Asymmetric Torsional Mode Research Articles

Infrared and Raman spectra, conformational stability, vibrational assignment, and ab initio calculations of 2-bromo-3-chloropropene

The infrared (3400-70 cm −1) and Raman (3300-20 cm −1) spectra of 2-bromo-3-chloropropene, H 2CC(Br)CH 2Cl, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded. Variable temperature (−60–−100°C) studies of the infrared spectrum (3400-400 cm −1) of 2-bromo-3-chloropropene dissolved in liquid xenon have been performed. Utilizing two sets of conformer doublets, the enthalpy difference has been determined to be 197 ± 20 cm −1 (563 ± 57 cal mol −1), with the anti conformer the more stable form (Cl atom cis to the double bond). Both anti and gauche conformers have been identified in the liquid phase, with the gauche conformer remaining in the annealed solid. The fundamental asymmetric torsional mode for the anti conformer is observed at 88.9 cm −1. A complete vibrational assignment is proposed based on group frequencies and relative infrared and Raman intensities. The conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with the RHF/LANL-1DZ, MP2/LANL-1DZ and RHF/4-31G∗/MIDI-4∗ basis sets. Normal coordinate analyses have also been performed with force fields determined from the RHF/4-31G∗/MIDI-4∗ basis set. The calculated frequencies support the proposed vibrational assignment. These data are compared to corresponding data for some similar molecules.

Spectra and structure of organophosphorus compounds Part LII. Raman and infrared spectra, conformational stability, and ab initio calculations for methoxydichlorophosphine

The Raman (3500 to 30 cm−1) and infrared spectra (3500 to 50 cm−1) of gaseous and solid methoxydichlorophosphine, CH3OPCl2, have been recorded. An infrared study of CH3OPCl2 dissolved in xenon as a function of temperature, as well as a temperature study of the Raman spectra of all phases, definitively shows that only the trans conformer (methyl group trans to the phosphorus lone pair) is present in all phases. The far-infrared spectrum of the gas has been recorded from 350 to 30 cm−1 at a resolution of 0.10 cm−1. The fundamental asymmetric torsional mode has been observed at 120 cm−1, and the frequency of the methyl torsional mode is estimated to be 58 cm−1 from a combination band. A complete vibrational assignment is given for both the normal and d3-isotopomer which is supported by ab initio calculations employing the RHF/3-21G* basis set. The structural parameters and relative conformer stabilities have been obtained with the RHF/6-31G* and MP2/6-31G* basis sets. From these calculations, the gauche conformer is estimated to be more than 3.6 kcal mol−1 less stable than the trans conformer which is consistent with the observation of only the trans conformer in the Raman spectrum of the gas at ambient temperature. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 3‐bromo‐2‐methylpropene

AbstractThe Raman (3200–20 cm−1) and infrared (3200–35 cm−1) spectra of 3‐bromo‐2‐methylpropene, H2CC(CH3)CH2Br were recorded for the gas and solid. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. The fundamental asymmetric torsional mode for the gauche conformer is observed at 81.5 cm−1 with one excited state falling at a lower wavenumber but the asymmetric torsion is not observed for the higher energy s‐cis conformer. Utilizing the gauche observed torsional wave numbers, the gauche dihedral angle, the enthalpy difference between conformers and the calculated torsional fundamental wavenumber for the s‐cis conformer, the potential function governing the interconversion of the rotamers was estimated. This potential function gives values of 170 cm−1 (486 cal mol−1) (1 cal = 4.184 J), 2659 cm−1 (7.60 kcal mol−1) and 725 cm−1 (2.07 kcal mol−1) for the s‐cis to gauche, gauche to gauche and gauche to s‐cis barriers, respectively, and it is compared with tht obtained with the RHF/STO‐3G* basis set. From the methyl torsional wavenumber of 172 cm−1 for the gauche conformer, the threefold barrier of 695 cm−1 (1.99 kcal mol−1) was calculated for the methyl group. A complete vibrational assignment is proposed based on Raman : depolarization data, group wavenumbers and relative infrared and Raman intensities. The conformational energy difference and optimized geometries of both conformers were also obtained from ab initio calculations with the RHF/STO‐3G*, RHF/LANL1DZ and MP2/LANL1DZ basis sets. Normal‐coordinate analyses were also performed with force fields determined from both the RHF/STO‐3G* and RHF/LANL1DZ basis sets. The calculated wavenumbers support the proposed vibrational assignment. These data are compared with the corresponding data for some similar molecules.

