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Internal Rotation and the Chlorine Nuclear Quadrupole Coupling Tensor of 1-Chloropropane
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
184, 60–77 (1997)
MS977298
Internal Rotation and the Chlorine Nuclear Quadrupole Coupling Tensor of 1-Chloropropane Ana de Luis, M. Eugenia Sanz, Felipe J. Lorenzo, Juan C. Lo´pez, and Jose´ L. Alonso 1 Departamento de QuıB mica FıB sica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain Received November 1, 1996; in revised form March 7, 1997
The rotational spectra of the trans and gauche forms of 1-chloropropane have been analyzed in the frequency range 8–40 GHz using Stark, waveguide FTMW, and pulsed molecular beam FTMW spectrometers. The spectra of 35Cl, 37Cl, and the three 13C isotopomers have been observed for both conformers. The complete quadrupole Cl-coupling tensors have been determined and discussed within the structural considerations. For the gauche form the barrier to internal rotation of the methyl group has been determined to be 2908(4) cal/mol from A–E splittings observed in the first excited torsional state. Two A–E splittings have been observed only in the ground state of the trans form. From these splittings a barrier of 2760 cal/mol has been estimated. q 1997 Academic Press
group. Furthermore the precision of the frequency measurements achieved with these techniques allows in many cases the determination of the off-diagonal elements of the quadrupole coupling tensor. The principal axis coupling tensor elements can then be easily calculated by diagonalization.
INTRODUCTION
1-Chloropropane has been investigated by both rotational (1–5) and vibrational (6–8) spectroscopy and by gas electron diffraction (5, 9). These studies have shown that 1chloropropane exists in two stable rotational isomers, trans and gauche (Fig. 1). The microwave spectrum of both conformers, using conventional Stark modulation spectroscopy, was first studied by Sarachman ( 1 ) , who reported the rotational and quadrupole coupling constants for the 35Cl and 37Cl isotopomers of the gauche form and the B and C rotational constants of the trans form. This author also observed the spectra of the first two excited states of the CH2Cl – C skeletal torsion and the first excited state of the methyl torsion vibration for the gauche form as well as the first excited state of the skeletal torsion for the trans form. The centrifugal distortion effect for the gauche conformer was later studied by Kaushik ( 3, 4 ) . The structure was analyzed by Kuchitsu and co-workers ( 5 ) using a joint analysis of electron diffraction and rotational data. They reported rotational, quadrupole coupling, and some quartic centrifugal distortion constants for the 35Cl and 37 Cl isotopomers of the trans form and the electric dipole moment for both conformers. In neither case have A – E splittings due to the internal rotation of the methyl group been observed. We decided to reinvestigate the spectra of the two rotational isomers of 1-chloropropane with the higher resolution and sensitivity of the currently available microwave Fourier transform techniques to resolve the methyl torsion fine structure and determine the barrier hindering internal rotation of the methyl 1
EXPERIMENTAL
A commercial sample of 1-chloropropane, in natural isotopic composition, was used without further purification. The microwave spectrum was recorded in the region 8–40 GHz using different spectrometers. Computer-controlled Stark modulation spectrometers (10, 11) were used for the range 26.5–40 GHz. The Stark cell was cooled at temperatures of about 260 K with sample pressures of 10–20 mTorr. Radiofrequency microwave double resonance (RFMWDR) (12) was also used for the assignment of excited state spectra of the trans form. The accuracy of frequency measurements is estimated to be better than 50 kHz. A waveguide Fourier transform microwave spectrometer (WG-FTMW) (13) covering the frequency range 8–18 GHz was used to investigate the quadrupole coupling hyperfine structure and to search for internal rotation splittings in the ground state spectra. Microwave pulses of 20–120 ns duration with peak power of 1–5 W polarized the molecules in the 12m waveguide cell. The spectra were taken at pressures down to 1 mTorr with temperatures of about 240 K. The frequency measurements have an estimated accuracy of 10 kHz. The spectra of the 37Cl and the 13C isotopomers were observed in natural abundance using a pulsed molecular beam Fourier transform microwave spectrometer (MB-FTMW) (14). The sample entering the nozzle, mounted in one of the cavity mirrors, was approximately 1% 1-chloropropane and 99% argon at pressures of 1–2 atm. The gas expansion leads to extreme
To whom correspondence should be addressed. 60
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FIG. 1. The trans (a) and gauche (b) forms of the 1-chloropropane molecule.
vibrational and rotational cooling. Only the ground vibrational state for rotational levels with a low J quantum number is populated. This simplifies the spectrum and allows the observation of low natural abundance isotopes. As the molecular beam traveled parallel to the direction of microwave propagation, all lines were doubled by the Doppler effect. Linewidths of ca. 7 kHz were obtained with an accuracy of frequency measurements better than 5 kHz.
RESULTS AND DISCUSSION
Ground State Spectra 1-Chloropropane is a prolate asymmetric rotor for both trans ( k Å 00.99) and gauche ( k Å 00.90) conformers. Cs symmetry constrains the trans form electric dipole moment to lie along the ab inertial plane. All three electric
FIG. 2. WG-FTMW spectrum of the 8 0,8 R 7 1,7 transition for the ground state of trans-1-chloropropane showing a methyl internal rotation A – E splitting for each quadrupole coupling component. A 7-MHz section out of the 50-MHz range of the power spectrum is given. Number of cycles, 5 million ( measuring time 198 s ) at a sample interval of 10 ns. The 1024 data points were supplemented by 3072 zeros prior to the FFT transformation.
