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Microwave spectrum of deutero propargyl mercaptan HCCCH2SD
JOUKNAI.
OP
MOLECULAR
Microwave
SPECTROSCOPY 63, 509-520
Spectrum
investigation .I. M.
of Deutero
of the Internal MIRRI,
(1976)
F.
Propargyl
Rotation
SCAPPINI,
R.
Mercaptan
in the -SH
and -SD
CERVELLATI,
AND
HCCCH,SD
Isotopic
Species
P. G. FAWRO
The microwave spectrum of the deuterated propargyl mercaptan was recorded and measurements in the torsional levels 0’ and l* are reported. The internal rotation analysis is carried further using the new data obtained and a more specific Hamiltonian. With the mean values of the rotational constants for the two isotopic species in the states O”, a ro-structure is proposed.
IKTKODUCTIOS Preliminary investigations on the gauche configuration of propargyl mercaptan have been carried out by Bolton and Sheridan (I) who measured the microwave spectrum of the normal isotopic species, including some a- and b-type close doublets due to the pure rotational transitions in the Of and O- torsional sublevels and some c-type torsiorotational transitions of the series Jl~(0’) +- Jo$(O*). No assignment of the individual components of the doublets was attempted b\. Bolton and Sheridan. Scappini, Mgder, and Sheridan (2) reported in a more recent paper a list of new lines (Jf the normal species and some a-type lines of the -SD isotopic species. Moreover, the!used a torsio-rotational Hamiltonian based on the symmetric internal top approsimation. The experimental torsional splitting A(O*) = 6891.76 MHz and the splittings of the rotational doublets were fitted in order to obtain information on the torsional potential function, which was expressed as a truncated Fourier expansion of the t!‘pe c (lT,li2)(1 %=I
- co:, nu).
They obtained a Vcis potential barrier of 470 f 10 cm-’ for the normal species, but the assignment of some b-type doublets, indirectly checked through the fitting process, was found to be incorrect by Scappini, Favero, and Cervellati (3). who checked several rotational doublets by MW-MW double resonance experiments. In the present work we continue the torsio-rotational analysis on the parent molecule and estend it also to the -SD isotopic species. In the case of the -SD species, some h-rotational and c-torsio-rotational transitions in the ground O* and first excited li torsional levels were also identified in order to obtain further information on the molecular structure and on the internal rotation. so9 ('opyright @ 1976 hy Academic Press. Inc. .\I1 rightnf relnKJuction in an\-form reserved.
MIRRI
510
ET AL.
The analysis of the internal rotation for both isotopic species was carried out with an effective Hamiltonian of the type used by Meakin, Harris, and Hirota (4), modified by the use of an instantaneous principal axis system which leads, at least in this case, to much better results than the Hamiltonian of Ref. (2). EXPERIMENTAL
-SD propargyl mercaptan was prepared sulphide obtained in the following way:
DETAILS
as in Ref.
(5) using potassium
deutero-
K + DzO--t KOD + +Dz, Al&
+ 6DzO + 3D& + 2Al(OD)s,
KOD + D2S -+ KSD + DzO. H--CC-CHZSD was subsequently purified by vacuum distillation and the amount of deuteration checked by NMR spectra. After proper conditioning of the microwave absorption cell, the intensity of the lines was found to be almost twice the intensity obtained in the case of deuteration of the normal species with DzO directly into the cell. The microwave spectra were recorded with a HP-MRR 8400 C-type spectrometer in the range 840 GHz. All measurements were made at dry-ice temperature. The presence of transitions of the normal species, which could not be eliminated, and of excited torsional and vibrational states with energies on the order of 100-200 cm-’ for both isotopic species, gives rise to very crowded spectra. Only the lines that had been checked by MW-MW double resonance technique as described in Ref. (3) were therefore used in the final least-squares fitting calculations. Large discrepancies between experimental and calculated effective molecular constants and large r.m.s.d. are in fact obtained if all the lines which were measured and seemed at first to be correctly assigned, are included in the calculations.
0’ FIG.
1. Double resonance map for HCCCH8D.
0The drawing is not in scale.
TORSION
ROTATION
SPECTRUM
OF HCCCHsSD
13500
FIG. -7. Torsion-rotation
transitions
in HCCCH&D
showing
5 11
MHz
the handhead
close to 13 250 MHz
From preliminary calculations the torsional splittings of the deuterated species were predicted to be A(O*) = 150-250 MHz and A(l*) = 11000-13 000 MHz. To obtain A(O*) the MW-MW double resonance technique was used, aiming at the identification of a number of c-type transitions connecting the 0+ and the O- torsional sublevels. Figure 1 shows a map of double resonance connections involving the observed c-type transitions pumped by the 2& + lol* a-t>Fe unresolved doublet. To obtain A(l*) several low-resolution spectra in the range of 8000-18 000 MHz were recorded in search of the torsio-rotational bandhead due to the J~J’ +-Jo.,* c-type transitions. A characteristic pattern of lines was found to converge to a bandhead at approximately 13 250 MHz (see Fig. 2), the convergency being due to the rotational contribution which tends to zero as J increases. The bandhead on the high-frequency. side is due to the relative signs of the rotational and centrifugal contributions. A similar
512
MIRRI
EET ;1L.
pattern was observed for the same kind of transitions in the ground state of the normal species (I). The results concerning the torsional splittings obtained as described above are to be found in Table VI. STRUCTURE
The mean values of the rotational constants corresponding to the ground-state torsional sublevels for the two isotopic species, listed in Table IV, were used in leastsquares fitting calculations in order to obtain information on the structural parameters. In Table I the results of such calculations are given. Structure I was obtained by fitting four parameters and assuming all the others, the choice of the parameters being based on their effect on the potential function determination. The --SH bond length is considerably shorter than in methyl mercaptan (7); therefore, a second calculation assuming the same --SH bond length as in CHsSH was performed, obtaining Structure II. While (SCC is practically the same in the two cases, considerably different values are obtained for (CSH and as. Structure I seems to be more reliable if the differences between experimental and calculated rotational constants are considered and also on the basis of the discrepancies between the experimental and calculated values of all the other molecular constants, as will be discussed later. The SH bond moment was evaluated to be 0.79 D in propargyl mercaptan compared TABLE Bond Lengths
in Angstroms, Bond Angles in Degrees, Differences between Calculated Rotational Constants in Megahertz
structure (AexP-I 6 exp
-2
(5exP-c (XexP-i; CC exP-F
talc talc talc ca1c
ca1c
)
structure
I
)H
-5.2
-3.2
-5.6
-11.7
)
)
H D
II
0.01
0.05
2.9
7.1
3.1
-8.4
1.301 + 0.015
(CCC
113.02
+ 0.01
(CSH c
104.3
+2
119.8
+3
a) acr;'~med as in CH,SH
[I]
as in propargyl-chloriUe
and
II
-O-C5
)
ag
c1.33aa 113.01
(HCC
(111.5)'
CH
(1.091)a
C-C
(,.460)b
EC:
(,.207jb
SC-H
(,.057jb
C-S
(1.81Yja
+ 0.01
98
+1
124
+3
[S]
1s the intercal rotation angle measured g starting fron be trans configuratron.
