65 -12) 21154.52 -4) 17493.31 -13) 20716.92 -4) 17021.37 -15) 20244.63 -9) 16517.43 (- -16) 19740.48 51 15984.31 .13) 19206.99 81 15424.99 -7) 18647.28 15) 14842.58 -2) 18064.59 34)
+-+ +-+ +_+
34070.6OJ
;
( 6) ( ( 01
i-
i
_- b)
_-
4)
“I
38857.87( 42083.78(
15) 19)
19629.29( -6) 19746.24( 29) 19496.90(-13) 19613.721 8) 19321.72( 6) 19438.49( 23) 19104.22( 10) 19220.85( 12) 18845.42(-14) 18962.27( 11)
18210.77(-20) 18327.52( -5) 17838.11( -2) 17954.84( -9) 17431.19(-21) 17547.901-11) 16992.25(-16) 17108.92(-12) 16523.73(-14) 16640.33(-14) 16028.30( -5) 16144.83(-121 15508.69( 1, 15625.18(-10) 14968.03( 18) 15084.51( 6)
29061.83( 33602.96( 337111.80(
40) 11) 35)
15676.421 9) 18933.29( 7) 15517.33( 7) 18774.35( 24) 15306.88( 0) 18563.59(-26) 15046.69( 1) 18313.741 -1) 14738.441 -91 17994.00( -3) 14384.51(-13) 17640.18( -4) 13987.48(-14) 17243.00( -7) 13550.30(-14) 16803.62( -6) 13076.22(-18) 16331.35( -4) 12568.96(-18) 15823.85( -2) 12032.44(-16) 15287.09( 4) 11470.901-111 14725.24( 9) 10888.731 -5) 14142.04( 24) 10290.54( 2) 13544.37( 36)
16030.24( 1) 19162.30( 3) 15878.23( 3) 19010.22( 5) 15677.02( -3) 18808.98( 1) 15428.06( -5) 18563.031 6) 15132.94( -9) 18264.16( -61 14793.74(-11) 17924.86( -6) 14412.76(-10) 17543.67( -6) 13992.62(-131 17123.29( -9) 13536.381-113 16666.71(-14) 13047.26( -9) 16177.32(-10) 12523.78(-12) 15658.64( -1) 11984.92( -?I 15114.42( ;j 11419.561 4) 14548.69( 9) 10836.99( 17) l3965.S2( 28)
17111.07( 6) 20318.86(-U) 16998.33( -21 20206.23( 0) 16849.011 8) 20056.85( 14) 16663.391 -5) 19871.17( 7) 16442.68( -7) 19651.67f-11) 16187.83; -41 19395.10( 4) -- ') 19106.971 -5) 15580.5Ol -41 18787.231 -7) 15230.91( -II)) 18437.48( -0) 14853."6( -6) 18059.35( -2) 14448.66( -9) 17654.71( 0) 1401Y.ATi -71 17225.64( 8) 13568.81( i) 16774.28( 17) 13097.77( 81 16302.90( 23)
25097.73( 28354.70( 29808.72( 33065.67( 34574.26(
25370.79( 28) 28502.84( 31) 30041.42( 32) 33173.17( 17) 34763.13(-44) 10123.95( 2)
25564.91( 23) 28772.19(-42) 29791.85( 9) 32999.19(-39)
8) 181 -7) 9) 9)
MICROWAVE
SPECTRA TABLE
transition
parity
514
- 404
-';
707
- 615
-';
'08
- '16
;;
909
- 817
-';
"OlO-
'18
-';
~2012-1~110
-';
13013-12111
-';
514 616 515 615 3) b)
transitions - 414 - 413 514 - 515
due
to
the
331
II-Continued
CH3CH2CH2SD
CH3CH2CD2SH
cH3CD2CH2SH
15017.20(-11) 15133.94( 3) 19156.01(-111 19272.67( -6) 23240.00( 25) 23356.25(-101 27266.03( 6) 27382.55( -2) 31232.50( 13) 31348.74(-24) 35136.59( 18) 35252.94( -7)
CD3CH2CH2SH
11576.82( 2) 14831.73( -3) 15716.84( 8) 18971.48(-16) 19789.01( 12) 23043.28(-19) 23790.05( 15) 27043.841-25) 27716.38( 15) 30969.88C -91 31564.31i 24j 34817.22( -9)
11177.87( 14308.39( 15304.96( 18435.28( 19368.09( 22498.10( 2336::3;;
1, -8) 1) -1) 1) 8) 6)
27290.34( 6) 30419.12(-16) 31142.73( 6) 34270.96(-19) 34918.01( 19)
11374.84( 14581.38( 15163.00( 18369.11( 18904.99( 22110.?1( 22599.30( 25804.62( 26244.18( 29448.86( 29837.44( 33041.63(
-4) -4) 5) -3) -3) -8) -5) -5) 6) -9) 10)
-1)
interaction 22449.39( 23047.311 27004.56( 27588.55(
'; -+ +-
Figures in parentheses to the last digit. Overlapped with other
MERCAPTAN
11) 49)
11133.58( -1) 12150.02( -1) 15373.26( -4) 16332.47( -1) 19555.37(-10) 20455.97( 7) 23678.32(-12) 24517.99( 8) 27739.78(-15) 28516.10( 12) 31737.21(-20) 32447.58( 191
';
~1011-1019
Extra
CH3CH2CH2SH 43692.17C 4692O.llC
OF PROPYL
indicate
differences
between
the
0) 2) 7) 1) observed
22292.77( 7) 22871.03(-15) 26822.42(-10) 27370.82( 3) and
calculated
frequencies
lines.
