480
The Journal of Physical Chemistry, Vot. 82, No. 4, 1978
L.
P. Thomas, N. S.True, and R. K. Bohn
Low Resolution Microwave Spectroscopy. 11. Conformations of Butyryl Fluoride, Chloride, and Bromide' Lawrence P. Thomas, Nancy S. True, and Robert K. Bohn" Department of Chemistry and Institute of Materials Science, The University of Connecticut, Storrs, Connecticut 06268 (Received July 2 1, 1977) Publication costs assisted by the University of Connecticut Research Foundation
-
Low resolution microwave spectra of butyryl fluoride, chloride, and bromide display a-type band series from syn-anti (rl(O=CCC) = 0", 72(CCCC) = 180') and syn-gauche (r1(0=CCC) = O", Q(CCCC) 70') conformers. and 1720(3)/1705(3) The syn-anti forms are characterized by B t C values of 3359(5),2446(3)/2399(3)(36Cl/37Cl), (79Br/81Br)MHz for the fluoride, chloride, and bromide, respectively. The syn-gauche forms are characterized and 1899(3)/1883(3)(79Br/81Br)MHz for the fluoride, by B t C values of 4114(7), 2804(3)/2746(5) (35C1/37C1), chloride, and bromide, respectively. Butyryl fluoride also displays band series from a skew-anti ( T ~ ( O C C C ) = 120°, T,(CCCC) = 180")conformer characterized by a B t C value of 3524(6) MHz. In all cases, the syn-anti and syn-gauche conformers have approximately equal stability. The skew-anti conformer of butyryl fluoride is marginally less stable than the syn-anti and syn-gauche forms.
Introduction Five stable conformers of butyryl halides may reasonably be postulated. They are designated syn-anti, syn-gauche, skew-anti, skew-gauche, and skew-gauche' according to their configuration about the OCCC torsion (syn (rl(OCCC) = 0'1, skew (71(OCCC) = 120') and about the CCCC torsion (anti ( T ~ ( C C C C=) B O " ) , gauche (r2(CCCC) 60"). Proposed conformations are displayed and torsional angles are defined in Figure 1. The conformations of propanal, propionyl halides, and propionic acid have been previously determined. Syn conformers of p r ~ p a n a lpropionyl ,~~~ f l ~ o r i d epropionyl ,~ ~ h l o r i d eand , ~ propionic acid6 have been assigned by high resolution microwave spectroscopy. Higher energy skew conformers of propanal (relative energy, 900 (100) cal/mol) and propionyl fluoride (relative energy, 1280(40) cal/mol) have also been observed. The presence of a skew conformer of propionyl chloride could not be established from microwave spectral data. Low resolution microwave (LRMW) spectroscopy is a powerful technique for conformational analysis of polar, prolate, volatile species. This study was undertaken to deduce the conformational species present in butyryl halides and to determine if results obtained for propionyl halides extend to homologous systems. This study determines the rotational isomers present in gaseous butyryl fluoride, chloride, and bromide and estimates their relative stabilities. Experimental Section All microwave measurements were made on a Hewlett-Packard Model 8460-A microwave spectrometer. Spectra of butyryl fluoride and bromide were obtained from 26.5 to 39 GHz and spectra of butyryl chloride were obtained from 18 to 39 GHz. For butyryl fluoride and chloride, spectra were recorded at 25 and a t ---63 "C. A spectrum of butyryl bromide was recorded a t 25 "C. In all cases the Stark voltage was 3200 V/cm and scan rates were 5 or 10 MHz/s with a 1-s detector time constant. Samples were distilled into the waveguide to pressures of 60-80 mTorr. In order to minimize errors in intensity measurements the detector crystal current was kept constant while each spectrum was being recorded. Samples of butyryl bromide and, to a lesser extent, butyryl chloride 0022-365417812082-0480$0 1.OOlO
tended to decompose in the waveguide necessitating frequent sample changes. 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 25 MHz for the samples studied. Samples of butyryl chloride and butyryl bromide were purchased from Aldrich Chemical Co. Butyryl chloride was fractionally distilled prior to use. Butyryl fluoride was prepared by direct exchange between butyryl chloride and thallous fluoride6 and was fractionally distilled under reduced pressure prior to use. Samples of butyryl chloride and fluoride were characterized by their 'H NMR and infrared spectra and their purity was established to be in excess of 98% by gas-liquid chromatograms on a 6-ft. 10% SE-30 column with 5-10-min retention times. The LRMW spectrum of butyryl bromide indicated the sample was contaminated with butyryl chloride.
