Stable Gauche Conformer Research Articles

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.

Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations for 3‐iodopropene

AbstractThe Raman spectra (3200—10 cm−1) of gaseous, liquid and solid and infrared spectra (3200—40 cm−1) of gaseous and solid 3‐iodopropene, H2CCHCH2I, were recorded. The fundamental asymmetric torsional transition of the most stable gauche conformer was observed in the Raman and far‐infrared spectra of the gas at 95 cm −1. In the Raman spectrum of the liquid a second CI stretch and two skeletal bending modes were identified for the less stable cis conformer. From a study of the Raman spectrum of the liquid at various temperatures, a value of 648±81 cm−1 (1.85±0.23 kcal mol−1)was determined for the enthalpy difference between the higher energy cis and preferred gauche conformers. These data, along with the dihedral angle for the gauche conformer obtained from an electron diffraction study, combined with results obtained from ab initio calculations, allow for an estimate of the potential function governing internal rotation about the CC bond. The values for the torsional potential coefficients are V1 = −208 ± 10, V2 = −532 ± 10, and V3 = 534 ± 8 cm−1. This potential is consistent with a dihedral angle (∠ CCCI) of 113.0° for the gauche conformer cis to gauche, gauche to gauche and gauche to cis barriers of 160, 904 and 738 cm−1, respectively, and an enthalpy difference between the conformers of 580 ± 30 cm−1 (1.66 ± 0.09 kcal mol−1). The asymmetric torsional potential surface, complete equilibrium geometries and vibrational frequencies were calculated using restricted Hartree‐Fock calculations with the STO‐3G*, 3‐21G*/medium, a medium‐sized combination basis set suitable for a DZ‐type contraction of the valence shell orbitals, and the LANL1DZ basis sets. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

Infrared and Raman spectra, conformational analysis, barriers to internal rotation, vibrational assignment, and ab initio calculations of trans-1,3-dichloropropene

The infrared (3200-50 cm -1 ) and Raman (3200-10 cm -1 ) spectra of gaseous and solid trans-1,3-dichloropropene, ClHC-CH(CH 2 Cl), have been recorded. Additionally, the Raman spectrum of the liquid has been obtained and qualitative depolarization ratios have been measured. The fundamental CH 2 Cl torsion for the more stable gauche conformer has been observed in the far-infrared spectrum of the gas and has a hybrid contour centered at 84.0 cm -1

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.

Ab initio molecular orbital study of dialuminum-ethylene complexes

The geometries of the various isomeric dialuminum-ethylene complexes Al 2C 2H 4 have been optimized using the STO-3G, 3-21G (*) and 6-31G (**) basis sets. Vibrational frequency calculations show that only two out of the six structures examined are thermodynamically stable. Unlike the case of Li 2C 2H 4, 1,2-dialuminoethane is most stable and more ethane-like than 1,2-dilithioethane perhaps due to the larger covalent character of the CAl bond. Besides, 1,2-dialuminoethane has been found, in contrast to ethane and 1,2-dilithioethane, to have no stable gauche conformer.

Conformational analysis of 1,2-dihalo-tetramethylethanes and 1,2-dihalo-tetramethyldisilanes by molecular mechanics calculations

Using non-bonding atom⋯atom interaction potentials derived from gas-phase data on related haloalkanes, torsional potentials of the halogen substituted (X=F, Cl,Br,I) molecules (CH 3) 2XCCX(CH 3) 2 and (CH 3) 2XSiSiX(CH 3) 2 have been calculated. These molecules have stable anti and gauche conformations. In all molecules anti has a lower energy than gauche. The rotational barrier heights corresponding to a transition from gauche to anti have values between 0.5 an 1.5 kcal mol −1 in the disilanes, while the corresponding values in the ethane derivatives are 3.5–5.5 kcal mol −1. Calculated results are compared with the gas-phase observations on (CH 3) 2XSiSiX(CH 3) 2.

Conformational analysis of cis,cis-2,2′- and 1,1′-halogen-substituted bicylcopropyl derivatives by molecular mechanics calculations

Torsional potential curves of the title compounds and bicyclopropyl itself have been calculated. The 1,1′-dihalobicyclopropyls have stable anti and gauche conformers. For the chloro, bromo and iodo substituted molecules gauche is the low energy conformer, while anti is the low-energy conformer in the fluoride and for bicyclopropyl itself. As the size of the halogen atom increases the energy of the anti form also steadily increases. The potential curves for the cis,cis-2,2′-dihalobicyclopropyls have only one conformational minimum corresponding to a staggered anti form.