c) a
Experimental
-0.01
H
SH
b) &xmmed
I
at the
gauche
configuration
ana
computed
TORSION ROTATION
SPECTRUM
OF HCCCH,SD
31 3
to 0.71 D in HzS and 1.10 D in CHSSH (8). Correspondingly, the bond lengths are 1.301 (from structure I), 1.323, and 1.336 A. Assuming an electron-donor inductive effect from the group attached to --SH as in Ref. (9) the sequence of the ratios (0.71i1.323) < (0.79/1.301) < (1.10,/1.336) results as expected, and the value of 1.301 D for the SH bond length in propargyl mercaptan does not contradict the results obtained in Ref. (8) and the assumption suggested in Ref. (9). TORSIO-ROTATIONAL
H.VMTLTO?r’L4N
,4 semirigid molecular model with only one degree of internal freedom was assumed in order to fit all the experimental data. The first approach was based on the torsiorotational Hamiltonian used by Scappini, Mader, and Sheridan (3) in the approximation of the symmetric internal rotor. The molecular constants F, pa, and Pa, which appear in their Hamiltonian, were precalculated from Structure I. The computation program described in Ref. (2) was used in order to fit to the rotational a- and b-type doublets, which had been checked by. MW-MW double resonance, and to the torsional splittings for each isotopic species separately, the potential constants Vq and Vs in the Fourier espansion
1’ = 2 (C’,/2) (1 - cos Ita) ,i=1 for (Ye= 120”, and the small differences of rotational constants corresponding to each torsional sublevel. We were not able to fit the present data reasonably with this Hamiltonian, which in this case is perhaps too drastically constrained to precalculated values of all the molecular constants involved in the expression of the simplified torsio-rotational interaction. Some considerably more perturbed transitions (compared to those used in Ref. (Z)] were in fact included in the least-squares fitting, that is the &,* + &I*, 6r6* - L*, and 606~ +- Sn,* doublets. The effective rotational Hamiltonian relative to the 0+ and O- torsional sublevels, as described by Meakin, Harris, and Hirota (4), was subsequently used. Such a Hamiltonian gives rise to matrix elements connecting the two torsional sublevels and has the following schematic form : 0+
ot
AfPa2 + B+Pb” +
C+P,2
o-
---
I &aP, +
&bh
+ iL(P,P,
+ P,Pd
i- iRbe(PbPc + O-
Details The A(O*) b-type of the
C.C.
!
pcpb)
A(O*) + A-P,* + B-P,” + C-P,” _____
are given in the papers by Hirota (10) and Schwoch and Rudolph (II). molecular constants A+, B+, C+, A-, B-, C-, an, 06, and the torsional splitting (for the -SD isotopic species) were determined by an empirical fit to the a- and transition frequencies and to some torsio-rotational c-type transition frequencies due to the high correlation two isotopic species. R,, an d Rbc were precalculated,
MIRRI ET AL.
514
of these constants to the rotational constants. The computation program which carries out the diagonalization of the effective Hamiltonian and the least-squares fitting was that already used for hydroxyacetonitrile (6). DJ and DJK were also included with assumed values of the order of magnitude as in molecules of the same type (6). The values of all these constants were finally compared with a second computation program which calculates the eigenvalues of the torsional Hamiltonian, given by Eq. (9) in Ref. (4), and the molecular constants of the effective Hamiltonian, that is R,,, Rbc, & &t,, and the effective rotational constants. Two different approaches were followed for the expression of the a-dependent parameters which were used in the computation of the effective molecular constants. The first approach is based on the use of a system of axes as defined by Quade and Lin (12) and it will be called a frame fixed system (FFS), although only its orientation is fixed with respect to the frame during the internal rotation. The second approach is based on the use of an instantaneous principal axis system (IPS). In this system, the vector position ri of each atom is a complicated function of LYin the kinetic energy expression 2T = g mi(ii + o X ri)‘, i-l ki = (drJdol)& if it is assumed that a! is the only internal With the notations
xi = dxqfdol, the following expression
Yi = dyi/da,
of the kinetic
degree of freedom.
zi =
&,/da,
energy is easily obtained:
2T = Ab2 + 2&(w,5 + C+,TJ + CO&)+ US-IO, where A = c mi(Xz
i- =
c
+ Yi2 + Z,“),
rni(YiXi -
Xbi),
and I is the diagonal instantaneous inertial tensor. If P, = dT/th, and p = aT/d& are evaluated, then
(1)
TORSION
that is
SPECTRUM
5 15
OF HCCCHzSD
pzpz I,, (P/A> -&/A> wz -h'A> p,(2) WY 7 C.C. Pz -pz I;, I[ Ii1 Pu
i
ROTATION
=
I,, -
(~‘/a)
- CWA)
({“/‘A)
where P, = (E/Q, P, = (v/A)P, Pz = (l/U. By substitution of d! = (l/A)@ - w,i - wy~ - wJ)
wz
in (1) and making
use of (21,
2T = C (Pi - pi)Lc+(Pj- pi) f (l/A>p”, i.2
where pij is an element of the inverse of I’ and I’ is the effective inertial in (2). The Hamiltonian
operator
The off-diagonal
terms
abbreviated
Ha + H RT can now be written
of I’ are due only
to torsio-rotational
tensor appearing
in the usual form coupling
(4).
terms.
In an
form,
Observed and Calculated Frequencies, in Megahertz, for the O+and O- Torsional States of HCCCHSH 0+ Observed frequency
calculated
freouency
0ohs.-talc.
Observed
frequency
camiatea
10,*00
a)
5 803.24
- 0.06
5 802.83
l,,%O
al
25
103.54
25 103.03
0.51
25 104.85
25
5 803.18
obso-calo.
frequency 5 802.57 104.91
0.26 -
0.06
l,Q-lO*
a)
19 588.15
19 588.36
0.39
19 589.86
19 590.01
- 0.15
2x1~110
d
71 894.98
11 895.51
- 0.53
11 893.94
lt 893.50
0.44
2,2-1,1
a)
11 317.92
11 318.40
- 0.48
11 318.22
11 318.19
0.03
202-10,
@.)
11 603.18
11 603.23
- 0.05
11 602.27
11 601.91
0.36
2,,-10%
d
30 618.50
30 618.18
0.32
30 619.89
30 620.54
- Oh65
2,x-202
8)
19 880.54
19 880.64
- 0.10
19 881e74
19
881.60
0.14
301-202
a)
17 396.65
17 396.71
- 0.06
17 395.39
17 394.80
0.59
31s~201
a)
35 990.46
35 990.36
0.10
35 992.66
35 993.99
- 1.33
311-303
a)
20 324.02
20 324.81
- 0.79
20 325.10
20 324.98
0.12
23 180.38
23
- 0.05
23
178e78
23 178.02
0.76
20 927.54
20 927.27
0.27
28 284,13 28 948.38
- 0.47 0.86
40r-305 415-404
a) a)
180.43
-_
--
51s'414
28 280.74
28 280.94
- 0.20
28 283.66
501-401
28 951.21
28 951.19
- 0.02
28 949.24
508’4x4
10 908.17
10 908.14
0.03
10 895.25
10 896.71
- la46
6,,-515
33 927.85
33 927.99
- 0.14
33 950.97
33 951.05
- 0.08
6~~~505
34 705.94
34 705.89
0.05
34 703.74
34 702.79
0.95
6c.,-51~
17 333.49
17 333.09
0.40
17 315.18
17 315.38
- 0.20
a) measurements from Ref. [pl and [Jl.