Results Butyryl Fluoride. Condensed LRMW R-band spectra of butyryl fluoride, shown in Figures 2A and 2B, display three a-type band series a t both 25 and --63 "C. B + C values are 3359(5), 4114(7), and 3524(6) MHz for conformers designated syn-anti, syn-gauche, and skew-anti, respectively. A fourth weak series attributed to the syn-gauche conformer is spaced a t constant 3536(3)-MHz intervals. Bands of this series index to integers +0.72 consistent with a type I1 band ~ e r i e s .The ~ corresponding 3536(3)-MHz spacing is approximately 2C. E and C for the syn-gauche conformer are therefore 2346(9) and 1768(3) MHz, respectively. A-type bands of the syn-anti and syn-gauche series are approximately 200-MHz wide. Bands of the skew-anti series are approximately 150-MHz wide. A t 25 "C the relative intensity ratio syn-anti:syn-gaucheskew-anti is 1:1:0.25 and a t --63 "C it is 1:1:0.2 indicating that the skew-anti conformer is marginally less stable than the syn-gauche and syn-anti species. Butyryl Chloride. Low resolution microwave spectra of butyryl chloride a t 25 and --63 'C display four a-type band series compatible with resolved chlorine isotopic species of two conformers. The conformers, designated syn-anti and syn-gauche, have B + C values 2446(3) 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Conformations of Butyryl Fluoride, Chloride, and Bromide
481
TABLE I: Rotational Constants, Band Intensities, and Relative Energies o f the Conformers of Butyryl Halides SA SG SkA B + C, MHz Re1 int (25 "C) Re1 int (-- 63 ' C) Re1 energy, kcal/mol
a. 'Ti- 0"
7i=l2O0
SY N
3523 ( 6 )
1
1
0.25
1
1
0.20
0
-0
0.3 (0.5)
Butyryl Chloride 2804 ( 3 ) /2746 ( 5 )
SKEW
2446 ( 3 ) B t C, /2399 ( 3 ) M H (35"cii ~
b.
3 7 ~ 1 )
Re1 int (25 " C ) Re1 int (m- 63 C) Re1 energy, kcal/mol
72.1 80"
72- 70"
GAUCHE
ANTI
Figure 1. Conformations of butyryl halides. (a) Newman projections of 7, = ' 0 and 7, = 120'. 7, = '0 when the ethyl group is syn eclipsed with the carbonyl group. (b) Newman projections of T~ = 70' and T~
18""
l 35
Butyryl Fluoride 4114 ( 7 ) 3359 ( 5 )
\
l
l
l
\
l
l
~
3 0 GHz
I
20"
I
SYn-
anti
I
1
0.4
1
0.4
0
-0
Butyryl Bromide B f C, 1720 ( 3 ) 1899 ( 3 ) MHz (79Br/ /1704 ( 3 ) /1883 ( 3 ) **Br) Re1 int 1 1 (25 " C ) Re1 int 1 1 (--63°C) Re1 energy, 0 -0 kcal/mol
+
"
I
'
"
I
30GHz
I
I
I
Figure 2. LRMW survey spectra of butyryl fluoride. (a) R band spectrum from 26.5 to 36 MHz swept at 10 MHz/s with a 1-s time constant. The band markers are calculated from ( J 4- l)(B C ) with B 4- C = 3359, 4114, and 3523 MHz for the syn-anti, syn-gauche, and skew-anti conformers, respectively. Type I1 bands are calculated from ( J 0.72)(3536). (b) Same as (a) except the temperature is --63 "C.
+
(2399(3), 37Cl) and 2804(3) (2746(5), 37Cl)MHz, respectively. Bands of both series are approximately 200-MHz wide. At both 25 and --63 'C the relative intensity ratio syn-anti:syn-gauche is 1:0.4 indicating that the two conformers are approximately equally stable. Butyryl Bromide. Four a-type band series corresponding to resolved bromine isotopic species of two conformers are present in LRMW R-band spectra of
butyryl bromide. B C values for the two conformers designated syn-anti and syn-gauche are 1720(3) (1705(3), 81Br)and 1899(3)(1883(3),'lBr) MHz, respectively. Bands of all four series are approximately 150-MHz wide. Accurate intensity measurements were precluded by rapid sample decomposition in the waveguide. The decomposition products, presumably butyric acid and HBr, do not yield LRMW spectra. The series have roughly equal intensity a t 25 'C. LRMW spectral data of butyryl fluoride, chloride, and bromide are summarized in Table I.
