Conformations of S-n-Propyl Thioesters
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
S. H. Butcher and E. B. Wilson, J . Chem. Phys., 40, 1671 (1964). H. M. Pickett and D. G. Scroggin, J. Chem. Pbys., 61, 3954 (1974). 0.L. Stiefvater and E. B. Wilson, J . Chem. Pbys., 50, 5385 (1969). H. Karlsson, J . Mol. Struct., 33, 227 (1976).
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(6) 0. L. Stiefvater,J . Chem. Phys., 62, 233, 244 (1975). (7) S. Borchert, J. Mol. Spectrosc., 57, 312 (1975). (8) S.Tsuchiya and T. Iijirna, J . Mol. Struct., 13, 327 (1972). (9) M. S. Farag and R. K. Bohn, J . Chem. Pbys., 62, 3946 (1975).
Low Resolution Microwave Spectroscopy. 13. Conformations of S-n-Propyl Thioesters Clarence J. Silvia, Nancy S. T r d , and Robert K. Bohn' Depatiment of Chemistry and Institute of Materials Science, The University of Connecticut, Storrs, Connecticut 06268 (Received August 22, 1977) Publication costs assisted by the University of Connecticut Research Foundation and the Petroleum Research Fund
Low resolution microwave spectra of S-n-propyl cyanothioformate, trifluorothioacetate, and chlorothioformate display band series from two conformational forms designated CA (compact-anti) and IA (intermediate-anti). S-n-Propyl fluorothioformate prodtxys band spectra from one conformational form designated CA. The CA conformers are characterized by B t C values of 1600.3(5),1088.8(5),1579.0(5) (1545.7(5),37Cl),and 2156.2(5) MHz for the cyanoformate, trifluorothioacetate, chlorothioformate, and fluorothioformate, respectively. The IA conformers are characterizedby B + C values of 1556(1),1064(1),and 1533(2)MHz for the cyanothioformate, trifluorothioacetate,and chlorothioformate, respectively. S-n-propyl cyanothioformate also displays band spectra from a conformer designated IG (intermediate-gauche)having a B + C value of 1760(1)MHz. For each S-n-propyl thioester, the CA conformer is compatible with a syn-gauche-anti (.T.~(OCSC) = O', Q(CSCC) go', Q(SCCC) = 180') structure. Each IA conformer is compatible with a gauche-gauche-anti (71(OCSC) 80', 72(CSCC) 210', 7&3CCC) = 180') structure and the IG conformer of S-n-propyl cyanothioformate is compatible with a gauche-gauche-gauche (q(0CSC) 80°, 72(CSCC) 210°, .r,(SCCC) 60') structure. In each case the CA conformer is the most8stable. Bands of the IA and IG conformers are anomalously intense. Extended syn-anti ( r l = 0', 7 2 = 180') conformers have not been observed in S-n-propyl thioesters. These results contrast to previous results for the corresponding n-propyl oxyesters where extended conformers as well as compact and intermediate conformers have been observed. N
N
Introduction Low resolution microwave (LRMW) spectroscopic studies of several S-ethyl thioestersl identified and characterized several conformational isomers. There are striking similarities as well as contrasts with the conformational isomers identified in the corresponding oxyAll of the observed species are assignable to three conformations: extended (E: syn-anti; 71(ococpr OCSC) = 0', r,(COCC or CSCC) = 180'), compact (C: syngauche; 7 1 OD, 72 go'), and intermediate (I: gauche-gauche; r1 45-90', 7 2 = 210-240'). See Figure 1. All three forms are observed in only 'a few of the compounds (see Table I) and the extended form is not observed in some thioesters as it is in the corresponding oxyesters. In the oxyesters the E and C forms are afways observed, are the most stable species, and have equal energy within 0.5 kcal/mol. In the thioesters the E and C forms have approximately equal energy in SLethyl fluorothioformate, the E form lies 0.5 kcal/mol highei. than the C form in the chlorothioformate, and tHe E form is not even observed in the cyanbthioformate and trifluorothioacetate. Thus the potential function to intepal rotation about ~ ( C o c or c CSCC) appears nearly unaffected by substitution in the acid moiety of oxyesters but changes dramatically upon substitution in the afjd moiety of thioesters. 0-Ethyl and S-ethyl ohhloroformates, cyanoformates, and trifluoroacetates display band series attributable to a species lying 1-2 kcal/mol higher in energy. These band series are all very broad, structureless, and anomalously intense. The B + C values of this conformer in each compound is consistent with the average structure r1 45-90' and 7 2 210-240'. S-n-Propyl thioesters are capable of displaying spectroscopically distinct rotational isomers about three bonds.