degenerate ones do not show any splittings within the resolution limit, we can safely assume that the differences of the rotational constants between (+) and (-) states are negligibly small. Furthermore, the splitting in the c-type transitions mainly arises from the pure internal rotational energy difference. To determine the parameters in the Hamiltonian (l), a least-squares analysis was carried out so as to fit all the observed transition frequencies having quantum number./ up to 15. In this calculation, the centrifugal distortion terms of d., and dJK were alone taken into account, because of low K transitions. Furthermore, we were not able to determine four parameters in Eq. (lc) independently, since D and E are highly correlated with Q and N, respectively. Thus, we neglected Q and N. Since the simultaneous adjustment of A, B, C, Av, dJ, dJK, D, and E by a leastsquares method led to divergence of the fitting, the E value was fixed to a certain value so as to obtain the smallest standard deviation, that was about 4.4 times smaller than the standard deviation for E = 0. In Table II, the observed and calculated frequencies are given for the species belonging to Groups (I) and (II). The spectroscopic constants thus obtained are shown in Table III. In addition to the allowed a- and c-type transitions, some of the forbidden transitions were observed due to an interaction between (+) and (-) states through the off-diagonal Hamiltonian (1~). In the case of CH,CD,CH,SH and CH&H&D$H, the 5t4 and 5, levels are in extremely near degeneracy (32 and 22 MHz for the unperturbed levels, respectively), where we label 5,, in the (+) state as 5:,, and so on. Thus, since the wavefunctions of 5:, and 5, levels are strongly mixed together by D and/or Q terms in (lc), four transitions, 5:,-4,, 5,-4&, 6,-5:,, and 6&,-5,, were able to be observed with relatively strong intensities. Furthermore, the intensities of the perturbed transitions appeared to be weaker compared to the transitions which did not suffer the influence. For example, the 5t4-4&
NAKAGAWA
332
AND HAYASHI
TABLE
III
Obtained Constants of Propyl Mercaptans (MHz) A
B
c
dj*103
Gi3CX@$H cTIQl$kI~SD CIl$+i&TI$H ~Q2~2= C1)@l&TH2SH aI~cH*cx!BH-1 Ui~cH$XcSEZ Q1$XEH2SH-1 cHQiEH2SH-2
23907.48(9) 2345.597(6) 2250.338(9) 0.22( 8) -5.7(151 22082.94(10) 2306.7X( 7) 2217.804(11) 0.21(10) -6.8(19) 19740.00(7) 2328.654(6) 2221.559(9) 0.22( 9) -4.9(14) 20007.63(71 2309.58416) 2207.319(9) 0.23( 7) -3.5(14) 20884.85(6) 2094.557(4) 2019.009(6) 0.21( 4) -2.3( 9) 21587.27(29) 2336.990(U) 2235.399(10) 0.19(12) -6.5(24) 21647.72(30) 2339.929(U) 2233.443(10) 0.21(13) -5.9(25) 21769.30(27) 2329.602(11) 2225.685(9) 0.24(10) -5.0(24) 21784.11(30)2327.638(11) 2228.657(10) 0.21(12) -5.6(24)
aQ$X$H CH&!H&!H$D
23845.24(22)2393.469(12) 2269.621(X) 0.51(14) 22769.40(15)2335.348(8) 2207.905(10) 0.47( 9)
a) b) C) d)
Aub'
dmx103
DC'
$'
1613.01(4) 1.326(24) 57.9 58.30(5) --X28.47(4) 1.193(7) 43.0 1566.09(4) 0.682(11) 47.0 1604.02(31 0.241(6) 45.5 ------------
Figures in parentheses indicate 99% reliability interval attached to last significant figures. Energy difference between the (+) and (-) states. see text. Coefficient Of (PbP,+P,Pb) term. see text. Coefficient of (PaPc+P,Pa) term. Fixed value. see text.