516
MlRRI
ET
AL.
a! is step-wise changed in the FFS, starting from cr = 0 as defined in Ref. (4), and the inertial tensor is diagonalized for each value of (Yin order to obtain I(a), TV, and fi(a). The same kind of procedure as that used for the FFS has then been followed in order to e?tpress the a-dependent parameters as series expansions in terms of cos(ncr) or sin(na) according to the symmetry of each parameter with respect to Q (4, 10). We found that the convergency of the Fourier expansions is slower in this system, but this disadvantage is compensated by the much smaller values of the effective molecular constants Rat, (4, IO, II), Ii,,, Rb,, aa, and &b and therefore by the smaller absolute uncertainty on the precalculated values of those constants which cannot be obtained by the fitting pracess, as R,,, Rbc, and Rat,. ANALYSIS
OF THE
TORSIO-ROTATIONAL
SPECTRA
The least-squares fitting of the rotational transitions for the normal species and of the torsio-rotational spectrum for the deuterated species, carried out with R,, and Rbc precalculated in both systems of axes, gives the same equally good results, as far as the deviations of the calculated from the observed transition frequencies are concerned. TABLE Observed
Transition
2OZ’lOl 30s~202 31z-211 313-202
El) a) a,b) d a,b)
III
and Calculated Frequencies, in Megahertz, Torsional states of HCCCHISD Observed
frequency
for the Of and O-
Calculated frequency
ohs.-talc.
11 457.87
11 457.97
-0.10
17 178.30
17 178.48
-0.18
17 610.66
17 610.81
-0.15
34 019.94
34 020.06
-0.12
888.85
22 888.84
0.01
22 931.40
22 931.19
0.21
22 920.80
22 920.74
0.06
22 953.95
22 953.89
0.06
2:, -1:,
28
28
709.08
0.16
2,,-101
28 708038
28 708-35
0.03
1:0-l:,
17 807.90
17 807.84
0.06
1,0-l ;I
17 808.80
17 808.74
0.06
4or-303 4,x-330 421-322 422-321
a,b) a,b) a,b)
22
709.24
17 917.80
17 917.76
17 139.71
17
139-73
-0.02
29 Y3Y.06
29 939.17
-0.11
29
29
-0.03
163.11
163.14
0.04
17 638.04
17 638.90
-0.06
16 862.60
17 862.64
-0.04
36 091.98
36 091.94
0.04
35 316.03
35 316.03
0.00
a) unsplit b) measurements from Ref.
[PI.
TORSION
ROTATION
SPECTRUM
TABLE Molecular
Constants
IV
from Least-Squares
Tables
II and
III
OF HCCCHzSD
Fittings
of the Data
in
(in Megahertz)
H-EC-CH,-SH A+
22 35Oa4b + U,j3
i)+
3 045.98 If. O.U4
c+ :.-
22 342.15 + O.jL
2 151.32 + O.cl4
I:-CIC-CH,-Sj
2li534~23 + 3
0.11
ciO5.67+ 0 02
2 725.62 + (,C5 2"
533~25 + i lb
5
3 045.43 + G.L5
: L05,64 + b.L3
_)-
2 757.44 + o,v:
2 725.Uj + L'.Lj
336.1 + 2.
14.7 + 0.s
-4ti.6 + :.
_d
(-12.2j2 (0.41a (0.002)" (-0.05)O (6891.76)'
+2i
(-13.)x
ii.JJa (o.oo2)” (-0.03)" 358.5 + 0.1
4 Calculatedfrom structure
I in the IPS b) Assumed from simlar molecules (see ref. [gj) C) From ref. [?I.
However, the discrepancies between the experimental and calculated values of the other barrier dependent parameters, that is AA = A+ - A-, AB = B+ - B-, AC = C+ - C-. &, and 06, are much larger in the case of FFS. In the IPS the uncertainties on the calculated values of all the molecular constants are perhaps smaller due to their small values. In particular the uncertainty is considerably reduced in the process of diagonalization of the 0+ and O- blocks due to the negligible rotation of axes associated with this diagonalization (4, 10, 11). We found, in fact, Rrrb* 10 MHz, while in the case of the FFS R,h cx 9000 MHz. The observed and calculated rotational and torsio-rotational transition frequencies used in the fitting process for the normal and deuterated species, respectively, arc reported in Tables II and III. In Table IV, the experimental molecular constants for the two isotopic species arc listed. We tried to fit to the torsional splittings, A(O*) = 6891.76 MHz for the normal species, A(O*) = 388.5 MHz and A(l*) = 13 250 MHz for the deuterated species, C-2and 113,for different values of (Ye,but no reasonable fitting could be obtained. Since a considerable interaction could be possible between the torsional motion and some other low-energy vibrations, as the -C=C-H bending mode, and such interaction should principally affect the first excited torsional level, we tried to fit only the two ground-state splittings. The trend of the results obtained by changing T/Z and L’s, shows that the fitting would be possible only for I/itruns < Vllauche, while this possibility seems to be excluded by the experimental results.
518
MIRRI
ET AL.
TABLE Barrier
Dependent
Constants for HC= C-CH&H (IPS VI, VZ, V3 in cm-‘; Other Constants in Megahertzabe
Experimental
8.35 f
0.60
AB
0.55 f
0.09
AC
&a2 + Q*” VT
System) ; VT,Vcis,
Calculated with Structure I
AA
;:
V
Molecular
-0.12
&
0.08
-48.6 186.1
z!z f
25
36 980
f
1230
Calculated Structure
with II
vcis = 435 V, = 40
443 -342
448 -711
452 - 1067
451 -506
458 -1262
T/z = 20 va = 440
-171 400
-356 359
-
-
-
10.4
0.35
10.8
10.2
0.4
0
0
-29 208
-27.3 196
43 300 178
39 160 18.5
Ilcja = 44Pf 1’1=-8OOYt300 i’, = - 400 f 1._3= 349 f
0.35 0 -26.7 191.4 37 340 191
533 319
46 537
409 460
10.3
5.3
5.5
0.4
0.3
0.3
0
0
0
-26.3 188
-26 126
-25.5 131
36 036 195
17 840 19.5
19 000 204
4 180 30
n R,, and Rbc are found to be practically independent of the potential constants has been explored. since our experimental data are little sensitive b vtlWl* has not been reported c The values of the potential constants are based on the convention V(Q) = 0.
in the range to such
which
constants.
