Chloromethyldichloromethylsilane and chloromethyldimethylchlorosilane: structure, conformational composition and torsional potential determined by gas-phase electron diffraction and ab initio molecular orbital calculations

Abstract

The molecular structure and conformation of chloromethyldichloromethylsilane (TCS), ClH 2CSiCl 2CH 3, and chloromethyldimethylchlorosilane (DCS), ClH 2CSiCl(CH 3) 2, have been studied using gas-phase electron diffraction at 24°C and ab initio molecular orbital calculations. These molecules exist in the gas phase as a mixture of two conformers, anti (A, torsion angle ∅(ClCSiX) = 180°, X = CH 3 (TCS) or Cl (DCS)) and gauche (G, torsion angle (∅(ClCSiX) close to 60°). Some relevant structural parameters for TCS (gauche) are: bond lengths ( r g), r( SiC(Cl)) = 1.877(5) A ̊ , r( SiCH 3) = 1.854(5) A ̊ , r( CCl) = 1.787(7) A ̊ , r( 〈 SiCl 〉 ) = 2.046(3) A ̊ (average value); bond angles (∠ α), ∠CSiC = 113.6(23)°, ∠SiCCl = 110.4(6)°, ∠ 〈 CSiCl 〉 = 108.8(5)°, ∠ClSiCl = 107.9(12)°; torsion angle, ∅(G) = 51(5)°. For TCS the experimental gas-phase composition (%) was (anti/gauche) 16 84 with error limits ± 15%. With ΔS° A-G = −R ln 2 + ΔS∗ , where ΔS∗ is the entropy difference from vibrational and rotational partition functions calculated from ab initio, an estimated conformational energy difference ΔE° A-G = 0.6(± 0.7) kcal mol −1 was obtained for TCS. Some relevant structural parameters for DCS (anti) are: bond lengths ( r g), r( SiC(Cl)) = 1.888(3) A ̊ , r( SiCl) = 2.078(5) A ̊ , r( CCl) = 1.793(14) A ̊ ; bond angles (∠ α), ∠ CSiC = 111.5°(assumed), ∠SiCCl = 110.7(10)°, ∠ClSiC(Cl) = 104.0°(assumed), ∠SiCH = 112.3(15)°; torsion angles, ∅(A) = 161(3)°, ∅(G) = 70(8)°. For DCS the anti conformer showed an apparent deviation from the exactly staggered value of 180°, but this is a result of large-amplitude torsional motion about the SiC bond. For DCS the experimental composition was (anti/gauche) 59 41 with error limits ± 16%. Using ΔS° A-G = −R ln 2 + ΔS∗ an estimated conformational energy difference ΔE° A-G = −0.6(± 0.4) kcal mol −1 was obtained from this composition. Error limits are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). Full geometry optimizations were performed for all conformers of both molecules employing ab initio molecular orbital HF/6–31G(d) level of theory, while conformational energy calculations were performed at the MP2(fc)/6–311 + G(d,p)//HF/6–31G(d) level, with scaled zero-point vibrational energy corrections from frequency calculations at the HF/6–31G(d) level. The theoretical energies and geometries are compared with the experimental results.