was much weaker than the 5,-4, transitions for these two species, while the same transitions showed approximately equal intensities for the normal and CD,CH,CH,SH species. For the SD species, there is no source from which to determine the parameters in the off-diagonal Hamiltonian, because of the lack of an accidental degeneracy. Actually, when we tried to determine the E constant, the smallest standard deviation was obtained for E = 95 MHz, that was only 1.1 times smaller than that for E = 0 MHz. Thus, we neglected all the off-diagonal parameters and the leastsquares analysis was carried out so as to fit all the observed frequencies. The differences between the observed and calculated frequencies are shown in Table II. The spectroscopic constants are given in Table III. The microwave spectra of species belonging to Group (III) can be well fitted by an ordinary rigid rotor expression with appropriate centrifugal distortion corrections because of the lack of equivalent configurations. Thus, the rotational and two centrifugal distortion constants given in Table III were obtained by a leastsquares analysis from the observed frequencies which are shown in Table IV. MICROWAVE
SPECTRA
OF THE tram ISOMER
Since the tram isomer is a slightly asymmetric prolate top molecule, the assignment of the a-type transitions would be easy if there were no overlapping in the region. Unfortunately, however, the strong u-type transitions due to the gauche isomer (in both the ground and excited states) mask some of the a-type transitions of the tram isomer. This situation makes the assignment more difficult. A rough observation shows that the intensity of the u-type transitions of the tram isomer is about 10 times weaker than that of the gauche isomer at dry ice temperature. As for the CH,CH,CH,SD species, the assignment became further difficult from the fact that a trace of the normal species was included in the sample as an impurity because of an exchange of the deuterium atom with the hydrogen atom in the waveguide cell.
MICROWAVE
SPECTRA
OF PROPYL
TABLE
Observed Frequencies transition
MERCAFTAN
333
IV
of Group (III) Species of ga&ze
Propyl Mercaptan” (MHz)
CH3CH2CHDSH-1
CHJCH2CHDSH-2
CH3CBDCH2SH-1
CH3CHDCH2SH-2
413 414 312 313 505 4404 515 I 14 514 - 413 %6 606 - 505 515 615 - 514 707 - 606 717 - 616 716 - 615
9144.42( 3043.24( 9246.43( 13715.61( 13564.64( 13863.30( 18285.52( 18085.59( 18431.91( 22853.95( 22606.23( 23114.03( 27420.17( 27126.23( 27735.74( 31383.39( 31645.74( 32356.72(
9146.311 9040.36( 9253.17( 13718.39( 13560.23( 13873.57( 18283.12( 18073.68( 18505.611 22858.16( 22533.631 23131.05( 27424.63( 27117.16( 27755.93( 31388.70( 31634.35( 32330.17(
11 9) -7) 6) 11) -1) 8) 5) 4) 13) -6) -5) 1) -2) -5) -4) -5) -8)
13664.26 t 13509.02 13821.46 18216.30 ( 18012.45 18428.06 22768.49 ( 22514.73 23034.20 27317.01 27016.31 27640.01 31863.34 31517.65 ( 32245.03 (
13667.40 4) 13520.26 7) 13817.17 4) 18221.48 12) 18026.52 6) 18422.41 3) 22773.37 22532.22 -1:; 23027.15 -31 27324.41 -1) 27037.61 -121 27631.58 1) 31872.72 1) 31542.43 -11) 32235.37 8)
212 313 414 6515 -
13143.33( 20) 18938.15( I) 18738.32( 10) 18550.63( 18256.68( -61 5) 17318.40(-10) 17537.34( -3) 17117.49( -3) 16659.31(-11) 16168.16(-12) 15645.45(-14) 15095.37( -8) 14521.57( 0) 13328.00( 13)