Moreover, the trend of the calculated values of C&, &b, and AA shows that the experimental splittings could be reproduced only for (@a2+ &b2)oalc21 2(aa2 + ab2)exp, AA(H) > 20 MHz, and AA (D) < 0. On the other hand it has been found in many cases (12, 13), that the potential barriers can be different for normal and deuterated internal tops, due to the different average positions of the hydrogen and deuterium atoms relative to the rest of the molecule, if other internal motions are taken into account. We therefore decided to fit separately the experimental data corresponding to each isotopic species. Since only one torsional splitting is available in the case of the normal species, the calculated values of the effective molecular constants corresponding to the experimental torsional splitting were compared with the experimental values in order to determine I/Z and Vs. In Table V are listed the barrier dependent quantities for the normal species calculated with Structures I and II as functions of Vz and V3 in the IPS. The (ga2 + &b2) value is to be compared with the corresponding experimental value, since in this work the transformation corresponding to the diagonalization of the o+ and O- blocks was not applied to the calculated values of &a and Qb. The best set of effective molecular constants calculated with Structure I corresponds to the interpolated value Vcis = 449 f 5 cm-‘, where the uncertainty is relative to the standard error on the experimental value of aa + Gb2. It can be seen from the same
TORSION ROTATION
SPECTRUM
OF HCCCHzSD
519
table that larger discrepancies are obtained with Structure II. The trend of the calculated value of &a2 + ob2 is such that an acceptable value can be obtained for YT = ET( l+) - ET(@) considerably larger than 205 cm-‘. From a recent infrared paper (14) on alkyl derivatives of methyl mercaptan, no significant change of the torsional frequency was obtained when the methyl group hydrogen atoms are different]) substituted. VT was observed to be 190 cm-l, and 189 cm-’ for the gauche conformers of CH&HzSH and (CHz)KHSH, respectively, and 193 cm-’ for (CH,)&SH, the splittings due to the torsional sublevels being unresolved in all three cases. It is therefore reasonable to assume VT ,s 195 cm-’ for propargyl mercaptan if the v’ciJ potential barrier is also taken into account. vcgp was in fact evaluated (14) to be 600 and 550 cm-’ for CHKH2SH and (CH&CHSH, respectively, that is, considerabl> higher than for H-C=C-CHzSH. It can also be seen from Table VI that ti and &,Levaluated with Structure I for the deuterated species, with potential constants obtained from the two torsional splittings. agree satisfactorily with the experimental values. The uncertainty on Llciu = 415 & 5 cnrl is relative to the experimental uncertaint! on the A(l*) torsional splitting. It is impossible to reproduce the experimental torsional splittings with Structure II unless AA << -3 MHz and L’frans < I’(lauche. In Table VII are listed the experimental and calculated values of the same constants in the FFS, relative to Structure I. Evident are the large discrepancies between the experimental and calculated values and the much greater calculated absolute value of each constant if compared with the corresponding quantity in the IPS. Other calculations were carried out with Structure 1 for (Ye= 123” and cxg = 117’, in order to cover the range of uncertainty of this angle. For cxI/= 123” a reasonable agreement of the experimental with the calculated effective molecular constants can be obtained onl! in the case of the normal species. For o(~ = 11 i” for I/lrans < l/s,,,Leheand VT > 200 crb, TABLE VI Barrier
Dependent
Molecular
Constants
for HC=C-CH&D, in Megahertz
(IPS System)
;
VT,Yciain cm-l, Other Constants
Calculated with Structure Vcia = 415 f 5 V, = - 267 f 80 I’? = - 133 f 40 [.T3= 67 f 10
Experimental
AA AB AC
1 f 0.03 f -0.02 f
;;:
-814.7
A (O*t) A(l+) VT
f
388.5 13 250 -
0.2 0.05 0.06
In
0.6 0.06 0
200.5
-2.5 l_,
& 0.1 f3oO
388 13 300 138
a With Structure II VL,,~, < Voo,,chr. lJ In the IPS the diagonalization of the O+ and O- blocks does not practically change the values of the molecular constants. Q. and 06 can therefore significantly be compared with the corresponding experimental values.
520
MIRRI
ET AL.
TABLE
VII
Barrier Dependent Molecular Constants in the FFS. vcia in cm-l, Other Constants in Megahertz
H-CEC-CH,SH Experimental
it-L:+c-Cil,ss CPlculated
Expermmta.1
vcis=44y
11.5 + 0.7 + 0.1
0.4
-0.1
+ 0.1
0.
214
?13
7
+5
" 46 000 (-172ja (16.3ja 6 891.76+0,02
vcis=415
8u
-".,
5.1 + 0.2 -0.6
i 0.04
c.
2 0.04
400
31.7
3bO
-8.3
290
000
=
-172 1tio3
15. 0. 0.
?; 0.5
41.
_e 0.1 1 100
(-321ja (31.41a
6 851
__
Calculated
388.5+0.1 13 250+300
37. =
3 050 -321 31.4 388 13 300
the same type of agreement is possible for VT1: 200 cm-l, but 1/s < 0, and this leads to a very distorted shape of the potential function in the region of q,. We may conclude that the best set of calculated effective molecular constants is obtained for Structure I and 01~= 120”; moreover, the uncertainty on this angle seems to be considerably smaller than the standard error as obtained by the least-squares fitting of the rotational constants. ACKNOWLEDGhIEN’I’ The authors wish to thank Professor B. Lunelli for his helpful deutero-propargyl mercaptan.
suggestions
in the preparation
of
RECEIVED: April 7, 1976 REFERENCE 1. K. B~LTON ANDJ. SIIEKIDAN,Spectrockim. Ado A 26, 1001 (1970). 2. F. SCAPPINI,H. M;~DER, ANU J. SHERIDAN,Z. Na2z~rforsch.28a, 77 (1973). Misprints in Tab. III have been corrected in Z. Nat~rforsch. 30a, 922 (1975). 3. F. SCAPPIM, P. G. FAVERO, AND R. CERVELLATI,Chem. Phys. Lell. 33, 499 (1975). -!. P. MEAKIN, D. 0. HARRIS, AND E. HIROTA, J. Chem. Phys. 51, 3775 (1969). 5. G. W. COUKLINANDR. C. MORRIS, U.S. Patent 2,707,714 (1955). 6. G. CAZZOLI,D. G. LISTER, ANDA. M. MIRRI, J. Chem. Sot. Faraday Trans. II 69, 569 (1973). 7. T. KOJIMAANDT. NISHIKAWA,J. Phys. Sot. Japalz 12, 680 (1957). 8. A. M. MIRRI, F. SCAPPINI,ANDH. MKDER, J. Mol. Spectrosc. 57, 264 (1975). 9. G. L. BENDAZZOLI,F. BERNARDI,ANDP. PALMIERI,J. Chem. Sot. Favaday II 69, 579 (1973). 10. E. HIROTA, J. Mol. Speclrosc. 34, 516 (1970). 11. D. SWOCHANDH. D. RUDOLPH,J. Mol. Spectrosc. 57, 47 (197.5). 12. C. R. QUADEAND C. C. LIN, J. Chem. Phys. 38, 540 (1963). 13. E. TANNEBAUM,R. J. MYERS, ANDW. D. GWINN, J. Gem. Phys. 25, 42 (1956). 11. F. INAGAKI,I. HARADA, ANDT. SHIMANOUCHI, J. Mol. Specfrosc. 46, 381 (1973). 15. E. HIROTA ANDY. MOBINO, Bull. Chem. Sot. Jaban 34, 341 (1961).