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TABLE 1 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for 35Cl and 37Cl Species of trans-1-Chloropropane in the Ground Vibrational State
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TABLE 1 —Continued
dipole moment components are different from zero for the gauche form but mc is small (5) and c-type lines have not been observed. Initial measurements of the 35Cl spectra were made using the Stark spectrometers. Predictions were based on the rotational and quadrupole coupling parameters previously reported (1, 5). The ground state rotational spectra for both conformers were easily identified and measured, but quadrupole coupling hyperfine structure was not completely resolved by the Stark modulation spectrometers used here. The waveguide Fourier transform spectrometer which has a superior resolution was subsequently used in order to resolve the quadrupole coupling structure. An additional splitting was observed for two b R transitions of the trans form ground state spectra (see Fig. 2). This splitting was attributed to the internal rotation of the methyl group. Each nuclear quadrupole coupling component was split into two components corresponding to the torsional A and E levels. No A–E splittings were observed for the gauche conformer. In the initial stages of the analysis, centrifugal distortion and quadrupole coupling were treated separately. First the quadrupole coupling constants xaa ( a Å a, b, c) were determined from the quadrupole splittings and then a set of center frequencies was calculated. These center frequencies were then fitted to derive the rotational and centrifugal distortion constants. In the final calculations we considered all the observed individual transitions which were fitted using the CALPGM program of Pickett (15). This program allows the direct diagonalization of the Hamiltonian including cen-
trifugal distortion and quadrupole coupling operators. The A-reduced semirigid rotor Hamiltonian of Watson in the I r representation was selected (16). Lines measured with the waveguide FTMW spectrometer were assigned uncertainties of 10 kHz. For the Stark spectrometers measurement uncertainties of 50 kHz were used for resolved hyperfine components and 100 kHz for the unresolved components. The observed frequencies of hyperfine components are given in Tables 1 and 2. All the measured components were first fitted to a Hamiltonian including only the diagonal elements of the quadrupole coupling tensor. For the trans form all the measured lines fit to this Hamiltonian within the estimated uncertainties with the exception of some components of the transitions 21,2 R 11,1 and 30,3 R 20,2 . These transitions were measured using the FTMW spectrometer and showed deviations between 100 and 300 kHz from the calculated frequencies. We calculated the energy of the levels involved in these transitions in order to search for near degeneracies giving rise to second-order quadrupole coupling effects. The difference in energy between the 11,1 and 30,3 levels was found to be 119 MHz and these levels can be connected by c-type symmetry operators. This is just the symmetry of the only nonzero off-diagonal element of the quadrupole coupling tensor xab . When we considered this element in the Hamiltonian a good fit was obtained for all the measured components and ÉxabÉ was determined with a standard error of 5%. Using the high resolution and accuracy of the MB-FTMW instrument to perform new measurements of low- J transitions, the
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DE LUIS ET AL.
TABLE 2 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for 35Cl and 37Cl Species of gauche-1-Chloropropane in the Ground Vibrational State
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TABLE 2 —Continued
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TABLE 3 Rotational Constants, Quartic Centrifugal Distortion Constants, and Chlorine Quadrupole Coupling Constants for Different Isotopomers of trans-1-Chloropropane in the Ground Vibrational State
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TABLE 4 Rotational Constants, Quartic Centrifugal Distortion Constants, and Chlorine Quadrupole Coupling Constants for Different Isotopomers of gauche-1-Chloropropane in the Ground Vibrational State
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FIG. 3. F Å 7/2 R 5/2, 5/2 R 3/2, and 1/2 R 1/2 quadrupole coupling components of the 20,2 R 10,1 transition for the 13C3 isotopomer of gauche1-chloropropane recorded using an MB-FTMW spectrometer. Sample, 0.5% in argon. Stagnation pressure, 1 atm. Number of cycles, 1665. Doppler effect splitting, 40 kHz.
ÉxabÉ quadrupole coupling element was finally determined with a standard error of 0.4%. This spectrometer has been also used to measure the spectra of the 37Cl species. The spectroscopic constants derived for the trans form are given in Table 3. The centrifugal distortion constant dK was found not determinable from the set of observed transitions and was fixed to zero. For the 37Cl species only transitions observed in this work were fitted and the centrifugal distortion constants DK and dK were fixed to the values found for the 35Cl isotopomer. The final results for the gauche conformer are presented in Table 4. In this case the quadrupole coupling tensor has the three off-diagonal elements different from zero. We performed different fits of the spectrum considering separately each off-diagonal element in order to see if they were determinable. The inclusion of the elements ÉxabÉ and ÉxbcÉ led to small improvements in the quality of the fit. No improvement in the standard deviation of the fit was found when considering ÉxacÉ but it was determined with a 25% error. In Table 4 the results of the fit including all three elements are presented. The reliability of the values obtained is discussed below. For the 37Cl species the centrifugal distortion constants were fixed to the 35Cl species values. Based on the assignment of the 35Cl species and an assumed structure, the isotopic shifts for the 13C species could be readily predicted. Using the MB-FTMW spectrometer, the rotational and vibrational cooling greatly simplified the spectrum and the transitions were easily found. Figure 3 shows one of these transitions. The com-
plete set of assigned components are listed in Table 5 and the derived rotational and quadrupole coupling constants in Tables 3 and 4. Only few transitions were measured so the centrifugal distortion constants and the off-diagonal elements were fixed to the 35Cl values in all the cases. For the 13C3 species of the trans conformer we also fixed in the fit the value of ( xbb 0 xcc ) . The planar moments Pcc Å Si mi c 2i listed in Table 3 for the trans conformer provide some insight into the symmetry of the molecule. When isotopic substitution occurs in a symmetry plane, the planar moment is not affected by substitution in that plane. The near identity of the Pcc value for all the isotopomers can be seen. This result implies that all isotopic substitutions occur in a symmetry plane. It is not possible to obtain substitution structures for the heavy atom skeleton due to the proximity of the Cl and C atoms to the inertial axes of both conformers (see Fig. 1). The structures reported by Kuchitsu and co-workers (5) reproduce the experimental rotational constants obtained in this work with only small changes within the quoted errors in Ref. (5). We have used these structures in the discussion about the nuclear quadrupole coupling tensor and in the calculations to determine the methyl group internal rotation barrier. Quadrupole Coupling Tensor The observed quadrupole coupling constants shown in Table 3 for the trans conformer refer to the principal inertial
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TABLE 5 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the 13C Isotopomers of trans- and gauche-1-Chloropropane
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TABLE 6 Quadrupole Coupling Tensor of trans-1-Chloropropane in the Inertial and Main Axis Systems
a
Standard error in parenthesis in units of the last digit. c principal inertial axis and y gradient field axis have been assumed to be coincident. c Angle, in degrees, between the z gradient field axis and the a principal inertial axis. d Angle, in degrees, between the C1-C1 bond and the a principal inertial axis. b