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0022-3654/78/2082-0483$0 1.OO/O
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TABLE I: Observed Conformers of Ethyl and n-Propyl Oxy and Thioesters Formate Thio formate 0 0 Ii I1 X X-C-0-R X-C-S-R ~~
R = Ethyl
H F
c1 CN CF3
ha
Eb Ed Ed Ed
C C C C C
I I
I
EC C E C C I C C I C C I
R = n-Propyl F CI CN CF,
EA,EG(' EA,EGe EA,EGe (EA,EGe
CA,CG CA CA IA,IG CA,CG 1A)g
CAf CAf IA CAf IA,IG CAf IA
a Reference 4. Reference 3. Reference 1. Reference 2. e Reference 5. This work. The LRMW spectrum of n-propyl trifluoroacetate was so complex and the signal/noise ratio sufficiently small that these assignments dre not necessarily reliable.
Proposed conformers are displayed in Figure 1. No structural studies of S-n-propyl thioesters have been reported. S-n-Propyl oxyesters have been observed and characterized by LRMW spectro~copy.~ Observed conformational species are included in Table I. Paralleling the results for ethyl oxyesters, the observed conformers of n-propyl oxyesters are compatible with extended, compact, and intermediate geometries which are coupled with gauche or anti configurations of the OCCC chain. Extended and compact conformers of n-propyl oxyesters have roughly equal energies and intermediate conformers
0 1978 American Chemical Society
484
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
C. J. Silvia, N. S. True, and R. K. Bohn
I
I 25
I
I
antl(A)
(13 = 180")
gaucho(G)
(73 - 60")
Flgure 1. Conformations of S-n-propyl thioesters. (a) Configurations about T,(OCSC) and T*(CSCC). (b) Configurations about r3(SCCC).
have 2-3 kcal/mol more energy. This study was undertaken to provide information on rotational isomerism in n-propyl thioesters, to determine if the results obtained for ethyl thioesters are common to homologous systems, and to compare conformational isomerism in S-n-propyl thioesters to n-propyl oxyesters. Four S-n-propyl thioesters, S-n-propyl fluorothioformate, chlorothioformate, cyanothioformate, and trifluorothioacetate, have been studied by LRMW spectroscopy.