OP
MOLECULAR
Microwave
SPECTROSCOPY 63, 509-520
Spectrum
investigation .I. M.
of Deutero
of the Internal MIRRI,
(1976)
F.
Propargyl
Rotation
SCAPPINI,
R.
Mercaptan
in the -SH
and -SD
CERVELLATI,
AND
HCCCH,SD
Isotopic
Species
P. G. FAWRO
The microwave spectrum of the deuterated propargyl mercaptan was recorded and measurements in the torsional levels 0’ and l* are reported. The internal rotation analysis is carried further using the new data obtained and a more specific Hamiltonian. With the mean values of the rotational constants for the two isotopic species in the states O”, a ro-structure is proposed.
IKTKODUCTIOS Preliminary investigations on the gauche configuration of propargyl mercaptan have been carried out by Bolton and Sheridan (I) who measured the microwave spectrum of the normal isotopic species, including some a- and b-type close doublets due to the pure rotational transitions in the Of and O- torsional sublevels and some c-type torsiorotational transitions of the series Jl~(0’) +- Jo$(O*). No assignment of the individual components of the doublets was attempted b\. Bolton and Sheridan. Scappini, Mgder, and Sheridan (2) reported in a more recent paper a list of new lines (Jf the normal species and some a-type lines of the -SD isotopic species. Moreover, the!used a torsio-rotational Hamiltonian based on the symmetric internal top approsimation. The experimental torsional splitting A(O*) = 6891.76 MHz and the splittings of the rotational doublets were fitted in order to obtain information on the torsional potential function, which was expressed as a truncated Fourier expansion of the t!‘pe c (lT,li2)(1 %=I
- co:, nu).
They obtained a Vcis potential barrier of 470 f 10 cm-’ for the normal species, but the assignment of some b-type doublets, indirectly checked through the fitting process, was found to be incorrect by Scappini, Favero, and Cervellati (3). who checked several rotational doublets by MW-MW double resonance experiments. In the present work we continue the torsio-rotational analysis on the parent molecule and estend it also to the -SD isotopic species. In the case of the -SD species, some h-rotational and c-torsio-rotational transitions in the ground O* and first excited li torsional levels were also identified in order to obtain further information on the molecular structure and on the internal rotation. so9 ('opyright @ 1976 hy Academic Press. Inc. .\I1 rightnf relnKJuction in an\-form reserved.
MIRRI
510
ET AL.
The analysis of the internal rotation for both isotopic species was carried out with an effective Hamiltonian of the type used by Meakin, Harris, and Hirota (4), modified by the use of an instantaneous principal axis system which leads, at least in this case, to much better results than the Hamiltonian of Ref. (2). EXPERIMENTAL
-SD propargyl mercaptan was prepared sulphide obtained in the following way:
DETAILS
as in Ref.
(5) using potassium
deutero-
K + DzO--t KOD + +Dz, Al&
+ 6DzO + 3D& + 2Al(OD)s,
KOD + D2S -+ KSD + DzO. H--CC-CHZSD was subsequently purified by vacuum distillation and the amount of deuteration checked by NMR spectra. After proper conditioning of the microwave absorption cell, the intensity of the lines was found to be almost twice the intensity obtained in the case of deuteration of the normal species with DzO directly into the cell. The microwave spectra were recorded with a HP-MRR 8400 C-type spectrometer in the range 840 GHz. All measurements were made at dry-ice temperature. The presence of transitions of the normal species, which could not be eliminated, and of excited torsional and vibrational states with energies on the order of 100-200 cm-’ for both isotopic species, gives rise to very crowded spectra. Only the lines that had been checked by MW-MW double resonance technique as described in Ref. (3) were therefore used in the final least-squares fitting calculations. Large discrepancies between experimental and calculated effective molecular constants and large r.m.s.d. are in fact obtained if all the lines which were measured and seemed at first to be correctly assigned, are included in the calculations.
0’ FIG.
1. Double resonance map for HCCCH8D.
0The drawing is not in scale.
TORSION
ROTATION
SPECTRUM
OF HCCCHsSD
13500
FIG. -7. Torsion-rotation
transitions
in HCCCH&D
showing
5 11
MHz
the handhead
close to 13 250 MHz
From preliminary calculations the torsional splittings of the deuterated species were predicted to be A(O*) = 150-250 MHz and A(l*) = 11000-13 000 MHz. To obtain A(O*) the MW-MW double resonance technique was used, aiming at the identification of a number of c-type transitions connecting the 0+ and the O- torsional sublevels. Figure 1 shows a map of double resonance connections involving the observed c-type transitions pumped by the 2& + lol* a-t>Fe unresolved doublet. To obtain A(l*) several low-resolution spectra in the range of 8000-18 000 MHz were recorded in search of the torsio-rotational bandhead due to the J~J’ +-Jo.,* c-type transitions. A characteristic pattern of lines was found to converge to a bandhead at approximately 13 250 MHz (see Fig. 2), the convergency being due to the rotational contribution which tends to zero as J increases. The bandhead on the high-frequency. side is due to the relative signs of the rotational and centrifugal contributions. A similar
512
MIRRI
EET ;1L.
pattern was observed for the same kind of transitions in the ground state of the normal species (I). The results concerning the torsional splittings obtained as described above are to be found in Table VI. STRUCTURE
The mean values of the rotational constants corresponding to the ground-state torsional sublevels for the two isotopic species, listed in Table IV, were used in leastsquares fitting calculations in order to obtain information on the structural parameters. In Table I the results of such calculations are given. Structure I was obtained by fitting four parameters and assuming all the others, the choice of the parameters being based on their effect on the potential function determination. The --SH bond length is considerably shorter than in methyl mercaptan (7); therefore, a second calculation assuming the same --SH bond length as in CHsSH was performed, obtaining Structure II. While (SCC is practically the same in the two cases, considerably different values are obtained for (CSH and as. Structure I seems to be more reliable if the differences between experimental and calculated rotational constants are considered and also on the basis of the discrepancies between the experimental and calculated values of all the other molecular constants, as will be discussed later. The SH bond moment was evaluated to be 0.79 D in propargyl mercaptan compared TABLE Bond Lengths
in Angstroms, Bond Angles in Degrees, Differences between Calculated Rotational Constants in Megahertz
structure (AexP-I 6 exp
-2
(5exP-c (XexP-i; CC exP-F
talc talc talc ca1c
ca1c
)
structure
I
)H
-5.2
-3.2
-5.6
-11.7
)
)
H D
II
0.01
0.05
2.9
7.1
3.1
-8.4
1.301 + 0.015
(CCC
113.02
+ 0.01
(CSH c
104.3
+2
119.8
+3
a) acr;'~med as in CH,SH
[I]
as in propargyl-chloriUe
and
II
-O-C5
)
ag
c1.33aa 113.01
(HCC
(111.5)'
CH
(1.091)a
C-C
(,.460)b
EC:
(,.207jb
SC-H
(,.057jb
C-S
(1.81Yja
+ 0.01
98
+1
124
+3
[S]
1s the intercal rotation angle measured g starting fron be trans configuratron.