19201.92 19043.66 18834.23 18574.35 18267.31 17313.56 17515.92 17076.34 (16593.63 16087.34 15543.43 14371.97 14376.79 13762.11
14) 9) 7) 7) -71 -8) -6) 10) -9) 12) 12) -7) 1) 7)
11533.91( 15782.20( 13908.16( 23968.90( 27961.81( 31883.21(
11485.18 15654.71 19759.13 23732.80 27755.40 31642.23
-7) 2) -7) -2) 6) 3)
212 202 211 303 313 404 312
-
101 111 110 202 212 303 211
202 303 404 205
716 17 - 706 07 818 - 808 913 - 903 10 110-10010
5) 3) 4) 7) 9) -21 2)
1)
-3) 12) 4) -4) 3) 01 -5) -3) -4) -7)
-5) -5) 1) -5) 12) 2)
9) 9)
-2) I -3) -1) I -7) 8) -3) I-13) 3) I -1) 4) I -9) 0) 9)
13ial.a4 ( 6) 18977.31 -1) -2) 18724.0z -- ) 18077.32 8) 17688.96 -6) 17259.33 ( -2) 16791.80 (- 14) 16233.75 -3) 15756.35 14)
0)
13210.90( 17) 19015.88( 5) 18774.34( 1) 18487.66( 2) 18157.43( -4) 17785.81( 0) 17374.80(-12) 16927.23(-12) 16445.81( -8) 15333.57( -4) 15393.781 -1) 14823.34( 2) 14245.87( 18)
23548.33( -2) 27510.41( 4) 31338.67( -2)
23672.00( 5) 27665.83( -4) 31531.23( 2)
I
I
I
14004.311
a)
Figures in parentheses indicate the differences between the observed and calculated
b)
Overlapped with other lines.
frequencies to the last digid.
The 11 u-type and 15 b-type transitions were assigned for the normal species with J 5 10 and K_, 5 1. For the SD species, 11 u-type and 13 b-types were assigned. In spite of the weakness of the transitions, b-type Q-branch series showed a typical Q-branch type Stark pattern and the assignment could be done without any ambiguity. As the influence of the centrifugal distortion effect seemed to be small, the rotational constants were determined by a least-squares method from all the observed transition frequencies given in Table V with a modified rigid rotor expression which included the dJ term. The rotational constants obtained are shown in Table III. MOLECULAR
STRUCTURE
OF galrche AND zrans ISOMER
As the numbers of the isotopically substituted species sufficient to determine the rs structures for two isomers, were estimated.
measured were not plausible structures
334
NAKAGAWA
AND HAYASHI
TABLE V Observed Frequencies transition 202 212 211 303 313
-
of tram Propyl Mercaptana (MHz) CH3CH2CH2SH
CH3CH2CH2SD
932_5;7;,( 10)
9085.94(
312
-
404 414
-
101 111 110 202 212 211 303 313
505 413 515 514
-
404 312 414 413
23304.49( 18898.76( -1) -9) -- b) 23622.43( -3)
-18426.6~/ -- b) 23032.06(
616 606 615 707 717 716
-
515 505 514 606 616 615
2795_9:3;,( -9) 28345.37( 4) 3261_0;9;,( -6)
27238.3~! -27636.62( 31768.88( 31348.40( 32240.26(
9449.92( -9) 13981;1;,( 10) 3) 1417_4y3/ -- b)
33067.37(
8)
4)
13627.3&( -11 -13820.57( 7) 1816!;9;)( -4)
2, 4) l) -2) -1) 2) 01
211 312 413 514 615 716 817 g18 1019
- 202 303 404 505 606 707 808 909 - 10010
21700.01( 1) 21887.52( -6) 22139.60( 5) 22457.60( 10) 22843.42( -2) 23299.73( 2) 23829.14( 8) 24434.59( 1) 25119.57(-12)
-- b) 20882.75( 5) 21142.36( 3) 21470.23( 3) 21868.56( 4) 22339.92( 3) 22887.41( 4) 23514.34( -3) 24224.60( -6)
212 313 707 808 909 10010
-
30654.06( 35131.52( 12368.30( 17426.56( 22531.23( 27678.29(
-- b) 33745.25(-12) 12576.54( 1) 17523.42( 2) 22516.58( -4) 27552.09( 1)
a) b)
101 202 616 717 818 919
-2) -4) 2) -3) 6) -2)
Figures in parentheses indicate differences between the observed and,calculated frequencies to the last digid. Overlapped mth theotherstrong lines.