Far-IR spectra, barriers to internal rotation, r0 structural parameters and ab initio calculations of chloromethyl methyl ether

The far-IR spectra from 350 to 50 cm −1 of gaseous chloromethyl methyl ether, ClCH 2OCH 3, along with three of the deuterium isotopomers, have been recorded at a resolution of 0.10 cm −1. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 180 and 133 cm −1, respectively. An estimate is given for the potential function governing the asymmetric torsion. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 556 ± 5 cm −1 (1.59 ± 0.01 kcal mol −1). Utilizing previously reported rotational constants for seven isotopomers, r 0 structural parameters are determined. The heavy atom parameters (distances in ångströms angles in degrees) are: r(C 1−Cl) = 1.818 ± 0.003; r(C 1−O) = 1.371 ± 0.004; r(C 2−O) = 1.430 ± 0.001; ∢ClC 1O = 113.01 ± 0.07;∢ClOC 2 = 114.03 ± 0.10 and dihedral ClC 1OC 2=70.20 ± 0.15. A complete vibrational assignment is given for the ClCD 2OCD 3 molecule. The fundamental vibrational frequencies, barrier to internal rotation and structural parameters which have been obtained experimentally are compared with those obtained from ab initio Hartree-Fock calculations employing the 3–21G * and 6–31G * basis sets along with the 6–31G * basis set with electron correlation at the MP2 level. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Far-IR spectrum, conformation stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-chloro-2-methylpropene

The far-IR spectrum from 375 to 30 cm −1 of gaseous 3-chloro-2-methylpropene, CH 2=C(CH 3)CH 2Cl, has been recorded at a resolution of 0.10 cm −1. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm −1 with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm −1 with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): V 1=189±12, V 2=−358±11, V 3=886±2 and V 4=−12±2 with an enthalpy difference between the more stable gauche and s-cis conformers of 150 ±25 cm −1 (430 ± 71 cal mol −1). This function gives values of 661 cm −1 (1.89 kcal mol −1), 1226 cm −1 (3.51 kcal mol −1) and 812 cm −1 (2.32 kcal mol −1), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm −1 for the gauche conformer, the threefold barrier of 678 cm −1 (1.94 kcal mol −1) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G * and 6–31G * basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G * basis set. These data are compared with the corresponding data for some similar molecules.

Conformational stability, barriers to internal rotation, vibrational assignments and Ab Initio calculations of 3‐methylbut‐1‐ene

AbstractThe Raman spectra (3200–10 cm−1) of gaseous, liquid and solid and infrared spectra (3200–40 cm−1) of gaseous and solid 3‐methylbut‐1‐ene, H2CCHCH(CH3)2, were recorded. From the vibrational spectra of gaseous 3‐methylbut‐1‐ene, below 200 cm−1, the asymmetric torsional mode of the more stable trans and higher energy gauche conformers were assigned. From studies of the Raman spectrum of the gas with variable temperatures the trans conformer has been determined to be more stable than the gauche form by 155 ± 31 cm−1 [443 ± 89 cal mol−1 (1 cal = 4.184 J)]. Similar studies for the liquid indicate that the two conformers are nearly equivalent in energy and for the annealed solid only the gauche conformer persists. Optimized structural parameters, obtained from ab initio calculations with the MP2/6–31G basis set, were used to obtain the kinetic energy terms with structural relaxation for the asymmetric internal rotation. The coefficients of the potential function governing the conformational interchange are V1 = 198 ± 15, V2 = 68 ± 6, V3 = 748 ± 1 and V4 = ‐55 ± 2 cm−1, and this potential has trans to gauche, gauche to gauche and gauche to trans barriers of 816, 787 and 657 cm−1, respectively. Barriers to internal rotation of the methyl rotors of 1169 cm−1 for the trans conformer and 1290 ± 30 and 1525 ± 50 cm−1 for the gauche conformer were determined from the low‐frequency Raman and far‐infrared spectra of the gas. Vibrational assignments are provided which are based on infrared band contours. Raman depolarization values, group frequencies and normal coordinate calculations. The conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which were determined from experimental data are compared with those obtained from ab initio calculations.