Experimental Section All microwave measurements were made on HewlettPackard Model 8460-A microwave spectrometers. Spectra of S-n-propyl chlorothioformate were obtained from 26.5 to 39.5 GHz. Spectra of S-n-propyl fluorothioformate and S-n-propyl trifluorothioacetate were obtained from 26.5 to 38 GHz. Spectra of S-n-propyl cyanothioformate were obtained from 18 to 26.5 GHz. All spectra were recorded a t two temperatures, room temperature and a lower temperature, --63 "C, with the sample cells packed in dry ice. Since the sample of S-n-propyl cyanothioformate tended to condense in the sample cells a t this low temperature, an additional spectrum of this compound was recorded a t --30 "C. In all cases the Stark voltage was 3200 V/cm and the scan rate was 10 MHz/s with a 1-s detector time constant. Samples were distilled into the waveguide to pressures of -80 mTorr. In order to minimize errors in intensity measurements, the sample pressure and detector crystal current were kept constant while each spectrum was being recorded. Frequency measurements are frequencies of the band maxima averaged over forward and reverse scans. Frequency accuracy, which is dependent on the shape and width of the bands, ranges from about 5 to 50 MHz for the samples studied. S-n-Propyl chlorothioformate was purchased from ICN-K&K Chemical Co. S-n-Propyl fluorothioformate was prepared by direct exchange between S-n-propyl thiochloroformate and thallium fluoride according to the procedure suggested by Nakanishi et ale6 S-n-Propyl cyanothioformate was prepared by direct exchange between potassium cyanide and S-n-propyl chlorothioformate using crown ether phase transfer catalysis.' S-n-Propyl trifluorothioacetate was prepared from n-propyl mercaptan and trifluoroacetic anhydride. Each sample was characterized by infrared and lH NMR spectroscopy. The S-n-propyl cyanothioformate was contaminated by small amounts of S-n-propyl chlorothioformate. The purity of all the other samples was established to be in excess of 97% by gas-liquid chromatography on a 5-ft. SE-30 column with 5-10-min retention times.
I
I
I
I
I I 20GHz
I
I
I
I
I
I
I
IG
I
I
I
=A
I
Figure 2. Condensed survey LRMW K band spectrum of S-n-propyl thiocyanoformate ---30 "C. The spectrum was scanned at 10 MHz/s with a I-stime constant. The band markers are calculated from (J 1)(8-k C) with B C = 1600,1556,and 1760 MHz for the CA, I A , and IG conformers, respectively.
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Results S-n-Propyl Cyanothioformate. LRMW K band spectra of S-n-propyl cyanothioformate a t 20 and --30 "C display three a-type band series characterized by B C values of 1600.3(5), 1556(1), and 1760(1) MHz. Based on analogy with n-propyl oxyesters, the three band series are designated CA (compact-anti), IA (intermediate-anti), and IG (intermediate-gauche), respectively. A condensed survey spectrum of S-n-propyl cyanothioformate a t --30 "C is shown in Figure 2. Bands of the three series are structurally different. Each CA band is sharp, narrow (-50 MHz), and displays a resolved vibrational satellite band displaced to the low frequency side of each main band. The vibrational satellite of the CA series has a B + C value of 1593.0(5) MHz. It is estimated that the vibrational satellite is approximately 70 cm-l higher in energy than the ground state. No other resolved vibrational satellite bands of the CA conformer are present although it is estimated that a second satellite with a relative energy of -200 cm-l would be observable. Bands of the IA conformer are smooth, structureless, and 175-MHz wide. Bands of the IG conformer are smooth, structureless, and 250-MHz wide. Relative energies of the three conformers may be estimated from the temperature dependence of their band intensities. At 20 "C the band intensity ratio CA:IA:IG is 1:4.0:4.2. When the sample is cooled to --30 "C the band intensity ratio is 1:2.5:2.9 indicating that the CA conformer has the lowest energy. The IA and IG conformers have approximately 1kcal/mol more energy than the CA conformer. LRMW spectral data of S-n-propyl cyanothioformate