axis system. There is only one nonzero off-diagonal constant ÉxabÉ (the sign cannot be determined) because of the Cs symmetry. Note that the ratio xcc ( 35Cl)/ xcc ( 37Cl) is 1.2687(17) in excellent agreement with the ratio of the nuclear quadrupole moments 35Q/ 37Q Å 1.26878 (17). The xcc constants determined for the 13C1 and 13C2 isotopomers are also equal within the quoted errors to that determined for the 35Cl species. For direct comparison of quadrupole coupling constants of different chlorine-containing molecules it is necessary to transform the coupling tensor into its own principal axis system. The results of this transformation for the trans conformer are shown in Table 6. A rotation of an angle uz ,a around the c principal axis must be performed in this case. The coupling constant xcc is equal to one of the diagonal values, say xyy . The angle ua between the a inertial axis and the C–Cl bond axis is that reported from the structure by Kuchitsu and co-workers (5). The angles uz ,a and ua are the same within the quoted errors and the z axis may be considered to lie along the C–Cl bond. The small value of the quantity h Å ( xxx 0 xyy )/ xzz which is a measure of the deviation from the cylindrical symmetry of the x tensor
around the z axis indicates that the x tensor of chlorine has nearly axial symmetry around the C–Cl bond. The magnitude of the coupling found here for the trans form of 1chloropropane is typical for 35Cl in molecules with single bonds (18). The values also compare very well with those found for related molecules as 1-chloroethane (19), 1chloro-1-fluoroethane (20), 1-chloro-1,1-difluoroethane (21), chloromethane (22), chlorofluoromethane (23), and chlorodifluoromethane (24). For the gauche form the off-diagonal elements of the quadrupole coupling tensor have been determined for the 35 Cl and 37Cl species (see Table 4), but their values are not precise. The diagonalization of the quadrupole tensor gives values of xxx , xyy , and xzz consistent with those determined for the trans form when xac is given a sign opposite to that of xab and xbc . However, the errors of these constants are very high as a consequence of the imprecision of the ÉxabÉ, ÉxbcÉ, and ÉxacÉ values. In order to test the reliability of the quadrupole coupling constants determined for the gauche form, these have been calculated from quadrupole coupling constants xxx , xyy , and xzz calculated for the trans form and the structure given in
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TABLE 7 Quadrupole Coupling Tensor Elements of trans- and gauche-1-Chloropropane in the Inertial Axis System Calculated from the Principal Quadrupole Coupling Constants of the trans-Conformer (Table 6) and the Structure of Ref. ( 5)
TABLE 8 Observed and Calculated Differences between the 13C and Parent Species Quadrupole Coupling Tensor Diagonal Elements
a
Dxaa Å xaa ( 13C) 0 xaaa ( 35C1) Calculated from the values in Tables 3 and 4. c Calculated from the values in Table 7. b
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TABLE 9 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the First Methyl Torsion Excited State of gauche-1-Chloropropane
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TABLE 10 Spectroscopic Constants for the First Methyl Torsion Excited State of gauche-1-Chloropropane A/MHz B C
11882.355(79)a 3309.482(11) 2844.856(12)
DJ /kHz DJK DK dJ dK
2.35(20) 013.98(41) 143.8(97) 0.5216(55) 8.55(39)
xaa /MHz (xbb-xcc)
018.3(11) 040.46(24)
Nb J max sc/MHz
83 20 0.069
xbb /MHz xcc
011.1(17) 29.4(17)
a
Standard error in parenthesis in units of the last digit. Number of fitted quadrupole components. c Standard deviation of the fit. b
Ref. (5) assuming that the z axis is collinear with the C– Cl bond. The same calculations have been performed for the different isotopomers of both forms. For the 37Cl species the quadrupole coupling factor 35Q/ 37Q Å 1.26878 (17) has been used. The results of these calculations are presented in Table 7. For the trans form the xcc element has been omitted in the table. The calculated values for the gauche form are consistent with the observed values given in Table 4, indicating the reliability of the determined off-diagonal quadrupole coupling constants. The same conclusion comes from the comparison between the observed and calculated values of both conformers for the 37Cl species. For the 13C isotopomers only the diagonal elements of the quadrupole coupling tensor have been determined. These elements show shifts with respect to the values determined for the parent species which are noticeable for the gauche form. The observed and calculated differences between the diagonal quadrupole coupling constants of the 13C species and those for the normal species are compared in Table 8. The agreement between the observed and calculated values is excellent, and may be taken as a proof for the assignments of the quadrupole hyperfine structure in the 13C species spectra for which only a few transitions have been measured. Vibrational Excited States One of the main purposes of this work was to determine the methyl group internal rotation barrier of 1-chloropropane. For the gauche form we concentrated our efforts on
the excited state assigned by Sarachman ( 1 ) to the first excited methyl torsion state. The rotational spectrum of this state was assigned using Stark spectroscopy on the basis of the rotational constants previously reported ( 1 ) by assuming the same quadrupole coupling constants as the ground state. A few A – E splittings due to internal rotation of the methyl group were observed for high- J bQ lines, confirming the assignment of this state to the methyl torsion ( 1 ) . The observed hyperfine components are shown in Table 9 and the corresponding rotational parameters are listed in Table 10. These parameters have been obtained by using the A-level components for the lines showing internal rotation splittings. Since only two A–E splittings were observed in the ground state of the trans conformer, the excited vibrational states have been investigated. With the help of RFMWDR, we assigned the a R spectra of three excited states which were initially labeled A, B, and C. The state labeled A was the most intense and occurs on the high-frequency side of the ground state transitions. The state labeled B was found close to the ground state lines at higher frequencies. The state C is the weakest and occurs on the low-frequency side. No A–E splittings were observed for the a R lines belonging to these states. Trials to assign the bQ spectra using the WGFTMW spectrometer failed due to their weakness and the presence of many lines arising from the quadrupole hyperfine structure. The observed hyperfine components are given in Table 11 and the rotational parameters obtained for the states A, B, and C of trans-1-chloropropane are given in Table 12.
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TABLE 11 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the Excited States of trans-1-Chloropropane
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TABLE 12 Spectroscopic Constants for the Excited States of trans-1-Chloropropane
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TABLE 13 Internal Rotation Parameters for IAM Analysis of the Ground State of trans-1-Chloropropane (a) and the First Methyl Torsion Excited State of gauche-1-Chloropropane (b)
r b (rad) g (rad) ˚ 2) Ia (amu A F (GHz) V3 (cal mol01) s s b (kHz) nc
TABLE 15 V3 Barriers to Internal Rotation for Some Propane Derivatives V3 (cal/mol) CH3CH2CH3 CH3CH2CH2F (gauche)
(a)
(b)
[0.15261] a [0.03620] [0.] [3.21] [183.7] 2760.(20)d,e 70.0(5) 12.0 2
[0.03542] [0.57303] [0.32317] [3.23] [166.234] 2908.(4)d 84.1(1) 32.0 6
a
Parameters in square brackets were kept constant in the fit. Standard deviation of the fit. c Number of splittings used in the fit. d As the necessary structural parameters were kept fixed in the fit, the calculated errors do not reflect possible errors introduced by the assumed structure. e The barrier for the trans form has been obtained from only two A–E splittings and should be considered as an estimate of V3 .