gauche Isomer Since the microwave spectra of seven isotopic species were assigned for the isomer, a fairly reliable structure could be estimated from the observed moment of inertia. As the first trial, the rs structural parameters of gauche ethyl mercaptan (I) and propane (12) were transferred. Usually, when the rs structure is well determined, the differences between the observed and calculated moments of inertia are small positive values and approximately equal for all the measured species. Then, taking this fact into account, some of the parameters in the skeletal part were so adjusted as to reproduce the observed moments of inertia. The fitted and transferred parameters are shown in Table VI, and the observed and calculated moments of inertia are given in Table VII. As seen from the Tables VI and VII, it appeared that only two adjusted parameters were sufficient to reproduce the moments of inertia and almost all the parameters could be fixed to the corresponding rs structural parameters of gauche ethyl mercaptan and propane. The adjusted parameters are the C-C bond length gauche
MICROWAVE
SPECTRA
OF PROPYL
TABLE Structure
Parameters
of gauche
CH3C
r(C-C) r(C-H) a(~-C-H) a(H-C-C)
CCH2C
r(C-CH2) r(C-H) ci(C-C-C) a(H3C-C-S) a(H-C-CH2) CL(H-C-H) ‘I
C-CH2-S e,
(C-S)
r(C-H) (u(C-C-S) a(H1-C-S) u(H2-C-S) a(H1-C-C) a(H2-C-C 'u(Hl-C-H 1 ) a(C-S-H)
Dihedral Angle r(CCSH) a) b) C) d) e) f)
335
VI
and trans Isomers
{r(S-HI
SH
MERCAPTAN
of Propyl Mercaptana
gauche
trans
,'I;;; :; 107056'C) lll~oo'c)
;:;;4" f; 107'56") lll~OO'c)
1.528
1.529 1.092
:i~Z~qvb) 108°22'd' 110037' 106'55'
;;;:$
,, .
110035' 108'07'.
1.814 1.089 113037' 104053' 109'16' 1 110'42' llla19' 1 106'35'
1.820 1.090 108'34'
1.336 96O 0'
1.336 f, 96'13'
109'26' 110014' 108054'
61445'
180'
Parameters without any notice were taken from the rs structures of corresponding isomers of ethyl mercaptan. Parameters fitted to the moments of inertia obtained. Taken from the rs structure of propane with an assumption of cylindrical symmetry along the C-C bond. Calculated from u(C-C-C), a(H-C-CH2) and cr(H-C-H). The symbols Hl and H2, denote the hydrogen atoms which situate the same and opposite direction to the hydrogen atom of the thiol group with respect to the C-C-C-S plane. Taken from the structure of the gauche isomer.
in the CH,-CH, part and the C-C-C angle, which are 0.01 A longer and 40’ smaller than the corresponding Y, parameters of propane, respectively. Pans Isomer
Because of the weakness of the spectra of trans isomer, we tried to assign two species, the normal and SD species. Thus, since the r,, structure of this isomer is less reliable than that of the gauche isomer, the structural parameters were transferred from the rS structure of trans ethyl mercaptan (13) and the corresponding parameters of the gauche isomer. Then, the observed and calculated moments of inertia were compared. The resultant parameters are shown in Table VI. The calculated moments of inertia agree with the observed ones as shown in Table VII. DIPOLE
MOMENT
AND STARK
EFFECT
The dipole moments and their components were determined from the Stark effect measurements of several transitions with J S 3 for the gauche CH3CH2CH2SH, CHJZHJH.$D, and trans CH,CH,CH,SH species. The electric field was calibrated using the OCS molecule as a standard (14).
336
NAKAGAWA
AND HAYASHI
TABLE VII Observed and Calculated Moments of Inertia of Propyl Mercaptan (amu. A’)
(AI,) a1
1, gauche
CH3CH2CH2SH CH,CH.,CH?SD CH;CH;CD;SH CH3CD2CH2SH CD3CH2CH2SH CH3CHZCHDSH-1 CH3CH2CHDSH-2 CH3CHDCH2SH-1 CH$HDCH*SH-2 tram
(AI,)=)
1,
21.1388 22.8854 25.6016 25.2592 24.1982 23.4108 23.3455 23.2151 23.1993
21.1940 22.1954
(0.0503) (0.0380) (0.0368) (0.0546) (0.0435) (0.0631) (0.0225) (0.0690) (0.0332)
215.4573 219.0890 217.0249 218.8169 '241.2806 216.2508 215.9792 216.9366 217.1197
(0.4862) (0.5637) (0.4807) (0.3234) (0.4127) (0.5019) (0.4541) (0.4965) (0.2967)
224.5778 227.8723 227.4871 228.9547 250.3089 226.0787 226.2766 227.0654 226.7626
0.3742) 0.4524) 0.31301 0.2190) 0.2886) 0.3481) 0.3691) 0.3150) 0.3127)
(0.0425) (0.0515)
211.1479 216.4029
(0.3401) (0.2744)
226.6698 228.8939
(0.1810) (0.0921)
isomer
CH3CH2CH2SH CH3CH2CH2SD a)
(AIb)='
Ib
isomer
AIg=(Iglobs -(Ig&lc
I
where
(Ig)calc
is calculated
from the structure
in TableVI.