are summarized in Table 11. S-n-Propyl Trifluorothioacetate. LRMW R band spectra of S-n-propyl trifluorothioacetate a t 20 "C are dominated by a band series having a B + C value of 1064(1) MHz designated IA. Bands of a second series having a B C value of 1088.8(5)MHz, designated CA, are barely discernible at 20 "C but are clearly discernible when the sample is cooled to --63 "C. Bands of the CA series are sharp and narrow (-50 MHz). Displaced to lower frequencies from each CA band is a second weaker band having a B + C value of 1084(1) MHz. This weaker band series is most likely a vibrational satellite band series of the CA conformer, The vibrational frequency is estimated to be 100 cm-l. No other vibrational satellite band series are present in LRMW spectra of S-n-propyl thiotrifluoroacetate. Bands of the IA series, in contrast, are broad (-300 MHz), smooth, and do not display resolved vibrational satellite structure. At 20 "C the CA:IA band intensity ratio is -1:20 and at -63 "C it is 1:5, indicating that the IA conformer has approximately 1.5 kcal/mol more energy than the CA form. LRMW spectral data of
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 405
Conformations of S-n-Propyl Thioesters
TABLE 11: Rotational Constants, Band Intensities, and Relative Energies of t h e Conformers of S-n-Propyl Thioesters CA IA IG B t C, MHz ( v = 0) B t C,MHz ( v = 1) Re1 int, 25 " C Re1 int,
S-n-Propyl Cyanothioformate 1600.3 ( 5 ) 1556 (1) 1760 (1) 1593.0 ( 5 ) 1
4.0
4.2
1
2.5
2.9
--30°C
Re1 energy, kcalimol B t C, MHz (v = 0 ) B + C, MHz ( v = 1) RR1 int, 25 " C Re1 int, --63 " C Re1 energy, kcal/mol B t C, MHz
0
-1
-1
S-n-Propyl Trifluorothioacetate 1088.8 ( 5 ) 1064 (1)
20
1
5
0
-1.5
5'-n-Pro] y l Chlorothioformate 1579.0 5)/1545.7 ( 5 ) 1533 ( 2 )
(u= 0)
( 35C1/37C1) B t C,MHz 1571 1)/1533 ( 2 ) ( u = 1) (35C1/37C1) 1 Re1 int, 25 " C Re1 int, 1 --63 " C 0 Re1 energy, kc al/m ol
B t C, MHz (v = 0 ) B t C, MHz ( u = 1)
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Discussion For each molecule values of B + C were calculated as functions of the three torsional angles ~~(ocsc), ~~(cscc), and 73(sccc)defined in Figure 1. Only anti (~~(sccc) = 180") and gauche (T,(sccc) 60") configurations were considered for 73. Geometrical parameters reported for the corresponding ethyl thioformate& coupled with a methylene to methyl C-C distance of 1.528 A and a CCC angle of 111" were used in these calculations. All parameters were held fixed except the torsional angles. For each molecule, calculations of B + C as a function of 72(CSCC), holding T1(ocsc)fixed at the 0" syn structure and holding T,(CSCC) at 60 and M O O , are displayed graphically in Figure 3a. B + C values for the observed conformers are indicated by horizontal lines. Error bars correspond to an estimated 3% uncertainty in calculated B + C values due to uncertainties in the assumed structured parameters excepting the torsional angles. This uncertainty has been arbitrarily assigned to the observed values for easy visualization. Closed circles indicate assignments. For the cyanothioformate (Figure 3a), trifluorothioacetate (Figure 3b), and chlorothioformate (Figure 3c) it is evident that the B + C value of the CA conformer of each is compatible with both a compact-anti ( T ~= O", T~ 80°, T~ = 180") and an extended-gauche ( T ~= O", T~ = BO", 7, 60") conformer. The CA conformer of S-npropyl fluorothioformate is compatible with a compact-anti conformer and marginally compatible with an extended-gauche conformer (Figure 3d). The band series has been assigned to the compact-anti conformation for three reasons. No band series corresponding to extended-anti conformers of S-n-propyl thioesters, except a very marginal series in the fluorothioformate, have been observed and it is therefore unlikely that large quantities of extended-gauche conformers would be present in these samples. For each S-n-propyl thioester studied, bands of the CA conformer are sharp and narrow. This observation is consistent with calculated K (Ray's asymmetry parameter; K = (2B - A - C)/(A - C ) ) values of --0.96 for