Raman and far‐infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2‐bromo‐3‐fluoropropene

AbstractThe far‐infrared spectrum (375–30 cm−1) of gaseous 2‐bromo‐3‐fluoropropene, CH2C(CH2F)Br, was recorded at a resolution of 0.10 cm−1. The fundamental asymmetric torsional mode is observed at 112.6 cm−1 with four excited states falling to lower frequency for the s‐cis (fluorine atom eclipsing the doubel bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is tentatively assigned to a very weak band at 93.0 cm−1. From these data the asymmetric torsional potential was calculated. The potential function coefficients are calculated to be V1 = 306 ± 5, V2 = −120 ± 4, V3 = 928 ± 6, V4 = 118 ± 2 and V6 = 83 ± 2 cm−1, with an enthaply difference between the more stable s‐cis and higher energy gauche conformers of 200 ± 30 cm−1 [572 ± 86 kcal mol−1 (1 kcal = 4.184 kJ)]. This function gives values of 1009 cm−1 (2.88 kcal mole−1), 1024 cm−1 (2.93 kcal mol−1) and 799 cm−1 (2.88 kcal mol−1), for the s‐cis to gauche, gauche to gauche, and gauche to s‐cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 548 cm−1 (gauche) and 678 cm−1 (s‐cis) as a function of temperature, the enthalpy difference is found to be 196 ± 23 cm−1 (560 ± 66 cal mol−1), and a similar study of the liquid gives a value of 66 ± 4 cm−1 (189 ± 11 cal mol−1) with the s‐cis conformer the more stable rotamer in both of these phases. The s‐cis conformer is the rotamer present in the crystalline solid. The Raman spectrum of the gas was recorded from 3500 to 70 cm−1 and, utilizing these data and previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation and fundamental vibrational frequencies, which were determined experimentally, are compared with those obtained from ab initio gradient calculations employing the RHF/LANL1DZ basis set and/or this basis set with electron correlation at the MP2 level, and with the corresponding quantities for some similar molecules.

Vibrational spectra, conformational stability, barriers to internal rotation, r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH2OCH3, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm−1 in the 350 to 50 cm−1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm−1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm−1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm−1) spectra of the gas and solid and from the Raman (3200 to 10 cm−1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer0 geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in A, angles in degrees) arer(C1-F) = 1.395 ± 0.005;r(C1-O) = 1.368 ± 0.007;r(C2-O) = 1.426 ±0.003; ⦔FC1O = 111.33 ± 0.25; ⦔C1OC2 = 113.50 ± 0.18 and dih FC1OC2 = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Far infrared spectrum, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 2-chloro-3-fluoropropene

The far infrared spectrum (375 to 30 cm−1) of gaseous 2-chloro-3-fluoropropene, CH2=C(CH2F)CI, has been recorded at a resolution of 0.10 cm−1. The fundamental asymmetric torsional mode is observed at 117.5 cm−1 with ten excited states falling to low frequency for thes-cis (fluorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be at 94 cm−1. From these data the asymmetric torsional potential function has been calculated. The potential function coefficients are calculated to be in cm−1):V1=803±21,V2=−94±21,V3= 1025±10,V4=95±10, andV6=2±1, with an enthalpy difference between the more stables-cis and gauche conformera of 550±100 cm−1 (1.57±0.29 kcal/mol). This function gives values of 1227±50cm−1(3.51±0.14kcal/mol), 1266±200 cm−1 (3.62±0.57 kcal/mol), and 665±100 cm−1 (1.90±0.29 kcal/mol), for thes-cis to gauche, gauche to gauche, and gauche tos-cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 652 cm−1 (gauche) and 731 cm−1 (s-cis) as a function temperature, the enthalpy difference is found to be 565±96 cm−1 (1.62±0.27 kcal/mol). However, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas has been recorded from 3500 to 70 cm−1 and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies, and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21 G* and 6–31G* basis sets and to the corresponding quantities for some similar molecules.