Though Ohashi e? al. (6) have determined the dipole moment of the gauche isomer, their calculation was based on the assumed a rotational constant, which was about 500 MHz smaller than ours, and without any considerations of the torsional splitting. These facts make a fairly large discrepancy, especially for the c-dipole component. Since no electronic plane of symmetry exists in the gauche isomer, a-, b-, and c-dipole components are expected. The connection through a- and b-dipole components is within the same state, i.e., (+)-(+), or (-)-(-), whereas the c-dipole connects (+) and (-) states. Taking this fact into account, the Stark coefficients were calculated by second-order perturbation theory from the observed energy levels and the transition probabilities which were assumed to be the same as those of the rigid rotor model. For the normal species, the 3& level is nearly degenerate with 4, level (1905 MHz apart), while the separation between the 3, and 40’4levels is 5130 MHz. Thus, the c-components of the Stark coefficients concerned with 3,2 levels are not the same for the (+) and (-) states. For example, that of 3:,-2& (M = 0) is 0.860 (in lop5 MHz~D-~~V-~~~~~), which is 4 times bigger than that of 3,2, (M = 0). The other coefficients which are not concerned with the 3r2 levels are approximately equal within the experimental error limit. Actually, the Stark lobes of the 3,,-2,, transition for each M quantum number separate into two components due to the (+) and (-) states at high voltage, while the other transitions do not show any splittings. The dipole moments given in Table VIII were determined by a weighted least-squares fit, in which the weights were taken as to be proportional to the square inverses of the standard deviations of the Stark effect measurements. For the tram isomer, the c-dipole component is identically equal to zero because of the symmetry. The obtained dipole moment and components are; /& = 1.179 +- 0.018, ,.‘,, = 1.078 -e 0.075 and ptotal = 1.598 + 0.054 D. From the dipole components obtained, the direction of the dipole moment in the molecule was determined. Since the deuterated substitution rotates the inertia
MICROWAVE
SPECTRA
OF PROPYL
TABLE
MERCAPTAN
337
VIII
Dipole Moments and Stark Coefficients of Propyl Mercaptan= gaucheiscxnx cwx!~2~ Sta& coefficient(Av/E')~) absd. c&XI. lMtparity ____calcd~ 303 - 203 2 ti 0.659( 5) 0.634 1 -H -0.126( 4) -0.127 0 -H -0.391( 5) -0.390 312 - 211 0 ++ 0.193( 2) 0.195 -_ -0.024( 1) -0.029I 313 - 212 0 t+ -0.091( 2) -0.091 211 - 110 0 -H 1.692( 4) 1.680 202 - lo1 1 3 1.737( 5) 1.733 0 ti -2.085( 7) -2.119 212 - 111 0 ++ 1.723( 8) 1.720 313 - 303 3 -+ -6.581(66) -6.570 + -6.248(164)-6.520
transi%CWW"
Ctl3CV2M2SD c&d. 0.661( 2) 0.661 -0.132( 1) -0.139 -0.409( 5) -0.405 0.192
0.189
-0.091 -0.092 1.715 1.697 1.738( 91 1.742 -2.126(12) -2.095 1.752(23) 1.727 -6.895(145) -6.928 -6.895020) -6.928
IMl 303 - 202 0 1 2 312 - 211 1
312 - 303 3 2
cbsd._ 0.391( 7) -0.160( 6) -0.353(27) -2.299(24)
calal.