the CA structure of each thioester. Extended-gauche conformers of these molecular are calculated to be less prolate having K values ranging from -0.94 to --0.88 and are expected to have broader bands than those observed. The CA series of each S-n-propyl thioester displays a vibrational satellite band series having a B + C value smaller by several MHz than the ground state series. Large frequency shifts and low frequency disposition of vibrational satellites are characteristic of compact conformers of oxyesters, having been observed for compact ethyl formate,6 ethyl fluorof ~ r m a t eand , ~ ethyl cyanoformate* and for several pro~ a r g y ln, ~- p r ~ p y land , ~ allyl esters.1° In each case these satellites have been attributed to the 0-R torsion. Similar satellites have been observed for S-ethyl thi0esters.l In contrast, resolved vibrational satellites of the extendedgauche conformer of n-propyl formate were found to lie
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1084 (1) 1
present in the LRMW K band spectrum of S-n-propyl fluorothioformate. The series with a B + C value of 2156.2(5) MHz is designated the CA form. Based on analogy with other S-n-propyl thioesters, the less intense series with a B C value of 2140(1) MHz is most likely a vibrational satellite of the CA conformer. The spectrum is consistent with small amounts of the other conformers having B + C values of -1820 and -1957 MHz but spectral complexity and low signal/noise ratio precludes an unambiguous assignment of these two weak series. Spectral data of S-n-propyl fluorothioformate are summarized in Table 11.
1
-0
Higher
S-n-Propyl Fluorothioformate 2156.2 ( 5 )
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2140 (1)
S-n-propyl thiotrifluoroacetate appear in Table 11. S-n-Propyl Chlorothioformate. S-n-Propyl chlorothioformate produces three sharp narrow ( 50 MHz) a-type band series at 20 and --63 "C. These series are consistent with one conformer displaying resolved chlorine isotopic species and resolved vibrational satellite bands. The conformer, designated CA, has B C values of 1579.0(5) MHz, u = 0 (1545.7(5) MHz, u = 0, 37Cl),and 1571(1) MHz, u = 1. The vibrational satellite band series is displaced to lower frequencies relative to the ground state bands and has a relative energy of -100 cm-'. In addition a broad (-300 MHz) band series with a B C value of 1533(2) MHz is present in the 20 "C spectrum. This series is designated IA. A t room temperature the CA and IA series have approximately equal intensity. At --63 "C the broad IA band series almost disappears and the sharp 37Clisotopic series of the u = 1 satellite of the CA conformer, which has a B + C value of 1533(2) MHz, is clearly discernible. The IA conformer is at least 2 kcal/mol higher in energy than the CA conformer, assuming the intensity of the IA bands are equal to the spectral noise level at --63 "C. A few weak unassigned structured absorbances are also present in LRMW of S-n-propyl chlorothioformate. Spectra data are summarized in Table 11. S-n-Propyl Fluorothioformate. Two a-type band series having B + C values of 2156.2(5) and 2140(1) MHz are N
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
C. J. Silvia, N. S. True, and R. K. Bohn
F-7
S n-propyl cyanothio -
S n-propyl c hlorothio-
'
S n-propyl trifluorot hio-
\
1.3
acstatQ
\
I
0"
90" 72(C s c c )
180"
3E
270"
S n-propyl fluorothioformate
G
1.2
1.'
h
N
I1.C (3
v
0 +
m
I
0"
90"
180"
I
I
3
270"
+
I"
+
Figure 3. Observed and calculated 6 C values of S-n-propyl thioesters. The-horizontal lines correspond to observed B C values. The curves display calculated B C values as a function of the torsional angle T&SCC) for T,(O=CSC) = O", T~(SCCC)= 180" (lower curve). The closed circles indicate the assigned conformations. The error bars correspond to an estimated 3% uncertainty in the calculated values due to uncertainties in the assumed structural parameters: (a, upper left) S-n-propyl cyanothioformate; (b, lower left) S-n-propyl trifluorothioacetate; (c, upper right) S-n-propyl chlorothioformate; (d, lower right) S-n-propyl fluorothioformate.