c) a
Experimental
-0.01
H
SH
b) &xmmed
I
at the
gauche
configuration
ana
computed
TORSION ROTATION
SPECTRUM
OF HCCCH,SD
31 3
to 0.71 D in HzS and 1.10 D in CHSSH (8). Correspondingly, the bond lengths are 1.301 (from structure I), 1.323, and 1.336 A. Assuming an electron-donor inductive effect from the group attached to --SH as in Ref. (9) the sequence of the ratios (0.71i1.323) < (0.79/1.301) < (1.10,/1.336) results as expected, and the value of 1.301 D for the SH bond length in propargyl mercaptan does not contradict the results obtained in Ref. (8) and the assumption suggested in Ref. (9). TORSIO-ROTATIONAL
H.VMTLTO?r’L4N
,4 semirigid molecular model with only one degree of internal freedom was assumed in order to fit all the experimental data. The first approach was based on the torsiorotational Hamiltonian used by Scappini, Mader, and Sheridan (3) in the approximation of the symmetric internal rotor. The molecular constants F, pa, and Pa, which appear in their Hamiltonian, were precalculated from Structure I. The computation program described in Ref. (2) was used in order to fit to the rotational a- and b-type doublets, which had been checked by. MW-MW double resonance, and to the torsional splittings for each isotopic species separately, the potential constants Vq and Vs in the Fourier espansion
1’ = 2 (C’,/2) (1 - cos Ita) ,i=1 for (Ye= 120”, and the small differences of rotational constants corresponding to each torsional sublevel. We were not able to fit the present data reasonably with this Hamiltonian, which in this case is perhaps too drastically constrained to precalculated values of all the molecular constants involved in the expression of the simplified torsio-rotational interaction. Some considerably more perturbed transitions (compared to those used in Ref. (Z)] were in fact included in the least-squares fitting, that is the &,* + &I*, 6r6* - L*, and 606~ +- Sn,* doublets. The effective rotational Hamiltonian relative to the 0+ and O- torsional sublevels, as described by Meakin, Harris, and Hirota (4), was subsequently used. Such a Hamiltonian gives rise to matrix elements connecting the two torsional sublevels and has the following schematic form : 0+
ot
AfPa2 + B+Pb” +
C+P,2
o-
---
I &aP, +
&bh
+ iL(P,P,
+ P,Pd
i- iRbe(PbPc + O-
Details The A(O*) b-type of the
C.C.
!
pcpb)
A(O*) + A-P,* + B-P,” + C-P,” _____
are given in the papers by Hirota (10) and Schwoch and Rudolph (II). molecular constants A+, B+, C+, A-, B-, C-, an, 06, and the torsional splitting (for the -SD isotopic species) were determined by an empirical fit to the a- and transition frequencies and to some torsio-rotational c-type transition frequencies due to the high correlation two isotopic species. R,, an d Rbc were precalculated,
MIRRI ET AL.
514
of these constants to the rotational constants. The computation program which carries out the diagonalization of the effective Hamiltonian and the least-squares fitting was that already used for hydroxyacetonitrile (6). DJ and DJK were also included with assumed values of the order of magnitude as in molecules of the same type (6). The values of all these constants were finally compared with a second computation program which calculates the eigenvalues of the torsional Hamiltonian, given by Eq. (9) in Ref. (4), and the molecular constants of the effective Hamiltonian, that is R,,, Rbc, & &t,, and the effective rotational constants. Two different approaches were followed for the expression of the a-dependent parameters which were used in the computation of the effective molecular constants. The first approach is based on the use of a system of axes as defined by Quade and Lin (12) and it will be called a frame fixed system (FFS), although only its orientation is fixed with respect to the frame during the internal rotation. The second approach is based on the use of an instantaneous principal axis system (IPS). In this system, the vector position ri of each atom is a complicated function of LYin the kinetic energy expression 2T = g mi(ii + o X ri)‘, i-l ki = (drJdol)& if it is assumed that a! is the only internal With the notations
xi = dxqfdol, the following expression
Yi = dyi/da,
of the kinetic
degree of freedom.
zi =
&,/da,
energy is easily obtained:
2T = Ab2 + 2&(w,5 + C+,TJ + CO&)+ US-IO, where A = c mi(Xz
i- =
c
+ Yi2 + Z,“),
rni(YiXi -
Xbi),
and I is the diagonal instantaneous inertial tensor. If P, = dT/th, and p = aT/d& are evaluated, then
(1)
TORSION
that is
SPECTRUM
5 15
OF HCCCHzSD
pzpz I,, (P/A> -&/A> wz -h'A> p,(2) WY 7 C.C. Pz -pz I;, I[ Ii1 Pu
i
ROTATION
=
I,, -
(~‘/a)
- CWA)
({“/‘A)
where P, = (E/Q, P, = (v/A)P, Pz = (l/U. By substitution of d! = (l/A)@ - w,i - wy~ - wJ)
wz
in (1) and making
use of (21,
2T = C (Pi - pi)Lc+(Pj- pi) f (l/A>p”, i.2
where pij is an element of the inverse of I’ and I’ is the effective inertial in (2). The Hamiltonian
operator
The off-diagonal
terms
abbreviated
Ha + H RT can now be written
of I’ are due only
to torsio-rotational
tensor appearing
in the usual form coupling
(4).
terms.
In an
form,
Observed and Calculated Frequencies, in Megahertz, for the O+and O- Torsional States of HCCCHSH 0+ Observed frequency
calculated
freouency
0ohs.-talc.
Observed
frequency
camiatea
10,*00
a)
5 803.24
- 0.06
5 802.83
l,,%O
al
25
103.54
25 103.03
0.51
25 104.85
25
5 803.18
obso-calo.
frequency 5 802.57 104.91
0.26 -
0.06
l,Q-lO*
a)
19 588.15
19 588.36
0.39
19 589.86
19 590.01
- 0.15
2x1~110
d
71 894.98
11 895.51
- 0.53
11 893.94
lt 893.50
0.44
2,2-1,1
a)
11 317.92
11 318.40
- 0.48
11 318.22
11 318.19
0.03
202-10,
@.)
11 603.18
11 603.23
- 0.05
11 602.27
11 601.91
0.36
2,,-10%
d
30 618.50
30 618.18
0.32
30 619.89
30 620.54
- Oh65
2,x-202
8)
19 880.54
19 880.64
- 0.10
19 881e74
19
881.60
0.14
301-202
a)
17 396.65
17 396.71
- 0.06
17 395.39
17 394.80
0.59
31s~201
a)
35 990.46
35 990.36
0.10
35 992.66
35 993.99
- 1.33
311-303
a)
20 324.02
20 324.81
- 0.79
20 325.10
20 324.98
0.12
23 180.38
23
- 0.05
23
178e78
23 178.02
0.76
20 927.54
20 927.27
0.27
28 284,13 28 948.38
- 0.47 0.86
40r-305 415-404
a) a)
180.43
-_
--
51s'414
28 280.74
28 280.94
- 0.20
28 283.66
501-401
28 951.21
28 951.19
- 0.02
28 949.24
508’4x4
10 908.17
10 908.14
0.03
10 895.25
10 896.71
- la46
6,,-515
33 927.85
33 927.99
- 0.14
33 950.97
33 951.05
- 0.08
6~~~505
34 705.94
34 705.89
0.05
34 703.74
34 702.79
0.95
6c.,-51~
17 333.49
17 333.09
0.40
17 315.18
17 315.38
- 0.20
a) measurements from Ref. [pl and [Jl.