Discussion Characterization of the configurations of the various conformers is done by matching the observed B + C values with those calculated from models. Structural parameters consistent with Ao, Bo, and Co values reported in high resolution microwave studies of propionyl fluoride3 and chloride4 and structural parameters of acetyl bromide obtained from an electron diffraction investigation* were used in these calculations. A methylene to methyl carbon distance of 1.528 A and CCC angles of 114' were used for each butyryl halide. For each compound B + C was calculated as a function of T~(CCCC) holding T~(OCCC) a t 0 (syn) or 120' (skew). All other geometrical parameters except r2 were held constant. Results of these calculations for butyryl fluoride, chloride, and bromide are displayed graphically in Figures 3-5, respectively. Lower curves display the variation of B + C with TJCCCC) holding T ~ ( O C C Cfixed ) in the syn position and the upper curves display the variation of B + C with T~(CCCC)holding Tl(0cCc) fixed in the skew configuration. Horizontal lines represent observed B + C values and error bars indicate an estimated 2% uncertainty in the calculated B + C values exclusive of the torsional angles. The uncertainties in the calculated B + C values are arbitrarily assigned to the experimental B + C values solely for easy visualization. The experimental uncer-
402
L. P. Thomas, N. S. True, and R. K. Bohn
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
0"
60" 120" 180" 240" 300" 360"
72 (CC
T2(ccc c)
+
+
Figure 3. Observed and calculated B Cvaiues of butyryl fluoride. The horizontal lines correspond to observed B Cvalues. The two curves display calculated 5 C values as a function of the torsional angle T~(CCCC) for T~(O=CCC) = 0' (lower curve) and T,(O=CCC) = 120' (upper curve). The closed circles indicate the assigned configurations. The error bars correspond to an estimated 2% uncertainty in the calculated values due to uncertainties in the assumed structural parameters.
+
+
.
0"
60" 120" 180" 240" 300'
72 (CCc
cc)
c)
360"
Figure 4. Observed and calculated values of B -I- Cfor the 35CIand 37CIisotopic species of butyryl chloride. The horizontal lines correspond to observed B+ Cvalues. The lower curves display B+ Cas a function of T~(CCCC)with T , = 0'. The upper curves are similar except T , = 120'. The closed circles indicate the assigned configurations. The error bars correspond to an estimated 2% uncertainty in the calculated values due to uncertainties in the assumed structural parameters.
tainties of a few MHz are invisible on the scale of these diagrams. For each butyryl halide the B C value of the syn-anti conformer is consistent with a T~(OCCC)= O', T~(CCCC)= 180' structure. The skew-anti conformer of butyryl fluoride is consistent with a T~(OCCC) 120°, T~(CCCC)= 180' structure. For the butyryl halides, and especially the fluoride, calculated B + C values are very sensitive to T~(CCCC).B C values of the syn-gauche conformers of all three butyryl halides are consistent with a T~(OCCC) = O', T~(CCCC) BO' structure. As the error bars in Figures 3-5 indicate, the uncertainties in the 7 2 values are rather large, -20'. Since the major source of
+
-
+
-
Figure 5. Observed and calculated B C values of the "Br and "Br isotopic species of butyryl bromide. The horizontal lines correspond to observed B+ Cvaiues. The lower curves display B+ Cas a function of s2(CCCC)with T~ = . '0 The upper set of curves are similar except T , = 120'. Closed circles indicate the assigned configurations. The error bars correspond to an estimated 2% uncertainty in the calculated values due to uncertainties in the assumed structural parameters.
uncertainty is the assumed values of the structural parameters, and since the error is probably almost the same in each compound, the results do demonstrate that T~ is about the same in each compound. Of the butyryl halides studied only the fluoride produced spectra consistent with the presence of a skew conformer. These results parallel previous findings for propionyl halides. The skew conformer of propionyl fluoride has been assigned3 but the skew conformer of propionyl chloride has not.4 Interestingly, the skew-anti conformer of butyryl fluoride is only marginally less stable than the syn-anti and syn-gauche conformers whereas skew propionyl fluoride is 1.3 kcal/mol higher in energy than the syn form. Of the five proposed conformers of butyryl halides only three have been observed. Skew-gauche and skew-gauche' conformers have not been observed for any of the butyryl halides studied. In each butyryl halide the skew-gauche conformers are calculated to have much more positive K = (2B - A - C / A - C) values than the conformers observed, ranging from -0.73 for the fluoride to 0.0 for the bromide, and are therefore unobservable to LRMW spectros~opy.~ Thus, their presence cannot be ruled out by this study. The skew-gauche' conformations (T~(OCCC) 120°, 72(CCCC) 300') have K values estimated to be -0.89, -0.71, and -0.78 for butyryl fluoride, chloride, and bromide, respectively. The absence of band series indicates that these forms are not present in significant concentrations.
-
-
Acknowledgment. The authors are grateful to Professor E. Bright Wilson of Harvard University for granting use of his microwave spectrometer supported by NSF Grant GP-37066X. Computations were carried out a t the University of Connecticut Computer Center. Supplementary Material Available: Tables 11-IV containing band frequencies, J 1 values, and B + C values for butyryl fluoride, chloride, and bromide (3 pages). Ordering information is available on any current masthead page.
+
References and Notes (1) Paper 10 of this series: N. S. True and R. K. Bohn, J. Phys. Chem.,
preceding paper in this issue.
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).
483
(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 characterized by 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 = O', Q(CSCC) go', Q(SCCC) thioester, the CA conformer is compatible with a syn-gauche-anti (.T.~(OCSC) = 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.
- - -
-
-
0022-3654/78/2082-0483$0 1.OO/O
--
-
-
TABLE I: Observed Conformers of Ethyl and n-Propyl Oxy and Thioesters Formate 0
Thio formate
0 I1 X-C-S-R
Ii
X
X-C-0-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