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od the high frequency side of the main bands. These observations strongly suggest that the CA conformer of S-n-propyl thioesters has the compact-anti and not the extended-gauche structures. The fact that for each Sn-propyl thioester studied the vibrational satellite band series for the CA conformer terminates abruptly after u = 1 suggests that the barrier separating the two enantiomeric CA configurations in these molecules may be low. Ix conformers of S-n-propyl cyanothioformate, trifluorothioacetate, and chlorothioformate have been observed. In each case the IA conformer has a B + C value smaller by -3% than the CA form, is higher in energy thdn the CA form, and produces broad structureless bands. As can be seen in Figures 3a-c the IA conformers in each case are compatible with an extended-gauche ( T = ~ O", 7 2 = 180°,73 60") conformer as well as a structure having T~ = O', 7 2 loo", T 3 = 180". The latter structure is unreasonable since it would require the potential function
--
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for internal rotation about 7 2 to oscillate from a minimum at i2 80" through a maximum and then to another minimum at least 1 kcal/mol higher in energy in a range of only 20'. If T1(o=csc)is not constrained to the 0" syn-eclipsed configuration, the B C values of the IA conformers of S-n-propyl thioesters are compatible with the range of structures shown in Figure 4a for S-n-propyl cyanothioformate. Figure 4a displays calculated B + C values for S-n-propyl cyanothioformate as a function of Tl(0cSc) and ~ ~ ( c s c forc Ts(scCc) ) = 0". The calculated values of B + C as a function of the two torsional angles are indicated by the contours shown at 100-MHz intervals. Blank regions correspond to conformers too asymmetric to produce LRMW band spectra. Similar surfaces were calculated for the trifluorothioacetate and the chlorothioformate. The topography of the three surfaces is qualitatively similar and the IA conformer in each case has a B C value compatible with the same
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 407
Conformations of S-n-Propyl Thioesters
ethyl and COCN groups are brought into closer proximity. Increasing steric repulsion dictates that bond angles and bond lengths should increase thus increasing the moments of inertia and diminishing B C. Thus the effect of a relaxing framework as r2varies is to slow the rate of change of B + C flattening out the surface and expanding the separation between contours along a section for which T~ is a constant. Similarly, as T1(OCSC) is changed from 0 ' value in the reference configuration, steric repulsion decreases between the carbonyl oxygen atom and primary methylene group suggesting that the structure will contract increasing B C more rapidly than the rigid framework model. At T~ 90' steric repulsion is at a minimum and then increases again for larger values of 71 as the CN and CH2 groups approach each other. Thus sections along constant 72 values will be steeper in the regions 0 < T~ < 90' and flatter in the region 90 < T~ < 270'. Similar arguments apply to Figure 4b. We conclude that the topography of the B + C surfaces shown in Figure 4 assuming a rigid framework is modestly distorted from the correct surfaces. However the range of structures consistent with a given B + C value is not drastically different from each dashed contour in Figure 4. Since the B C value of the IA conformer of n-propyl thioesters is compatible with two reasonable models, extended-gauche (T1(O=CSC) = 0', T~(CSCC)= 180', T~(SCCC)= 60') and an intermediate-anti (T~(O=CSC) 80', T~(CSCC) 210°, 73 = M O O ) , a definite assignment cannot be made. Relative energy data and band shape data support the intermediate-anti configuration, based on analogy with intermediate conformers of other esters which have been characterized more definitively. The fact that no extended-anti conformers of n-propyl thioesters have been observed makes it unlikely that large quantities of extended-gauche conformers would be present. Extended-gauche conformers have been observed in several oxyesters. In each case this conformer produces structured bands and has energy comparable to the compact-anti form. Based on these observations, it is reasonable to assign the 1556-MHz