516
MlRRI
ET
AL.
a! is step-wise changed in the FFS, starting from cr = 0 as defined in Ref. (4), and the inertial tensor is diagonalized for each value of (Yin order to obtain I(a), TV, and fi(a). The same kind of procedure as that used for the FFS has then been followed in order to e?tpress the a-dependent parameters as series expansions in terms of cos(ncr) or sin(na) according to the symmetry of each parameter with respect to Q (4, 10). We found that the convergency of the Fourier expansions is slower in this system, but this disadvantage is compensated by the much smaller values of the effective molecular constants Rat, (4, IO, II), Ii,,, Rb,, aa, and &b and therefore by the smaller absolute uncertainty on the precalculated values of those constants which cannot be obtained by the fitting pracess, as R,,, Rbc, and Rat,. ANALYSIS
OF THE
TORSIO-ROTATIONAL
SPECTRA
The least-squares fitting of the rotational transitions for the normal species and of the torsio-rotational spectrum for the deuterated species, carried out with R,, and Rbc precalculated in both systems of axes, gives the same equally good results, as far as the deviations of the calculated from the observed transition frequencies are concerned. TABLE Observed
Transition
2OZ’lOl 30s~202 31z-211 313-202
El) a) a,b) d a,b)
III
and Calculated Frequencies, in Megahertz, Torsional states of HCCCHISD Observed
frequency
for the Of and O-
Calculated frequency
ohs.-talc.
11 457.87
11 457.97
-0.10
17 178.30
17 178.48
-0.18
17 610.66
17 610.81
-0.15
34 019.94
34 020.06
-0.12
888.85
22 888.84
0.01
22 931.40
22 931.19
0.21
22 920.80
22 920.74
0.06
22 953.95
22 953.89
0.06
2:, -1:,
28
28
709.08
0.16
2,,-101
28 708038
28 708-35
0.03
1:0-l:,
17 807.90
17 807.84
0.06
1,0-l ;I
17 808.80
17 808.74
0.06
4or-303 4,x-330 421-322 422-321
a,b) a,b) a,b)
22
709.24
17 917.80
17 917.76
17 139.71
17
139-73
-0.02
29 Y3Y.06
29 939.17
-0.11
29
29
-0.03
163.11
163.14
0.04
17 638.04
17 638.90
-0.06
16 862.60
17 862.64
-0.04
36 091.98
36 091.94
0.04
35 316.03
35 316.03
0.00
a) unsplit b) measurements from Ref.
[PI.
TORSION
ROTATION
SPECTRUM
TABLE Molecular
Constants
IV
from Least-Squares
Tables
II and
III
OF HCCCHzSD
Fittings
of the Data
in
(in Megahertz)
H-EC-CH,-SH A+
22 35Oa4b + U,j3
i)+
3 045.98 If. O.U4
c+ :.-
22 342.15 + O.jL
2 151.32 + O.cl4
I:-CIC-CH,-Sj
2li534~23 + 3
0.11
ciO5.67+ 0 02
2 725.62 + (,C5 2"
533~25 + i lb
5
3 045.43 + G.L5
: L05,64 + b.L3
_)-
2 757.44 + o,v:
2 725.Uj + L'.Lj
336.1 + 2.
14.7 + 0.s
-4ti.6 + :.
_d
(-12.2j2 (0.41a (0.002)" (-0.05)O (6891.76)'
+2i
(-13.)x
ii.JJa (o.oo2)” (-0.03)" 358.5 + 0.1
4 Calculatedfrom structure
I in the IPS b) Assumed from simlar molecules (see ref. [gj) C) From ref. [?I.
However, the discrepancies between the experimental and calculated values of the other barrier dependent parameters, that is AA = A+ - A-, AB = B+ - B-, AC = C+ - C-. &, and 06, are much larger in the case of FFS. In the IPS the uncertainties on the calculated values of all the molecular constants are perhaps smaller due to their small values. In particular the uncertainty is considerably reduced in the process of diagonalization of the 0+ and O- blocks due to the negligible rotation of axes associated with this diagonalization (4, 10, 11). We found, in fact, Rrrb* 10 MHz, while in the case of the FFS R,h cx 9000 MHz. The observed and calculated rotational and torsio-rotational transition frequencies used in the fitting process for the normal and deuterated species, respectively, arc reported in Tables II and III. In Table IV, the experimental molecular constants for the two isotopic species arc listed. We tried to fit to the torsional splittings, A(O*) = 6891.76 MHz for the normal species, A(O*) = 388.5 MHz and A(l*) = 13 250 MHz for the deuterated species, C-2and 113,for different values of (Ye,but no reasonable fitting could be obtained. Since a considerable interaction could be possible between the torsional motion and some other low-energy vibrations, as the -C=C-H bending mode, and such interaction should principally affect the first excited torsional level, we tried to fit only the two ground-state splittings. The trend of the results obtained by changing T/Z and L’s, shows that the fitting would be possible only for I/itruns < Vllauche, while this possibility seems to be excluded by the experimental results.
518
MIRRI
ET AL.
TABLE Barrier
Dependent
Constants for HC= C-CH&H (IPS VI, VZ, V3 in cm-‘; Other Constants in Megahertzabe
Experimental
8.35 f
0.60
AB
0.55 f
0.09
AC
&a2 + Q*” VT
System) ; VT,Vcis,
Calculated with Structure I
AA
;:
V
Molecular
-0.12
&
0.08
-48.6 186.1
z!z f
25
36 980
f
1230
Calculated Structure
with II
vcis = 435 V, = 40
443 -342
448 -711
452 - 1067
451 -506
458 -1262
T/z = 20 va = 440
-171 400
-356 359
-
-
-
10.4
0.35
10.8
10.2
0.4
0
0
-29 208
-27.3 196
43 300 178
39 160 18.5
Ilcja = 44Pf 1’1=-8OOYt300 i’, = - 400 f 1._3= 349 f
0.35 0 -26.7 191.4 37 340 191
533 319
46 537
409 460
10.3
5.3
5.5
0.4
0.3
0.3
0
0
0
-26.3 188
-26 126
-25.5 131
36 036 195
17 840 19.5
19 000 204
4 180 30
n R,, and Rbc are found to be practically independent of the potential constants has been explored. since our experimental data are little sensitive b vtlWl* has not been reported c The values of the potential constants are based on the convention V(Q) = 0.
in the range to such
which
constants.