4.066(81) 2.054(91)
3.943 1.960
0.386 -0.168 -0.352 -2.323
Dipolewnsnt K&ye)
u.3
1.575(7) 0.115(82) 0.580(9) 1.68300)
!'b UC IUT I a) b)
1.566(11) 0.067(40) 0.594(13) 1.6771ll)
1.179v3) 1.078(75) Y.598154)
Figures in parentheses indicate the uncertainties calculated from 2.5 times standard deviations attached to the last significant figures. in 10s5 MHz (V/cm12 unit.
axes considerably for the gauche isomer, the direction was uniquely determined from the present measurement. In our coordinate system of the molecule, the signs of ,..&b and & are the same as and opposite to that of pa component, respectively. Taking this fact into account, it is concluded that the dipole moment for the normal species makes angles of 24”03’ and 2422’ with the C-S bond and the bisector of C-S-H angle, and it is almost in the C-S-H plane (the angle between the dipole moment and the plane is 3”14’ + 2”25’). Since the dipole moment of the tram isomer was determined for only the parent species, the direction could not be uniquely determined from the experimental data alone. There are two possible directions from the experimental data, direction (A) makes an angle of 17”53’ with the C-S bond in the apex of C-S-H, and direction (B) consists of the angle of 67”Ol’ with the C-S bond out of the apex. However, comparison with related molecules makes it possible to determine the direction. Taking consideration of the fact that the angles between the dipole moments and C-S bonds are 29”45’ and 19”20’ for methyl mercaptan (15) and tram ethyl mercaptan (13), it appears that direction (A) is more reasonable than direction (B). The dipole moments and their directions are compared with those of other normal alkyl mercaptans in Table IX. The amounts of the dipole moment in the tram CH,SH, CH,CH,SH, and CH3CH2CH2SH series and the gauche CH,CH,SH, CH$H,CH$H series show a quite regular increase upon the substitution of the smaller alkyl group by a bigger one. The direction of the dipole moment becomes more parallel to the C-S bond as the alkyl group becomes bigger. ENERGY
SEPARATION
BETWEEN
gauche AND truns ISOMERS
The energy separation of the two isomers was estimated from the relative intensity measurement at dry ice (195 K) and room (298 K) temperatures. A pair
NAKAGAWA
338
AND HAYASHI
TABLE IX Comparison of Dipole Moments and Directions in n-Alkyl Mercaptans u(Debye)
trans
CH3SH d) CH3CH2SH e, CHJCH2CH2SiI f, CH$H2SH g) CH3CH2CH2SH f,
gauche
a) b)
C) d) e) f) 9)
The angle between The angle between The angle between Ref. (i4). Ref. (13). Present study. Ref. (2).
a(uXC-S)
moment moment moment
b)
a(uxbis)
29045' 19020' 17053' 25000' 24"03'
1.532 1.560 1.598 1.61 1.683
the dipole the dipole the diDok
a)
u(vxC-S-H) C)
18"30' 28"46' 30013' 23"27' 24"22'
0 0 0 2056 3014'
and the C-S bond. and the bisector of a(C-S-H). and the C-S-H olane.
of transitions of f-303-202 (13 987.18 MHz) and g-3&-2& (13 930.26 MHz) was chosen, because these lines exist in a nearly equal frequency region and are well isolated from other transitions. The ratios Z(g)/Z(t) obtained were 9.32 k 0.50 and 5.84 2 0.56, at dry ice and room temperatures, respectively. After suitable corrections for the dipole moments, the line strengths and the partition functions, the energy separation AE = E(trans) - E(gauche) was determined to be 146 and 126 cm-‘, at dry ice and room temperatures. Thus, we obtained the average of 136 k 40 cm-’ for the energy separation. This value is very similar to that of ethyl mercaptan reported by Schmidt and Quade (Z), 142 cm-‘. POTENTIAL
FUNCTION
OF THIOL INTERNAL
ROTATION
Assuming a one-dimensional Hamiltonian for the thiol internal rotation, the potential function was determined from the observed (+)-(-) splittings of the normal and SD species and the energy difference of the two isomers of the normal species. The one-dimensional Hamiltonian is written as a Fourier series of the TABLE X Result for the Thiol Internal Rotation
Barrier "1 "2
-207(67) -386(27) 1305(20)
normal 10.501 -0.021 0.016 diff.
go- - go+ to - go,
a) b) C)
-207(assumed) -353( 6) 1310( 9)
-
(cm-l)
V,,(O) L_(l) L.,(2) Energy
CH3CH2SH a)
(cal/mol)"
v3
Kinetic
CH3CH2CH2SR
Normal 10.870 -0.011 0.020
SD 5.857 -0.040 0.004
0.046 0.006
(cm-l)') 0.05380( 136_+40(
-1) 0)
0.00194(
12)
0.05851( -31 142(-21)
0.00234( 20) 138(-90)
Ref. (1). Figures in parentheses indicate the 2.5 times standard deviations. Figures in parentheses indicate the deviations of the observed values the calculated ones attached to the last significant figures.