band series of S-n-propyl cyanothioformate, the 1533-MHz series of the chlorothioformate, and the 1064-MHz series of the trifluorothioacetate to the IA conformation. The 1760(1)-MHz conformer of S-n-propyl cyanothioformate is compatible with a range of structures including the (T~(OCSC) 80', r2(CSCC) 210°, T~(SCCC) 60') structure as can be seen from the contour diagram shown in Figure 4b. This diagram is analogous to Figure 4a and displays calculated B + C values for S-n-propyl cyanothioformate as a function of rl(O=CSC) and 72(cscc)for 73(SCCC) = 60'. Again this assignment is plausible but not unique. LRMW spectra of S-n-propyl thioesters may be interpreted in terms of CA, IA, and in the case of S-n-propyl cyanothioformate,also IG, conformers. These assignments are not unique, but by drawing analogies with S-ethyl esters which has one less torsional angle and for which the B + C values of reasonable compact and extended models are not accidentally degenerate and the intermediate band series B + C value is different from that of any possible model with a planar OCSC group. These spectra are not consistent with large quantities of EA conformers. Table I displays the similarities and differences in conformational properties of ethyl and n-propyl thio and oxyesters. One result of this study is that extended conformers are present in every ethyl and n-propyl oxyester but are absent in S-ethyl cyanothioformate,S-ethyl trifluorothioacetate, and
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360'
270'
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186
90
-2
0 0
v
rr
Figure 4. (a, upper) Contour diagrams of E -!- C for S-n-propyl cyanothioformate as a function of the torsional angles, T,(O=CSC) and T~(CSCC)with T,(SCCC) = 180'. The dashed contours indicate the observed E C values of species assigned to anti conformations about 7,. The labels CA and IA locate the most probable configurations of those species. The blank regions correspond to models too asymmetric to produce LRMW band spectra which are therefore unobservable by this technique. (b, lower) Similar to (a)except 7,(SCCC) = 60'. The dashed contour indicates the observed E C value of the IG species. The label IG indicates the most probable configuration of that species.
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range of T,(OCSC) and T~(CSCC),when T~(SCCC)is constrained to 180". In each case the most likely geometry for a nonplanar T1(O=CSC) conformer would be that structure furthest removed in 71,72 space from the CA conformer which still has a calculated B + C value compatible with the experimental B C value. For the IA conformers of S-n-propyl thioesters this structure has coordinates T1(0=CSC) = 80°, T~(CSCC) 210°, 73(SCCC) = 180'. Analogous nonplanar conformers have been observed in ethyl thioestersl and in most primary oxyesters.2 The B + C contours displayed in Figure 4 were calculated assuming no relaxation of the thio bond angles and bond lengths as the torsional angles vary. This is an assumption and there must be some adjustment of the framework as the torsional angles change. The magnitude of the relaxation is not known but the direction and its effect on the diagram can be described. Consider the EA form (71 = O', 7 2 = 180°, 7 3 = 180') as a reference configuration. As T~(CSCC) varies from 180°, the terminal
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The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
D. Raiem and W. H. Hamill
all the S-n-propyl thioesters which have been studied, indicating that the potential functions for internal rotation about T~(CXCC) in thio and oxyesters are different. Acknowledgment. The authors are grateful to Professor E. Bright Wilson of Harvard University and Marlin D. Harmony of the University of Kansas for allowing the use of the microwave spectrometers supported by NSF Grants GP-37066X and MPS 74-22178, respectively. Calculations were carried out a t t&-University of Connecticut Computer Center. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Supplementary Material Available: Tables 111-VI containing the observed band frequencies, J 1 values, and B + C values for S-n-propyl cyanothioformate, tri-
fluorothioacetate, chlorothioformate, and fluorothioformate (5 pages). Ordering information is available on any current masthead page.