Conformational stability, barrier to internal rotation, structural parameters, ab initio calculations, and vibrational assignment of cyclopropylcarbonyl chloride

The fundamental asymmetric torsional mode of cyclopropylcarbonyl chloride, c-C 3H 5-CClO, for the more stable cis conformer (carbonyl group oriented cis to the ring) is observed at about 54 cm −1. The corresponding mode for the high energy trans form is assigned to a Q-branch series with stronger intensity beginning at 87.4 cm −1, decreasing in frequency with successive excited states. From a study of the Raman spectra of the vapor at variable temperatures, it has been determined that the cis is more stable than the trans form by 171 ± 35 cm −1 (489 ± 100 cal mol −1) and that the cis conformer remains in the solid. From a similar study of the spectrum of the liquid, it is found that the trans form is slightly more stable than the cis form by 93 ± 27 cm −1 (266 ± 77 calmol −1). From these data, the potential function for internal rotation of the asymmetric top has been determined and the potential coefficients are: V 1 = 528 ± 14, V 2 = 1769 ± 27 and V 3 = −371 ± 15 cm −1. This potential is consistent with cis to trans and trans to cis barriers of 1934 and 1779 cm −1, respectively, and a conformational energy difference of 155 ± 56 cm −1 (443 ± 160 cal mol −1). The r 0 structural parameters have been obtained for the cis conformer from a combination of ab initio calculated parameters with appropriate off-set values and the fit of the previously reported microwave rotational constants for the two naturally occurring chlorine isotopes. The conformational stability, barriers to internal rotation and fundamental vibrational frequencies, which have been determined from experimental data, are compared with those obtained from ab initio Hartree—Fock gradient calculations with the 3-21G * and 6-31G * basis sets and results obtained for some similar molecules.

Vibrational spectrum, conformational stability, barriers to internal rotation, r0 structural parameters and ab initio calculations of bromomethyl methyl ether

AbstractThe Raman and far‐infrared spectra at a resolution of 0.10 cm−1 of gaseous bromomethyl methyl ether, BrCH2OCH3, and three of its deuterium isotopes, d2, d3 and d5, were recorded in the 350‐50 cm−1 region. The fundamental asymmetric torsional and methyl torsional modes for the d0 molecule are extensively mixed and were observed at 176 and 130 cm−1, respectively, for the stable gauche conformer with each mode having excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one‐dimensional model the barrier to internal rotation of the methyl moiety is determined to be 549 ± 8 cm−1 (1.57 ± 0.02 kcal mol−1). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500–50 cm−1) spectra of the gas and solid and Raman (3200–10 cm−1) spectra of the gas, liquid and solid is proposed. All of these data are compared with the corresponding quantities obtained from ab initio Hartree‐Fock gradient calculations employing the STO‐3G* basis set. Additionally, complete r0 geometries were determined from the combined previously reported microwave data and CH distances determined from infrared studies along with carbon—hydrogen angles transferred from the corresponding chloride. The heavy atom structural parameters (distance in Å, angles in degrees) are r(C1Br) = 1.996 ± 0.005; r(C1O)=1.359 ± 0.005; r(C2O) = 1.433 ± BrCO = 113.7 ± 0.3; ∢ C1OC2 = 113.5 ± 0.3 and dih BrC1OC2 = 70.9 ± 0.3. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Raman and far‐infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2,3‐dicholoropropene