CH3CH2CH2F (trans) CH3CH2CH2OH (trans) CH3CH2CH2CN (gauche) CH3CH2CH2CN (trans) CH3CH2CH2NC (gauche) CH3CH2CH2NC (trans) CH3CH2CF3 CH3CH2CH2Cl (gauche) CH3CH2CH2Cl (trans)
3166 2758 2874 2710 2719 2730 3107 3110 2894 2954 2635 2908 2760
(27) a (35) b (24) b (8) b (53) b (60) c (24) d (38) d (23) e (22) e (4) f (4) g (20) g
State vt vt vt vt vt vt vt vt vt vt vt vt vt
Å Å Å Å Å Å Å Å Å Å Å Å Å
0 0 1 0 1 0 0 0 0 0 0 1 0
b
The state labeled A is the most intense and was assigned by Sarachman (1) to the lowest frequency mode of the molecule, the CH2Cl–C torsion, observed at 130 cm01 in the gasphase Raman spectrum (7). The increment in the Pcc value with respect to the ground state is consistent with the motion of the Cl atom out of the ab plane. A similar behavior has been found for the planar moment Pcc for the equivalent C– C torsion in trans-1-fluoropropane (25). The methyl torsion vibration has been assigned for the trans form to a vibrational band observed at 231 cm01 (6) and the C–C–Cl
TABLE 14 Observed A–E Splittings and Differences (in MHz) with the Calculated Values for the Ground State of trans-1-Chloropropane and the First Methyl Torsion Excited State of gauche-1-Chloropropane J*
Ka*
Kc*
J9
R
Ka9
K9c
A–E
Diff
00.1774 00.1612
00.0087 00.0087
01.291 01.557 01.903 00.700 01.026 01.437
00.047 0.002 00.016 0.024 0.041 0.017
Trans form, ground state 7 8
0 0
7 8
6 7
1 1
6 7
Gauche form, vt Å 1 14 15 16 18 19 20
2 2 2 3 3 3
12 13 14 15 16 17
14 15 16 18 19 20
1 1 1 2 2 2
13 14 15 16 17 18
a G. Bestmann, W. Lalowski and H. Dreizler, Z. Naturforsch. A, 40, 271– 273 (1985). b Ref. [27]. c H. Dreizler and F. Scappini, Z. Naturforsch. A, 36, 1187–1191 (1981). d K. Vormann and H. Dreizler, Z. Naturforsch. A, 43, 338–344 (1988). e M. Kru¨ger and H. Dreizler, Z. Naturforsch. A, 47, 1067–1072 (1992). f S. Antolı´nez J. C. Lopes and J. L. Alouso, J. Chem. Soc. Faraday Trans. 93, 1291–1295 (1997). g This work.
deformation to bands observed close to 242 cm01 (6, 7). From their intensities the B and C states may be tentatively assigned to the CH3 torsion and C–C–Cl deformation, respectively. Methyl Group Internal Rotation Barrier The calculation of the barrier hindering internal rotation of the methyl group has been carried out independently using the internal axis method in the form given by Woods ( 26 ) . Observed A – E splittings are present for all the quadrupole components of each rotational transition so an arithmetic mean of the splittings for each quadrupole component was considered. For both conformers the parameters r, b, and g were constrained to the values arising from the reported structures ( 5 ) . The results obtained are summarized in Table 13. The mean splitting values are given in Table 14. The barrier height of V3 Å 2760(20) cal/mol for the ground state of the trans form should be taken as an estimate value since it has been derived from only two splittings. Both values for the trans and gauche conformers are compared to those obtained for other propane derivatives in Table 15. The barrier V3 Å 2908(3) cal/mol determined for the first excited torsional state of the gauche form is about 150 cal/ mol higher than the barrier obtained for the ground state of the trans form. This difference may arise from several factors. One of them is the fact that the value for the trans form was obtained from only two transitions. A second factor to
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consider is that V3 for the gauche form has been derived from A–E splittings in the first excited torsional state. In Table 10 the barriers determined for trans- and gauche-1fluoropropane (27) in the ground and first torsional excited states are given. These barriers have been determined using the same method used in the present work. For 1-fluoropropane (27) the barriers determined for the ground and £t Å 1 states of the trans form and for the ground state of the gauche form have about the same value. However, the barrier calculated for the £t Å 1 state of gauche-1-fluoropropane is about 125 cal/mol higher than the value for the ground state. This difference was discussed in Ref. (27) by an approximate treatment of the coupling between CH3 and C– C torsional motions. Using the values of F and V3 , the harmonic oscillator approximation ( 28 ) results in torsional oscillation frequencies of 235 cm01 for trans-1-chloropropane and 228 cm01 for gauche-1-chloropropane. The torsional frequency for the trans form is in good agreement with that of 231 cm 01 observed from infrared measurements ( 6 ) . However, the wavenumbers observed for the torsional fundamental of gauche-1-chloropropane from vibrational spectroscopy occur at values around 214 cm01 ( 6, 7 ) . An interaction between methyl torsion and other low-frequency skeletal vibrations is possible and may be the cause of these discrepancies. ACKNOWLEDGMENTS Research funds from the Direccio´n General de Investigacio´n CientıB fica y Te´cnica (DGICYT, Grant PB93-0224) and the Junta de Castilla y Leo´n (Grant VA19/94) are gratefully acknowledged.
REFERENCES 1. T. N. Sarachman, J. Chem. Phys. 39, 469–473 (1963). 2. W. E. Steimetz, F. Hickernell, I. K. Mun, and L. H. Scharpen, J. Mol. Spectrosc. 68, 173–182 (1977).