Conformational analysis of the halomethyl cyclopropanes and the halosilyl cyclopropanes by molecular mechanics calculations

Using non-bonding atom⋯atom interaction potentials derived from gas-phase data on haloalkanes, the torsional potentials in the molecules XH 2CC 3H 5 and XH 2SiC 3H 5 have been calculated. The molecules have stable gauche and syn conformations. Our calculations show that for FH 2CC 3H 5 syn is the conformer of lowest energy, while for the remaining XH 2CC 3H 5 molecules (X=Cl,Br,I) gauche is the low-energy form. In contrast to this, syn is the low-energy form of all XH 2SiC 3H 5 molecules X=(F,Cl,Br,I). The molecular mechanics results agree with the available observations on XH 2CC 3H 5 compounds (X=Cl,Br,I). Conformational energies, rotational barrier heights and torsional force constants have been calculated.

Phosphinodifluorophosphine (H 2PPF 2): Is there a stable gauche conformer? An ab-initio study

The effect of the use of different basis sets on the conformation stability and on the dipole moment of phosphinodifluorophosphine has been studied at the ab-initio HF-SCF-LCAO level. Geometry optimization has been carried out with the 6-31G** basis. Although the trans and gauche (70.8°) conformers are predicted to be equally stable relative to the cis conformer, the small barrier of 0.9 kcal mol −1 for the gauche— trans interconversion and zero-point energy arguments do not seem to agree with the experimental observation of a stable gauche conformer.

Calculation of the vibrational spectra of methyl vinyl sulfide and its deuterated derivatives

1. Vibrational spectra were obtained for methyl vinyl sulfide and its deuterated analogs and the band were assigned on the basis of normal mode analysis. 2. The major differences in the vibrational spectra of methyl vinyl sulfide and methyl vinyl ether result from differences in the geometrical parameters of the CSC and COC fragments, difference in the mass of the S and O atoms, and difference in the electronegativities of these atoms. 3. The vibrations whose frequencies are sensitive to rotational isomerism of methyl vinyl sulfide were established. The analysis shows that the C3SC1C2 torsional angle is about 135° in the less stable gauche conformer.

Spectra and structure of organophosphorus compounds. XXIII. Microwave spectra, electric dipole moment, and molecular structure of two conformers of ethyldifluorophosphine

The microwave spectrum of ethyldifluorophosphine, CH3CH2PF2, has been investigated in the region from 18.0 to 39.5 GHz. The spectra of two conformers have been identified. The ground state and seven vibrationally excited states have been assigned for the more stable gauche conformer (phosphorus lone pair with respect to CH3 group), whereas the ground state and one excited state have been assigned for the trans form. Rotational and most quartic centrifugal distortion constants have been determined with A=6784.501(3), B=2930.156(2), C=2352.576(2) MHz for gauche and A=5444.910(3), B=3290.133(3), C=2871.915(3) MHz for trans, both in the ground state. Electric dipole moments were obtained from ground state Stark effect measurements: ‖ μa‖=1.990(4), ‖ μb‖=0.855(4), ‖ μc‖=0.091(7), ‖ μt‖=2.168(2) D for gauche and ‖ μa‖=1.780(10), ‖ μb‖=0, ‖ μc‖=1.077(10), ‖ μt‖=2.081(10) D for trans. Using a new computer program, a diagnostic least-squares fit of the structural parameters suggests significant structural differences between the conformers, notably the C–C–P angle which changes from 110.7° in the gauche form to 114.9° in the trans form. From a temperature study of the microwave spectrum, it was found that the gauche conformer is at least 80 cm−1 (229 cal/mol) more stable than the trans conformer. These results are compared to similar quantities in some corresponding molecules.

Structure des associations nitrile—phénol: étude expérimentale et théorique

A theoretical study by STO—3G method of the hydrogen bond association between acetonitrile and phenol is reported. The results are compared to those previously obtained by static dielectric constant measures and CNDO/2 calculations. The latter two methods, experimental and theoretical, have made possible a comparative study of propionitrile—, fumarodinitrile— and succinonitrile—phenol complexes. The barriers to internal rotation in the isolated succinonitrile molecule are determined. In addition to the trans conformation, there is a stable gauche conformation, the existence of which appears to be more probable with the formation of the hydrogen bond.