AbstractThe far‐infrared spectrum (375 to 30 cm−1) of gaseous 2,3‐dichloropropene, CH2C(CH2Cl)Cl, was recorded at a resolution of 0.06 cm−1. The fundamental asymmetric torsional mode is observed at 91.9 cm−1 with eight excited states falling to low frequency for the s‐cis (chlorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be near 85 cm−1. Utilizing the s‐cis torsional frequency, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers was calculated. The determined potential function coefficients are V1 = 1047 ± 96, V2 = −619 ± 68, V3 = 1216 ± 50 and V4 = 26 ± 22 cm−1, with an enthalpy difference between the more stable s‐cis and gauche conformers of 245 ± 30 cm−1 (0.70 ± 0.09 kcal mol−1). This function gives values of 1035 ± 10 cm−1 (2.96 ± 0.03 kcal mol−1), 2015 ± 150 cm−1 (5.76 ± 0.43 kcal mol−1) and 787 ± 10 cm−1 (2.25 ± 0.03 kcal mol−1) for the s‐cis to gauche, gauche to gauche and gauche to s‐cis barriers, respectively. From the relative intensities of the Raman lines of the liquid at 1633 cm−1 (gauche) and 1649 cm−1 (s‐cis) as a function of temperature, the enthalpy difference is found to be 334 ± 39 cm−1 (0.95 ± 0.11 kcal mol−1), but now with the gauche conformer more stable. Also, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas was recorded from 3500 to 70 cm−1 and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies and structural parameters which were determined experimentally are compared to those obtained from ab initio Hartree—Fock gradient calculations employing both the 3‐21G* and 6‐31G* basis sets and to the corresponding quantities for some similar molecules.

Far-infrared spectrum, barriers to internal rotation, structural parameters, and vibrational assignment of cyclopropylcarbonyl fluoride

The far-infrared (400 to 50 cm −1) spectrum of gaseous cyclopropylcarbonyl fluoride, c-C 3H 5CFO, was recorded at 0.1 cm −1 resolution. The fundamental asymmetric torsion for the most stable conformer, which has the carbonyl group oriented cis to the ring, is observed at 83.51 cm −1 with five torsionally excited-state transitions proceeding to lower frequencies. For the high energy trans conformer the fundamental asymmetric torsional mode was assigned to a series of C-type Q branches in the region of 94 to 85 cm −1. From these data the potential function for internal rotation of the asymmetric top was determined and the potential coefficients are: V 1 =459±10, V 2=2131±47, V 3 = −320±18, V 4 = 16 ± 15 cm −1 and V 5 = 57±9. The cis to trans and trans to cis barriers were determined to be 2307 ± 28 and 2111 ± 26 cm −1, respectively, with an enthalpy difference between the conformers of 196 ± 75 cm −1 (560 ± 214 cal mol −1). The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree-Fock gradient calculations at the 3-21G and 6-31G* basis set levels. The structural parameters were determined for c-C 3H 5CFO from the ab initio values, with adjustments, and the microwave data. The results obtained are compared with the corresponding results for some similar molecules.

ChemInform Abstract: Infrared and Raman Spectra, Vibrational Assignment, and Conformational Stability of Methacryloyl Bromide.

Abstract The IR (3500 to 20 cm −1 ) and Raman (3200 to 10 cm −1 ) spectra have been recorded for gaseous and solid methacryloyl bromide, CH 2 C(CH 3 )C(O)Br. Additionally, the Raman spectra of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the s- trans conformer (the two double bonds are oriented trans to one another) is the thermodynamically preferred rotamer in the gaseous and liquid phases and the only rotamer present in the spectra of the annealed solid. A second high energy conformer is present in the fluid phases at ambient temperature and spectral features are consistent with it being the gauche conformer but the possibility that it is the s- cis conformer cannot be completely eliminated. From a temperature study of the Raman spectrum of the liquid an enthalpy difference of 326±12 cm −1 (932±34 cal mol −1 ) has been determined. The methyl and asymmetric torsional modes for the s- trans conformer have been assigned at 166.6 and 48 cm −1 , respectively, from the IR spectrum of the vapor. The threefold barrier to internal rotation of the methyl group has been calculated to be 662±2 cm −1 (1.89 kcal mol −1 ). A complete vibrational assignment is proposed based on IR band contours, depolarization values and group frequencies. These results are compared to similar quantities in some related molecules.