3. V. N. Kaushik, Spectrochim. Acta A 33, 463–466 (1977). 4. V. N. Kaushik, Spectrochim. Acta A 35, 851–855 (1979). 5. K. Yamanouchi, M. Sugie, H. Takeo, C. Matsumura, and K. Kuchitsu, J. Phys. Chem. 88, 2315–2320 (1984). 6. K. Tanabe and S. Sae¨ki, J. Mol. Struct. 27, 79–96 (1975). 7. Y. Ogawa, S. Imazeki, H. Yamaguchi, H. Matsuura, I. Harada, and T. Shimanouchi, Bull. Chem. Soc. Jpn. 51, 748–767 (1978). 8. M. Rasanen and V. E. Bondybey, J. Phys. Chem. 90, 5038–5044 (1986). 9. Y. Morino and K. Kuchitsu, J. Chem. Phys. 28, 175–184 (1958). 10. A. G. Lesarri, M. E. Charro, R. M. Villaman˜a´n, D. G. Lister, J. C. Lo´pez, and J. L. Alonso, J. Mol. Spectrosc. 149, 317–328 (1991). 11. A. G. Lesarri, J. C. Lo´pez, and J. L. Alonso, J. Mol. Struct. 273, 123– 131 (1992). 12. F. J. Wodarczyk and E. B. Wilson, Jr., J. Mol. Spectrosc. 37, 445– 463 (1971). 13. J. L. Alonso, A. Lesarri, S. Mata, J. C. Lo´pez, J.-U. Grabow, and H. Dreizler, Chem. Phys. 208, 391–401 (1996). 14. J. L. Alonso, F. Lorenzo, J. C. Lo´pez, A. Lesami, S. Mata, and H. Dreizler, Chem. Phys., in press. 15. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991). 16. J. K. G. Watson, in ‘‘Vibrational Spectra and Structure’’ (J. R. Durig, Ed.), Vol. 6, pp. 1–89, Elsevier, Amsterdam, 1977. 17. W. Gordy and R. L. Cook, ‘‘Microwave Molecular Spectra,’’ Appendix E, pp. 859–872, Wiley–Interscience, New York, 1984. 18. W. Gordy and R. L. Cook, ‘‘Microwave Molecular Spectra,’’ Chap. XI, pp. 391–449, Wiley–Interscience, New York, 1984. 19. M. Hayashi and T. Inagusa, J. Mol. Struct. 220, 103–117 (1990). 20. R. Hinze, A. Lesarri, J. C. Lo´pez, J. L. Alonso, and A. Guarnieri, J. Chem. Phys. 104, 9729–9734 (1996). 21. J. L. Alonso, J. C. Lo´pez, S. Blanco, and A. Guarnieri, J. Mol. Spectrosc. 182, 148–162 (1997). 22. J. Kraitchman and B. P. Dailey, J. Chem. Phys. 22, 1477–1481 (1954). 23. S. Blanco, A. Lesarri, J. C. Lo´pez, J. L. Alonso, and A. Guarnieri, J. Mol. Spectrosc. 174, 397–416 (1995). 24. S. Blanco, A. Lesarri, J. C. Lo´pez, J. L. Alonso, and A. Guarnieri, Z. Naturforsch A 51, 129–132 (1996). 25. W. Caminati, A. C. Fantoni, F. Manescalchi, and F. Scappini, Mol. Phys. 64, 1089–1103 (1988). 26. R. C. Woods, J. Mol. Spectrosc. 21, 4–24 (1966). 27. W. Kasten and H. Dreizler, Z. Naturforsch A 41, 944–954 (1986). 28. W. Gordy and R. L. Cook, ‘‘Microwave Molecular Spectra,’’ Chap. XII, pp. 569–645, Wiley–Interscience, New York, 1984).
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Internal Rotation and the Chlorine Nuclear Quadrupole Coupling Tensor of 1-Chloropropane Ana de Luis, M. Eugenia Sanz, Felipe J. Lorenzo, Juan C. Lo´pez, and Jose´ L. Alonso 1 Departamento de QuıB mica FıB sica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain Received November 1, 1996; in revised form March 7, 1997
The rotational spectra of the trans and gauche forms of 1-chloropropane have been analyzed in the frequency range 8–40 GHz using Stark, waveguide FTMW, and pulsed molecular beam FTMW spectrometers. The spectra of 35Cl, 37Cl, and the three 13C isotopomers have been observed for both conformers. The complete quadrupole Cl-coupling tensors have been determined and discussed within the structural considerations. For the gauche form the barrier to internal rotation of the methyl group has been determined to be 2908(4) cal/mol from A–E splittings observed in the first excited torsional state. Two A–E splittings have been observed only in the ground state of the trans form. From these splittings a barrier of 2760 cal/mol has been estimated. q 1997 Academic Press
group. Furthermore the precision of the frequency measurements achieved with these techniques allows in many cases the determination of the off-diagonal elements of the quadrupole coupling tensor. The principal axis coupling tensor elements can then be easily calculated by diagonalization.
INTRODUCTION
1-Chloropropane has been investigated by both rotational (1–5) and vibrational (6–8) spectroscopy and by gas electron diffraction (5, 9). These studies have shown that 1chloropropane exists in two stable rotational isomers, trans and gauche (Fig. 1). The microwave spectrum of both conformers, using conventional Stark modulation spectroscopy, was first studied by Sarachman ( 1 ) , who reported the rotational and quadrupole coupling constants for the 35Cl and 37Cl isotopomers of the gauche form and the B and C rotational constants of the trans form. This author also observed the spectra of the first two excited states of the CH2Cl – C skeletal torsion and the first excited state of the methyl torsion vibration for the gauche form as well as the first excited state of the skeletal torsion for the trans form. The centrifugal distortion effect for the gauche conformer was later studied by Kaushik ( 3, 4 ) . The structure was analyzed by Kuchitsu and co-workers ( 5 ) using a joint analysis of electron diffraction and rotational data. They reported rotational, quadrupole coupling, and some quartic centrifugal distortion constants for the 35Cl and 37 Cl isotopomers of the trans form and the electric dipole moment for both conformers. In neither case have A – E splittings due to the internal rotation of the methyl group been observed. We decided to reinvestigate the spectra of the two rotational isomers of 1-chloropropane with the higher resolution and sensitivity of the currently available microwave Fourier transform techniques to resolve the methyl torsion fine structure and determine the barrier hindering internal rotation of the methyl 1
EXPERIMENTAL
A commercial sample of 1-chloropropane, in natural isotopic composition, was used without further purification. The microwave spectrum was recorded in the region 8–40 GHz using different spectrometers. Computer-controlled Stark modulation spectrometers (10, 11) were used for the range 26.5–40 GHz. The Stark cell was cooled at temperatures of about 260 K with sample pressures of 10–20 mTorr. Radiofrequency microwave double resonance (RFMWDR) (12) was also used for the assignment of excited state spectra of the trans form. The accuracy of frequency measurements is estimated to be better than 50 kHz. A waveguide Fourier transform microwave spectrometer (WG-FTMW) (13) covering the frequency range 8–18 GHz was used to investigate the quadrupole coupling hyperfine structure and to search for internal rotation splittings in the ground state spectra. Microwave pulses of 20–120 ns duration with peak power of 1–5 W polarized the molecules in the 12m waveguide cell. The spectra were taken at pressures down to 1 mTorr with temperatures of about 240 K. The frequency measurements have an estimated accuracy of 10 kHz. The spectra of the 37Cl and the 13C isotopomers were observed in natural abundance using a pulsed molecular beam Fourier transform microwave spectrometer (MB-FTMW) (14). The sample entering the nozzle, mounted in one of the cavity mirrors, was approximately 1% 1-chloropropane and 99% argon at pressures of 1–2 atm. The gas expansion leads to extreme
To whom correspondence should be addressed. 60
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FIG. 1. The trans (a) and gauche (b) forms of the 1-chloropropane molecule.
vibrational and rotational cooling. Only the ground vibrational state for rotational levels with a low J quantum number is populated. This simplifies the spectrum and allows the observation of low natural abundance isotopes. As the molecular beam traveled parallel to the direction of microwave propagation, all lines were doubled by the Doppler effect. Linewidths of ca. 7 kHz were obtained with an accuracy of frequency measurements better than 5 kHz.