Moreover, the trend of the calculated values of C&, &b, and AA shows that the experimental splittings could be reproduced only for (@a2+ &b2)oalc21 2(aa2 + ab2)exp, AA(H) > 20 MHz, and AA (D) < 0. On the other hand it has been found in many cases (12, 13), that the potential barriers can be different for normal and deuterated internal tops, due to the different average positions of the hydrogen and deuterium atoms relative to the rest of the molecule, if other internal motions are taken into account. We therefore decided to fit separately the experimental data corresponding to each isotopic species. Since only one torsional splitting is available in the case of the normal species, the calculated values of the effective molecular constants corresponding to the experimental torsional splitting were compared with the experimental values in order to determine I/Z and Vs. In Table V are listed the barrier dependent quantities for the normal species calculated with Structures I and II as functions of Vz and V3 in the IPS. The (ga2 + &b2) value is to be compared with the corresponding experimental value, since in this work the transformation corresponding to the diagonalization of the o+ and O- blocks was not applied to the calculated values of &a and Qb. The best set of effective molecular constants calculated with Structure I corresponds to the interpolated value Vcis = 449 f 5 cm-‘, where the uncertainty is relative to the standard error on the experimental value of aa + Gb2. It can be seen from the same
TORSION ROTATION
SPECTRUM
OF HCCCHzSD
519
table that larger discrepancies are obtained with Structure II. The trend of the calculated value of &a2 + ob2 is such that an acceptable value can be obtained for YT = ET( l+) - ET(@) considerably larger than 205 cm-‘. From a recent infrared paper (14) on alkyl derivatives of methyl mercaptan, no significant change of the torsional frequency was obtained when the methyl group hydrogen atoms are different]) substituted. VT was observed to be 190 cm-l, and 189 cm-’ for the gauche conformers of CH&HzSH and (CHz)KHSH, respectively, and 193 cm-’ for (CH,)&SH, the splittings due to the torsional sublevels being unresolved in all three cases. It is therefore reasonable to assume VT ,s 195 cm-’ for propargyl mercaptan if the v’ciJ potential barrier is also taken into account. vcgp was in fact evaluated (14) to be 600 and 550 cm-’ for CHKH2SH and (CH&CHSH, respectively, that is, considerabl> higher than for H-C=C-CHzSH. It can also be seen from Table VI that ti and &,Levaluated with Structure I for the deuterated species, with potential constants obtained from the two torsional splittings. agree satisfactorily with the experimental values. The uncertainty on Llciu = 415 & 5 cnrl is relative to the experimental uncertaint! on the A(l*) torsional splitting. It is impossible to reproduce the experimental torsional splittings with Structure II unless AA << -3 MHz and L’frans < I’(lauche. In Table VII are listed the experimental and calculated values of the same constants in the FFS, relative to Structure I. Evident are the large discrepancies between the experimental and calculated values and the much greater calculated absolute value of each constant if compared with the corresponding quantity in the IPS. Other calculations were carried out with Structure 1 for (Ye= 123” and cxg = 117’, in order to cover the range of uncertainty of this angle. For cxI/= 123” a reasonable agreement of the experimental with the calculated effective molecular constants can be obtained onl! in the case of the normal species. For o(~ = 11 i” for I/lrans < l/s,,,Leheand VT > 200 crb, TABLE VI Barrier
Dependent
Molecular
Constants
for HC=C-CH&D, in Megahertz
(IPS System)
;
VT,Yciain cm-l, Other Constants
Calculated with Structure Vcia = 415 f 5 V, = - 267 f 80 I’? = - 133 f 40 [.T3= 67 f 10
Experimental
AA AB AC
1 f 0.03 f -0.02 f
;;:
-814.7
A (O*t) A(l+) VT
f
388.5 13 250 -
0.2 0.05 0.06
In
0.6 0.06 0
200.5
-2.5 l_,
& 0.1 f3oO
388 13 300 138
a With Structure II VL,,~, < Voo,,chr. lJ In the IPS the diagonalization of the O+ and O- blocks does not practically change the values of the molecular constants. Q. and 06 can therefore significantly be compared with the corresponding experimental values.
520
MIRRI
ET AL.
TABLE
VII
Barrier Dependent Molecular Constants in the FFS. vcia in cm-l, Other Constants in Megahertz
H-CEC-CH,SH Experimental
it-L:+c-Cil,ss CPlculated
Expermmta.1
vcis=44y
11.5 + 0.7 + 0.1
0.4
-0.1
+ 0.1
0.
214
?13
7
+5
" 46 000 (-172ja (16.3ja 6 891.76+0,02
vcis=415
8u
-".,
5.1 + 0.2 -0.6
i 0.04
c.
2 0.04
400
31.7
3bO
-8.3
290
000
=
-172 1tio3
15. 0. 0.
?; 0.5
41.
_e 0.1 1 100
(-321ja (31.41a
6 851
__
Calculated
388.5+0.1 13 250+300
37. =
3 050 -321 31.4 388 13 300
the same type of agreement is possible for VT1: 200 cm-l, but 1/s < 0, and this leads to a very distorted shape of the potential function in the region of q,. We may conclude that the best set of calculated effective molecular constants is obtained for Structure I and 01~= 120”; moreover, the uncertainty on this angle seems to be considerably smaller than the standard error as obtained by the least-squares fitting of the rotational constants. ACKNOWLEDGhIEN’I’ The authors wish to thank Professor B. Lunelli for his helpful deutero-propargyl mercaptan.
suggestions
in the preparation
of
RECEIVED: April 7, 1976 REFERENCE 1. K. B~LTON ANDJ. SIIEKIDAN,Spectrockim. Ado A 26, 1001 (1970). 2. F. SCAPPINI,H. M;~DER, ANU J. SHERIDAN,Z. Na2z~rforsch.28a, 77 (1973). Misprints in Tab. III have been corrected in Z. Nat~rforsch. 30a, 922 (1975). 3. F. SCAPPIM, P. G. FAVERO, AND R. CERVELLATI,Chem. Phys. Lell. 33, 499 (1975). -!. P. MEAKIN, D. 0. HARRIS, AND E. HIROTA, J. Chem. Phys. 51, 3775 (1969). 5. G. W. COUKLINANDR. C. MORRIS, U.S. Patent 2,707,714 (1955). 6. G. CAZZOLI,D. G. LISTER, ANDA. M. MIRRI, J. Chem. Sot. Faraday Trans. II 69, 569 (1973). 7. T. KOJIMAANDT. NISHIKAWA,J. Phys. Sot. Japalz 12, 680 (1957). 8. A. M. MIRRI, F. SCAPPINI,ANDH. MKDER, J. Mol. Spectrosc. 57, 264 (1975). 9. G. L. BENDAZZOLI,F. BERNARDI,ANDP. PALMIERI,J. Chem. Sot. Favaday II 69, 579 (1973). 10. E. HIROTA, J. Mol. Speclrosc. 34, 516 (1970). 11. D. SWOCHANDH. D. RUDOLPH,J. Mol. Spectrosc. 57, 47 (197.5). 12. C. R. QUADEAND C. C. LIN, J. Chem. Phys. 38, 540 (1963). 13. E. TANNEBAUM,R. J. MYERS, ANDW. D. GWINN, J. Gem. Phys. 25, 42 (1956). 11. F. INAGAKI,I. HARADA, ANDT. SHIMANOUCHI, J. Mol. Specfrosc. 46, 381 (1973). 15. E. HIROTA ANDY. MOBINO, Bull. Chem. Sot. Jaban 34, 341 (1961).