Spectra and structure of organophosphorus compounds: Part XLIII. Far-infrared spectra and ab initio calculations of ethylphosphine

The far-IR spectra (350 to 30 cm −1) of gaseous ethylphosphine and the P- d 2 derivative have been recorded at 0.1 cm −1) resolution. These data have been utilized to reinterpret and verify assignments to the methyl torsional fundamentals. The fundamental vibrational frequencies for both the more stable trans (methyl group oriented trans to the lone pair on the phosphorus atom) and high energy gauche rotamers are calculated by ab initio Hartree-Fock gradient calculations employing the 3–21G ∗ basis set and these frequencies are compared to those obtained experimentally. Potential surfaces for both the methyl and asymmetric torsional modes have been calculated utilizing both the 3–21G ∗ and 6–31G ∗ basis sets. The theoretically determined barriers for both torsional modes are compared to those determined from analysis of the torsional bands observed in the far-IR spectra. The optimized geometries for both conformations are compared to the structures obtained from microwave spectroscopy. It is concluded that at these levels of calculation the structure, potential surface and vibrational frequencies for ethylphosphine are well determined.

Infrared and raman spectra, vibrational assignment, and conformational stability of methacryloyl bromide

The IR (3500 to 20 cm −1) and Raman (3200 to 10 cm −1) spectra have been recorded for gaseous and solid methacryloyl bromide, CH 2C(CH 3)C(O)Br. Additionally, the Raman spectra of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the s- trans conformer (the two double bonds are oriented trans to one another) is the thermodynamically preferred rotamer in the gaseous and liquid phases and the only rotamer present in the spectra of the annealed solid. A second high energy conformer is present in the fluid phases at ambient temperature and spectral features are consistent with it being the gauche conformer but the possibility that it is the s- cis conformer cannot be completely eliminated. From a temperature study of the Raman spectrum of the liquid an enthalpy difference of 326±12 cm −1 (932±34 cal mol −1) has been determined. The methyl and asymmetric torsional modes for the s- trans conformer have been assigned at 166.6 and 48 cm −1, respectively, from the IR spectrum of the vapor. The threefold barrier to internal rotation of the methyl group has been calculated to be 662±2 cm −1 (1.89 kcal mol −1). A complete vibrational assignment is proposed based on IR band contours, depolarization values and group frequencies. These results are compared to similar quantities in some related molecules.

Far-infrared spectra and conformational stability of the epihalohydrins

The far-IR spectra of gaseous (360-50 cm −1) and solid (500-50 cm −1) epifluorohydrin, epichlorohydrin, and epibromohydrin, OCH 2CH(CH 2X), have been recorded and the asymmetric torsional modes of two conformations of the epichlorohydrin and epibromohydrin molecules have been observed and assigned. The asymmetric torsional modes for all three conformers of epifluorohydrin have been observed and assigned, and an asymmetric potential function governing internal rotation is calculated. The potential constants obtained are V 1=590±167, V 2=−245±203, V 3=1175±98, V 6=−60±40 cm −1 which are coefficients for the cosine terms, and V′ 1=775± 80, V′ 2=−365±216, V′ 3=−135±33 cm −1 which are terms associated with the sine portion of the function. The calculated enthalpy difference between the most stable gauche I and next more stable conformation, the cis form, is 269 cm −1 (769 cal mol −1) with the enthalpy difference between the gauche I and gauche II conformers calculated to be 1007 cm −1 (2.88 kcal mol −1). The results of a Raman temperature study of epifluorohydrin in the gaseous phase are also included. Some reassignments of the low frequency bending vibrations are suggested for the epihalohydrins as well. The results are compared to the corresponding quantities for the similar halomethylcyclopropane molecules.