RESULTS AND DISCUSSION
Ground State Spectra 1-Chloropropane is a prolate asymmetric rotor for both trans ( k Å 00.99) and gauche ( k Å 00.90) conformers. Cs symmetry constrains the trans form electric dipole moment to lie along the ab inertial plane. All three electric
FIG. 2. WG-FTMW spectrum of the 8 0,8 R 7 1,7 transition for the ground state of trans-1-chloropropane showing a methyl internal rotation A – E splitting for each quadrupole coupling component. A 7-MHz section out of the 50-MHz range of the power spectrum is given. Number of cycles, 5 million ( measuring time 198 s ) at a sample interval of 10 ns. The 1024 data points were supplemented by 3072 zeros prior to the FFT transformation.
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TABLE 1 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for 35Cl and 37Cl Species of trans-1-Chloropropane in the Ground Vibrational State
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TABLE 1 —Continued
dipole moment components are different from zero for the gauche form but mc is small (5) and c-type lines have not been observed. Initial measurements of the 35Cl spectra were made using the Stark spectrometers. Predictions were based on the rotational and quadrupole coupling parameters previously reported (1, 5). The ground state rotational spectra for both conformers were easily identified and measured, but quadrupole coupling hyperfine structure was not completely resolved by the Stark modulation spectrometers used here. The waveguide Fourier transform spectrometer which has a superior resolution was subsequently used in order to resolve the quadrupole coupling structure. An additional splitting was observed for two b R transitions of the trans form ground state spectra (see Fig. 2). This splitting was attributed to the internal rotation of the methyl group. Each nuclear quadrupole coupling component was split into two components corresponding to the torsional A and E levels. No A–E splittings were observed for the gauche conformer. In the initial stages of the analysis, centrifugal distortion and quadrupole coupling were treated separately. First the quadrupole coupling constants xaa ( a Å a, b, c) were determined from the quadrupole splittings and then a set of center frequencies was calculated. These center frequencies were then fitted to derive the rotational and centrifugal distortion constants. In the final calculations we considered all the observed individual transitions which were fitted using the CALPGM program of Pickett (15). This program allows the direct diagonalization of the Hamiltonian including cen-
trifugal distortion and quadrupole coupling operators. The A-reduced semirigid rotor Hamiltonian of Watson in the I r representation was selected (16). Lines measured with the waveguide FTMW spectrometer were assigned uncertainties of 10 kHz. For the Stark spectrometers measurement uncertainties of 50 kHz were used for resolved hyperfine components and 100 kHz for the unresolved components. The observed frequencies of hyperfine components are given in Tables 1 and 2. All the measured components were first fitted to a Hamiltonian including only the diagonal elements of the quadrupole coupling tensor. For the trans form all the measured lines fit to this Hamiltonian within the estimated uncertainties with the exception of some components of the transitions 21,2 R 11,1 and 30,3 R 20,2 . These transitions were measured using the FTMW spectrometer and showed deviations between 100 and 300 kHz from the calculated frequencies. We calculated the energy of the levels involved in these transitions in order to search for near degeneracies giving rise to second-order quadrupole coupling effects. The difference in energy between the 11,1 and 30,3 levels was found to be 119 MHz and these levels can be connected by c-type symmetry operators. This is just the symmetry of the only nonzero off-diagonal element of the quadrupole coupling tensor xab . When we considered this element in the Hamiltonian a good fit was obtained for all the measured components and ÉxabÉ was determined with a standard error of 5%. Using the high resolution and accuracy of the MB-FTMW instrument to perform new measurements of low- J transitions, the
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TABLE 2 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for 35Cl and 37Cl Species of gauche-1-Chloropropane in the Ground Vibrational State
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TABLE 2 —Continued
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TABLE 3 Rotational Constants, Quartic Centrifugal Distortion Constants, and Chlorine Quadrupole Coupling Constants for Different Isotopomers of trans-1-Chloropropane in the Ground Vibrational State
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TABLE 4 Rotational Constants, Quartic Centrifugal Distortion Constants, and Chlorine Quadrupole Coupling Constants for Different Isotopomers of gauche-1-Chloropropane in the Ground Vibrational State
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FIG. 3. F Å 7/2 R 5/2, 5/2 R 3/2, and 1/2 R 1/2 quadrupole coupling components of the 20,2 R 10,1 transition for the 13C3 isotopomer of gauche1-chloropropane recorded using an MB-FTMW spectrometer. Sample, 0.5% in argon. Stagnation pressure, 1 atm. Number of cycles, 1665. Doppler effect splitting, 40 kHz.
ÉxabÉ quadrupole coupling element was finally determined with a standard error of 0.4%. This spectrometer has been also used to measure the spectra of the 37Cl species. The spectroscopic constants derived for the trans form are given in Table 3. The centrifugal distortion constant dK was found not determinable from the set of observed transitions and was fixed to zero. For the 37Cl species only transitions observed in this work were fitted and the centrifugal distortion constants DK and dK were fixed to the values found for the 35Cl isotopomer. The final results for the gauche conformer are presented in Table 4. In this case the quadrupole coupling tensor has the three off-diagonal elements different from zero. We performed different fits of the spectrum considering separately each off-diagonal element in order to see if they were determinable. The inclusion of the elements ÉxabÉ and ÉxbcÉ led to small improvements in the quality of the fit. No improvement in the standard deviation of the fit was found when considering ÉxacÉ but it was determined with a 25% error. In Table 4 the results of the fit including all three elements are presented. The reliability of the values obtained is discussed below. For the 37Cl species the centrifugal distortion constants were fixed to the 35Cl species values. Based on the assignment of the 35Cl species and an assumed structure, the isotopic shifts for the 13C species could be readily predicted. Using the MB-FTMW spectrometer, the rotational and vibrational cooling greatly simplified the spectrum and the transitions were easily found. Figure 3 shows one of these transitions. The com-
plete set of assigned components are listed in Table 5 and the derived rotational and quadrupole coupling constants in Tables 3 and 4. Only few transitions were measured so the centrifugal distortion constants and the off-diagonal elements were fixed to the 35Cl values in all the cases. For the 13C3 species of the trans conformer we also fixed in the fit the value of ( xbb 0 xcc ) . The planar moments Pcc Å Si mi c 2i listed in Table 3 for the trans conformer provide some insight into the symmetry of the molecule. When isotopic substitution occurs in a symmetry plane, the planar moment is not affected by substitution in that plane. The near identity of the Pcc value for all the isotopomers can be seen. This result implies that all isotopic substitutions occur in a symmetry plane. It is not possible to obtain substitution structures for the heavy atom skeleton due to the proximity of the Cl and C atoms to the inertial axes of both conformers (see Fig. 1). The structures reported by Kuchitsu and co-workers (5) reproduce the experimental rotational constants obtained in this work with only small changes within the quoted errors in Ref. (5). We have used these structures in the discussion about the nuclear quadrupole coupling tensor and in the calculations to determine the methyl group internal rotation barrier. Quadrupole Coupling Tensor The observed quadrupole coupling constants shown in Table 3 for the trans conformer refer to the principal inertial