from
MICROWAVE
-
-
SPECTRA OF PROPYL MERCAFTAN
339
CH$H&H2SH
---- CH$H$3i$SD O180”
SH Dihedral
90”
Argle
62”
FIG. 1. Potential curve, energy levels, and transition frequencies
of thiol internal rotation.
internal rotation angle (Yas Hint = Z:l*‘n’(pzCOS nzQI+ COS flap:)+ ZV,/2(1 - COS flcY),
(2)
where the symbols have their usual meanings. For a molecule having an asymmetric internal rotor, it is necessary to take at least the first three Fourier coefficients in the potential, in contrast with the internal rotation of the threefold symmetric rotor like CH3 and SiHS groups. Unfortunately, since there are not sufficient experimental data available, we could not determine the first three coefficients independently. Thus, a least-squares analysis was carried out taking the V, and V, as parameters with the V, value fixed to that in ethyl mercaptan (I). The first three coefficients of the kinetic part in Eq. (2) were calculated from the structure listed in Table VII taking consideration of the tilt angle against the C-S bond in a manner similar to that in the previous analysis (Z). The results are shown in Table X, and the potential function and the energy levels of the normal and SD species are shown in Fig. 1. It is of some interest to compare the potential functions of the thiol internal rotation of propyl and ethyl mercaptans, though the comparison of potential functions for asymmetric rotors is more ambiguous than for symmetric rotors. This is because the potential function and the structure cannot be determined near the top of the potential, but are defined at near the equilibrium position of the molecule. For a symmetric rotor, we do not have to compare the whole of the potential but near the bottom. For an asymmetric rotor, however, the comparison should be made as a whole potential function, because there are two or more nonequivalent potential minima. Nevertheless, it is worthwhile to examine the transferability of the potential functions. As shown in Table X, V3 of propyl mercaptan is 1310 2 9 calimol, which is nearly equal to that of ethyl mercaptan, 1305 -+ 20 cal/mol. Also the agreement of the V2 values is very good. From the present result, it is concluded that the thiol internal rotation has a good transferability between ethyl and propyl mercaptans. RECEIVED: March 10, 1980
340
NAKAGAWA
AND HAYASHI
REFERENCES J. NAKAGAWA, K. KUWADA, AND M. HAYASHI, Bull. Chem. Sot. Japan 49, 3420-3432 (1976). R. E. SCHMIDT AND C. R. QUADE, J. Chem. Phys. 62, 3864-3874 (1975). J. H. GRIFFITHS AND J. E. BOGGS, J. Mol. Spectrosc. 56, 257-269 (1975). K. BOLTON AND J. SHERIDAN, Spectrochim. Acru A 26, 1001-1006 (1970); F. SCAPPINI, H. MADER, AND J. SHERIDAN, Z. Nafurforsch. A 28, 77-81 (1973). 5. F. INAGAKI, I. HARADA, AND T. SHIMANOLJCHI,.I. Mol. Spectrosc. 46, 381-396 (1973). 6. 0. OHASHI, M. OHNISHI, A. TAGUI, T. SAKAIZUMI, AND 1. YAMAGUCHI. Bull. Chem. Sot. Jmpan 50, 1749- 1753 (1977). 7. A. MURRAY III AND D. L. WILLIAMS, “Organic Syntheses with Isotopes,” Part II, p. 1341, Interscience, New York, 1958.
1. 2. 3. 4.
8. Reference (7) p. 1340. 9. A. STREITWIESER, JR., J. Amer.
Chem. Sot. 75, 5014-5018
(1953).
10. M. S. KHARASCH, M. C. MCNAB, AND F. R. MAYO, J. Amer. Chem. Sot. 55, 2531-2533 II. 12. IS. 14. 15.
J. NAKAGAWA AND M. HAYASHI, Chem. Lett., 1321-1322 (1979). D. R. LIDE, JR., J. Chem. Phys. 33, 1514-1518 (1961). M. HAYASHI, M. IMAISHI, AND K. KUWADA, Bull. Chem. Sot. Japan, J. S. MUENTER, J. Chem. Phys. 48, 4544-4547 (1968). T. KOJIMA, J. Phys. Sot. Japan IS, 1284-1291 (1960).
2382-2388
(1933).
(1974).