Spectra and structure of small ring compounds LIII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid chloride

The microwave spectrum of cyclobutylcarboxylic acid chloride, c-C 4H 7C(O)Cl, has been recorded from 26.5 to 39.0 GHz and only a-type transitions were observed. The R-branch assignments have been made for the ground and first two excited states of the asymmetric torsional mode of the 35Cl species. The ground state rotational constants determined from these assignments are consistent with an equatorial- gauche conformation and the values are: A = 4349.86 ± 0.17, B = 1414.78 ± 0.01, and C = 1148.24 ± 0.01 MHz. These values when used with a set of reasonably assumed heavy atom structural parameters are consistent with a dihedral angle of puckering of 20°. From the relative intensity measurements of the excited state transitions, the frequency of the asymmetric torsion was estimated to be 54 ± 10 cm −1. From a comparison of the infrared spectrum of the gas (3500 to 30 cm −1) and the Raman spectrum of the liquid (3200 to 100 cm −1) to the corresponding spectra obtained for the solid a second conformation can be shown to be present in the fluid phases at ambient temperature. From variable temperature studies of the Raman spectrum of the liquid the energy difference between the preferred equatorial- gauche and high energy (assumed to be equatorial- trans) conformations is determined to be 1.4 kcal mol −1. The ring puckering fundamental has been assigned to a weak band observed in the far infrared spectrum of the solid at 153 cm −1 and this assignment in addition to the assignments made for the remainder of the vibrational spectra are compared to those for some similar molecules.

Spectra and structure of phosphorus—boron compounds—XXVI. Infrared and Raman spectra, conformational stability and normal coordinate analysis of ethyldichlorophosphine-borane

The Raman (3500-10 cm −1) and infrared (3500-50 cm −1) spectra of solid ethyldichlorophosphine-borane, CH 3CH 2P(BH 3)Cl 2 and its deuterated analog, CH 3CH 2P(BD 3)Cl 2 have been recorded. Additionally, the infrared spectra of the gases and the Raman spectra of the liquids have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several distinct Raman lines disappear on going from the liquid to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers, with the trans conformer being more stable in the liquid phase, and the only one present in the solid phase. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be nearly zero. Based on Raman depolarization data, group frequencies, isotopic shift factors and infrared band contours, a complete vibrational assignment has been proposed for the trans conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The BH 3 torsion has been observed at 188 cm −1, while the BD 3 torsion was not observed. The methyl torsions in the spectra of the solids have been observed at 209 and 202 cm −1 for the “light” and deuterated species, respectively. From the torsional data, barriers to internal rotation have been calculated. The asymmetric torsional mode has been observed for the trans conformer in the infrared spectra of the gas phase at 108 and 104 cm −1 for the BH 3 and BD 3 species, respectively. These results are compared with similar quantities for some corresponding organophosphine—borane compounds.

Spectra and structure of small ring molecules. XLVII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid fluoride

The microwave spectrum of cyclobutylcarboxylic acid fluoride, c-C4H7CFO, has been recorded from 18.0 to 40.0 GHz. The a-type R-branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial-gauche conformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm−1. From the Stark effect the dipole moment components were determined to be: ‖μa‖ =2.97±0.02, ‖μb‖ =0.84±0.01, ‖μc‖ =0.35±0.01, and ‖μt‖ =3.11±0.01 D. The central line of an additional conformer has been identified with a B+C value of 3482 MHz which is consistent with the expected value for the equatorial-trans conformer. The infrared (3500 to 30 cm−1) and Raman spectra (3200 to 30 cm−1) have also been recorded for the gaseous and solid states of cyclobutylcarboxylic acid fluoride. Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. From the relative intensities of the Raman lines of the gas at 918 cm−1 (equatorial-trans) and 926 cm−1 (equatorial-gauche) as a function of temperature, the enthalpy difference was found to be 352±100 cm−1 (1.01 kcal/mol) with the equatorial-gauche being more stable. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. These results are compared to similar quantities for some related molecules.