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TABLE 5 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the 13C Isotopomers of trans- and gauche-1-Chloropropane
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TABLE 6 Quadrupole Coupling Tensor of trans-1-Chloropropane in the Inertial and Main Axis Systems
a
Standard error in parenthesis in units of the last digit. c principal inertial axis and y gradient field axis have been assumed to be coincident. c Angle, in degrees, between the z gradient field axis and the a principal inertial axis. d Angle, in degrees, between the C1-C1 bond and the a principal inertial axis. b
axis system. There is only one nonzero off-diagonal constant ÉxabÉ (the sign cannot be determined) because of the Cs symmetry. Note that the ratio xcc ( 35Cl)/ xcc ( 37Cl) is 1.2687(17) in excellent agreement with the ratio of the nuclear quadrupole moments 35Q/ 37Q Å 1.26878 (17). The xcc constants determined for the 13C1 and 13C2 isotopomers are also equal within the quoted errors to that determined for the 35Cl species. For direct comparison of quadrupole coupling constants of different chlorine-containing molecules it is necessary to transform the coupling tensor into its own principal axis system. The results of this transformation for the trans conformer are shown in Table 6. A rotation of an angle uz ,a around the c principal axis must be performed in this case. The coupling constant xcc is equal to one of the diagonal values, say xyy . The angle ua between the a inertial axis and the C–Cl bond axis is that reported from the structure by Kuchitsu and co-workers (5). The angles uz ,a and ua are the same within the quoted errors and the z axis may be considered to lie along the C–Cl bond. The small value of the quantity h Å ( xxx 0 xyy )/ xzz which is a measure of the deviation from the cylindrical symmetry of the x tensor
around the z axis indicates that the x tensor of chlorine has nearly axial symmetry around the C–Cl bond. The magnitude of the coupling found here for the trans form of 1chloropropane is typical for 35Cl in molecules with single bonds (18). The values also compare very well with those found for related molecules as 1-chloroethane (19), 1chloro-1-fluoroethane (20), 1-chloro-1,1-difluoroethane (21), chloromethane (22), chlorofluoromethane (23), and chlorodifluoromethane (24). For the gauche form the off-diagonal elements of the quadrupole coupling tensor have been determined for the 35 Cl and 37Cl species (see Table 4), but their values are not precise. The diagonalization of the quadrupole tensor gives values of xxx , xyy , and xzz consistent with those determined for the trans form when xac is given a sign opposite to that of xab and xbc . However, the errors of these constants are very high as a consequence of the imprecision of the ÉxabÉ, ÉxbcÉ, and ÉxacÉ values. In order to test the reliability of the quadrupole coupling constants determined for the gauche form, these have been calculated from quadrupole coupling constants xxx , xyy , and xzz calculated for the trans form and the structure given in
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TABLE 7 Quadrupole Coupling Tensor Elements of trans- and gauche-1-Chloropropane in the Inertial Axis System Calculated from the Principal Quadrupole Coupling Constants of the trans-Conformer (Table 6) and the Structure of Ref. ( 5)
TABLE 8 Observed and Calculated Differences between the 13C and Parent Species Quadrupole Coupling Tensor Diagonal Elements
a
Dxaa Å xaa ( 13C) 0 xaaa ( 35C1) Calculated from the values in Tables 3 and 4. c Calculated from the values in Table 7. b
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TABLE 9 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the First Methyl Torsion Excited State of gauche-1-Chloropropane
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TABLE 10 Spectroscopic Constants for the First Methyl Torsion Excited State of gauche-1-Chloropropane A/MHz B C
11882.355(79)a 3309.482(11) 2844.856(12)
DJ /kHz DJK DK dJ dK
2.35(20) 013.98(41) 143.8(97) 0.5216(55) 8.55(39)
xaa /MHz (xbb-xcc)
018.3(11) 040.46(24)
Nb J max sc/MHz
83 20 0.069
xbb /MHz xcc
011.1(17) 29.4(17)
a
Standard error in parenthesis in units of the last digit. Number of fitted quadrupole components. c Standard deviation of the fit. b
Ref. (5) assuming that the z axis is collinear with the C– Cl bond. The same calculations have been performed for the different isotopomers of both forms. For the 37Cl species the quadrupole coupling factor 35Q/ 37Q Å 1.26878 (17) has been used. The results of these calculations are presented in Table 7. For the trans form the xcc element has been omitted in the table. The calculated values for the gauche form are consistent with the observed values given in Table 4, indicating the reliability of the determined off-diagonal quadrupole coupling constants. The same conclusion comes from the comparison between the observed and calculated values of both conformers for the 37Cl species. For the 13C isotopomers only the diagonal elements of the quadrupole coupling tensor have been determined. These elements show shifts with respect to the values determined for the parent species which are noticeable for the gauche form. The observed and calculated differences between the diagonal quadrupole coupling constants of the 13C species and those for the normal species are compared in Table 8. The agreement between the observed and calculated values is excellent, and may be taken as a proof for the assignments of the quadrupole hyperfine structure in the 13C species spectra for which only a few transitions have been measured. Vibrational Excited States One of the main purposes of this work was to determine the methyl group internal rotation barrier of 1-chloropropane. For the gauche form we concentrated our efforts on
the excited state assigned by Sarachman ( 1 ) to the first excited methyl torsion state. The rotational spectrum of this state was assigned using Stark spectroscopy on the basis of the rotational constants previously reported ( 1 ) by assuming the same quadrupole coupling constants as the ground state. A few A – E splittings due to internal rotation of the methyl group were observed for high- J bQ lines, confirming the assignment of this state to the methyl torsion ( 1 ) . The observed hyperfine components are shown in Table 9 and the corresponding rotational parameters are listed in Table 10. These parameters have been obtained by using the A-level components for the lines showing internal rotation splittings. Since only two A–E splittings were observed in the ground state of the trans conformer, the excited vibrational states have been investigated. With the help of RFMWDR, we assigned the a R spectra of three excited states which were initially labeled A, B, and C. The state labeled A was the most intense and occurs on the high-frequency side of the ground state transitions. The state labeled B was found close to the ground state lines at higher frequencies. The state C is the weakest and occurs on the low-frequency side. No A–E splittings were observed for the a R lines belonging to these states. Trials to assign the bQ spectra using the WGFTMW spectrometer failed due to their weakness and the presence of many lines arising from the quadrupole hyperfine structure. The observed hyperfine components are given in Table 11 and the rotational parameters obtained for the states A, B, and C of trans-1-chloropropane are given in Table 12.