References and Notes (1) Paper 12 of this series: C. J. Silvia, N. S. True, and R. K. Bohn, submitted for publication. (2) N. S. True and R. K. Bohn, J. Am. Cbem. SOC.,96, 1188 (1976). (3) N. S. True and R. K. Bohn, 31st Symposiumon Molecular Structure and Spectroscopy, Columbus, Ohio 1976, Abstract WB’1. (4) J. M. Riveros and E. B. Wilson, J . Cbem. Phys., 46, 4605 (1967). (5) Paper 6 of this series: N. S. True and R. K. Bohn, accompanying DaDer in this issue. (6) S . ‘Nakanishi, T-C: Meyers, and E. V. Jensen, J. Am. Cbem. SOC., 77, 3099 (1955). (7) M. E. Childs and W. P. Weber. J. Ora. Cbem., 41, 3486 (1976). ( 8 ) R. D.Suenram, N. S.True, and R. K. bohn, J . Mol. Specfrhsc., in press. (9) N. S. True and R. K. Bohn, J . Am. Cbem. Sac., 99, 3575 (1977). (10) N. S. True and R. K. Bohn, J . Pbys. Cbem., 81, 1671 (1977).
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Activated and Activationless Localization and Impurity Trapping of the Electron in C2H50Hand C2H50D Dugan Raiem’ and William H. Hamill” Department of Chemistry and the Radiation Laboratory,‘ University of Notre Dame, Natre Dame, Indiana 46556 (Received July 14, 1977) Publication casts assisted by the U.S. Department of Energy
Dry electron localization by solvent and trapping by impurity acceptors have been examined for several molecules in C2H50Hand in C2H50Dfrom 150 to 300 K. The concentration of acceptor required to reduce the initial yield of electrons to 37% is C37= k,oc/kcin terms of rate constants for localization and electron trapping. As the temperature decreases, C37 decreases and becomes constant below 150 K. Each process involves two channels, one activated, the other activationless. Consequently, kloc = kloc,T+ hlo2 and he- = k,-,T + k;’. For impurity k;,T exhibits a negative isotope effect and k;O corresponds to the gas phase resonance. The compound negative ion of the solvent molecule is considered to be stabilized by solvation in the condensed phase as the “solvated electron”. Below 150 K, activationless processes dominate and correspond to those in low-temperature amorphous solids. Both are dependent upon the zero-point kinetic energy of strongly scattered electrons of short wavelength in disordered materials. Yields of solvated electrons in ice below -273 K appear to depend almost entirely on activated localization with Ead = 0.16 eV but electron attachment by the near-resonant process may occur for hot electrons with small yields since there is no zero-point kinetic energy, unlike aqueous glasses.
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Introduction Chase and Hunt3 observed that trapping dry electrons (e-) by toluene in propanol became much more efficient as the temperature decreased. This interesting effect suggested a more detailed investigation of similar systems. The discovery that the trapping efficiency is approximately linear in the concentration of chemically bound oxygen for water and alcohols3 seems to be related to solvent isotope effect^.^ These provide important supplementary means to study temperature dependence since dry-electron trapping by an impurity competes with localization by solvent. Ten molecules known to react slowly with solvated electrons (e;) were selected to facilitate time resolution of fast and slow reactions. Measurements below room temperature are also advantageous while C2H50Hand CzH50Dare suitable solvents, providing a relatively long lifetime for e- and more efficient t r a ~ p i n g .An ~ improved understanding of the temperature dependence is the objective of this work, which extends a preliminary examinati~n.~ 0022-3654/78/2082-0488$0 1.OO/O
Experimental Section Irradiations of 1krd were performed with 10-ns pulses of -8-MeV electrons. The efficiency of electron trapping was measured from the decreased absorbance of solvated electrons, relative to undoped solvent, measured a t A,, for each temperature. Other details have been de~cribed.~ Results are expressed in terms of C37,the concentration of electron scavenger required to reduce the electron yield to 37% of that in the undoped solvent.
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Results Of the dry-electron scavengers examined previously: ten have been selected for further work in the range 150-300 K in C2H50Hand CzH50D. Values of C37 decrease with decreasing temperature but tend to constants, C3,: at