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TABLE 11 The Observed and Calculated Frequencies (MHz) of Hyperfine Components for the Excited States of trans-1-Chloropropane
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TABLE 12 Spectroscopic Constants for the Excited States of trans-1-Chloropropane
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TABLE 13 Internal Rotation Parameters for IAM Analysis of the Ground State of trans-1-Chloropropane (a) and the First Methyl Torsion Excited State of gauche-1-Chloropropane (b)
r b (rad) g (rad) ˚ 2) Ia (amu A F (GHz) V3 (cal mol01) s s b (kHz) nc
TABLE 15 V3 Barriers to Internal Rotation for Some Propane Derivatives V3 (cal/mol) CH3CH2CH3 CH3CH2CH2F (gauche)
(a)
(b)
[0.15261] a [0.03620] [0.] [3.21] [183.7] 2760.(20)d,e 70.0(5) 12.0 2
[0.03542] [0.57303] [0.32317] [3.23] [166.234] 2908.(4)d 84.1(1) 32.0 6
a
Parameters in square brackets were kept constant in the fit. Standard deviation of the fit. c Number of splittings used in the fit. d As the necessary structural parameters were kept fixed in the fit, the calculated errors do not reflect possible errors introduced by the assumed structure. e The barrier for the trans form has been obtained from only two A–E splittings and should be considered as an estimate of V3 .
CH3CH2CH2F (trans) CH3CH2CH2OH (trans) CH3CH2CH2CN (gauche) CH3CH2CH2CN (trans) CH3CH2CH2NC (gauche) CH3CH2CH2NC (trans) CH3CH2CF3 CH3CH2CH2Cl (gauche) CH3CH2CH2Cl (trans)
3166 2758 2874 2710 2719 2730 3107 3110 2894 2954 2635 2908 2760
(27) a (35) b (24) b (8) b (53) b (60) c (24) d (38) d (23) e (22) e (4) f (4) g (20) g
State vt vt vt vt vt vt vt vt vt vt vt vt vt
Å Å Å Å Å Å Å Å Å Å Å Å Å
0 0 1 0 1 0 0 0 0 0 0 1 0
b
The state labeled A is the most intense and was assigned by Sarachman (1) to the lowest frequency mode of the molecule, the CH2Cl–C torsion, observed at 130 cm01 in the gasphase Raman spectrum (7). The increment in the Pcc value with respect to the ground state is consistent with the motion of the Cl atom out of the ab plane. A similar behavior has been found for the planar moment Pcc for the equivalent C– C torsion in trans-1-fluoropropane (25). The methyl torsion vibration has been assigned for the trans form to a vibrational band observed at 231 cm01 (6) and the C–C–Cl
TABLE 14 Observed A–E Splittings and Differences (in MHz) with the Calculated Values for the Ground State of trans-1-Chloropropane and the First Methyl Torsion Excited State of gauche-1-Chloropropane J*
Ka*
Kc*
J9
R
Ka9
K9c
A–E
Diff
00.1774 00.1612
00.0087 00.0087
01.291 01.557 01.903 00.700 01.026 01.437
00.047 0.002 00.016 0.024 0.041 0.017
Trans form, ground state 7 8
0 0
7 8
6 7
1 1
6 7
Gauche form, vt Å 1 14 15 16 18 19 20
2 2 2 3 3 3
12 13 14 15 16 17
14 15 16 18 19 20
1 1 1 2 2 2
13 14 15 16 17 18
a G. Bestmann, W. Lalowski and H. Dreizler, Z. Naturforsch. A, 40, 271– 273 (1985). b Ref. [27]. c H. Dreizler and F. Scappini, Z. Naturforsch. A, 36, 1187–1191 (1981). d K. Vormann and H. Dreizler, Z. Naturforsch. A, 43, 338–344 (1988). e M. Kru¨ger and H. Dreizler, Z. Naturforsch. A, 47, 1067–1072 (1992). f S. Antolı´nez J. C. Lopes and J. L. Alouso, J. Chem. Soc. Faraday Trans. 93, 1291–1295 (1997). g This work.
deformation to bands observed close to 242 cm01 (6, 7). From their intensities the B and C states may be tentatively assigned to the CH3 torsion and C–C–Cl deformation, respectively. Methyl Group Internal Rotation Barrier The calculation of the barrier hindering internal rotation of the methyl group has been carried out independently using the internal axis method in the form given by Woods ( 26 ) . Observed A – E splittings are present for all the quadrupole components of each rotational transition so an arithmetic mean of the splittings for each quadrupole component was considered. For both conformers the parameters r, b, and g were constrained to the values arising from the reported structures ( 5 ) . The results obtained are summarized in Table 13. The mean splitting values are given in Table 14. The barrier height of V3 Å 2760(20) cal/mol for the ground state of the trans form should be taken as an estimate value since it has been derived from only two splittings. Both values for the trans and gauche conformers are compared to those obtained for other propane derivatives in Table 15. The barrier V3 Å 2908(3) cal/mol determined for the first excited torsional state of the gauche form is about 150 cal/ mol higher than the barrier obtained for the ground state of the trans form. This difference may arise from several factors. One of them is the fact that the value for the trans form was obtained from only two transitions. A second factor to
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ROTATIONAL SPECTRUM OF 1-CHLOROPROPANE
consider is that V3 for the gauche form has been derived from A–E splittings in the first excited torsional state. In Table 10 the barriers determined for trans- and gauche-1fluoropropane (27) in the ground and first torsional excited states are given. These barriers have been determined using the same method used in the present work. For 1-fluoropropane (27) the barriers determined for the ground and £t Å 1 states of the trans form and for the ground state of the gauche form have about the same value. However, the barrier calculated for the £t Å 1 state of gauche-1-fluoropropane is about 125 cal/mol higher than the value for the ground state. This difference was discussed in Ref. (27) by an approximate treatment of the coupling between CH3 and C– C torsional motions. Using the values of F and V3 , the harmonic oscillator approximation ( 28 ) results in torsional oscillation frequencies of 235 cm01 for trans-1-chloropropane and 228 cm01 for gauche-1-chloropropane. The torsional frequency for the trans form is in good agreement with that of 231 cm 01 observed from infrared measurements ( 6 ) . However, the wavenumbers observed for the torsional fundamental of gauche-1-chloropropane from vibrational spectroscopy occur at values around 214 cm01 ( 6, 7 ) . An interaction between methyl torsion and other low-frequency skeletal vibrations is possible and may be the cause of these discrepancies. ACKNOWLEDGMENTS Research funds from the Direccio´n General de Investigacio´n CientıB fica y Te´cnica (DGICYT, Grant PB93-0224) and the Junta de Castilla y Leo´n (Grant VA19/94) are gratefully acknowledged.
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