Gas-phase Ab Initio Calculations Research Articles

The molecular structure and conformation of (trifluorosilylmethyl)cyclopropane as determined by gas-phase electron diffraction and ab initio calculations

The molecular structure and conformational stability of (trifluorosilylmethyl)cyclopropane (TFSMCP), C 3H 5CH 2SiF 3, with respect to the C ringCH 2 and the SiCH 2 axes have been studied by gas-phase electron diffraction and ab initio calculations using the basis sets 3-21G∗, 4-21G∗ and 6-31G∗∗. The electron diffraction conformational analysis with respect to the C ringC bond has provided an anticlinal:synperiplanar conformational composition of 75(12):25(12). A reasonable explanation for the preference of the anticlinal conformation is provided by the σ-π hyperconjugation effect. The occurrence of the syn conformer is most likely the result of attractive interactions between the fluorine atoms and the ring moiety. The major bond distances ( r a , Å) and angles (∠a, deg) for the anticlinal conformer obtained from the least-squares refinements with uncertainties estimated at 3σ are: r(CH) = 1.095(8), r(C ringC) = 1.548 (12), r(SiC) = 1.837 (5), r(SiF) = 1.593 (1); ∠F 6SiC = 111.3 (2.4), ∠SiCC = 115.0 (1.2), ∠CC(ring plane) = 129.5 (1.7), and the dihedral torsional angle τ SiCCX = 115.7 (2.9) (X is the center of the ring). All bonded distances in the syn conformer were assumed to be equal to the corresponding distances in the anticlinal form. Only the following angles for the syn conformer were refined: ∠F 7(8)SiC = 115.9 (6.4), ∠SiCC = 123.4 (3.7), ∠CC-(ring plane) = 124.6 (5.3). The potential barriers to internal rotation about the SiC and the CC ring axes have been calculated and compared to the corresponding barriers in various methyl cyclopropyl derivatives. It has been postulated that there is a peculiar similarity between the allyl and methyl cyclopropyl systems. Numerous derivatives of both systems have been calculated to demonstrate this intriguing resemblance.

Methylvinyldifluorosilane and Methylvinyldichlorosilane (H2CCHSiX2CH3, X = F, Cl): Structure, Conformation, and Torsional Potential As Determined by Gas-Phase Electron Diffraction and ab Initio Calculations

The molecular structures and conformations of methylvinyldifluorosilane (VFS), H2CCHSiF2CH3, and methylvinyldichlorosilane (VCS), H2CCHSiCl2CH3, have been studied by using gas-phase electron diffraction (GED) data at 25 °C. Ab initio molecular orbital (MO) calculations were used to establish constraints in the theoretical model used to analyze the GED data. These molecules exist in the gas phase as a mixture of two conformers, syn (S, torsional angle φ(CCSiC) = 0°) and gauche (G, torsional angle φ(CCSiC) close to 120°). Relevant structural parameters for VFS (syn) are as follows: bond lengths (rg), r(SiCvinyl) = 1.846(3) Å, r(SiCH3) = 1.851(3) Å, r(CC) = 1.344(5) Å, r(SiF) = 1.592(2) Å; bond angles (∠α), ∠CSiC = 113.4(11)°, ∠CCSi = 123.3(8)°; torsional angle, φ(G) = 117(14)°. For VFS the experimental gas-phase composition (%) was (syn/gauche) 35/65 (±41)%. An estimated conformational energy difference ΔE°G-S = 0.0(±1.2) kcal/mol was obtained for VFS. Relevant structural parameters for VCS (syn) are as follows: bond lengths (rg), r(SiCvinyl) = 1.843(4) Å, r(SiCH3) = 1.855(4) Å, r(SiCl) = 2.051(2) Å, r(CC) = 1.341(6) Å; bond angles (∠α), ∠CSiC = 111.6(17)°, ∠CCSi = 123.3(17)°; torsional angle, φ(G) = 121(16)°. For VCS the experimental composition was (syn/gauche) 45/55 (±64)%. An estimated conformational energy difference ΔE°G-S = 0.3(±1.8) kcal/mol was obtained from this composition. Error estimates are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data).

A comparative study of first hyperpolarizabilities of the acidic and basic forms of weak organic acids in water

Dynamic and static first hyperpolarizabilities (β) of a series of benzoic and cinnamic acids in water have been investigated. These acids exist in a mixture of acidic and basic forms in solution. The hyperpolarizability of the basic form is higher than that of the corresponding acid form which is attributed to the red-shift of the absorption maximum of the basic form relative to that of the acid form. A ZINDO-correction vector (CV) method including solvent effects through a self-consistent reaction field (SCRF) for computing β reproduce measured values within experimental errors. Static ab initio hyperpolarizabilities within SCRF are slightly lower than the ZINDO/CV/SCRF β’s. Semiemperical as well as ab initio gas phase calculations show the correct experimental trend in β but their magnitudes are much smaller than the measured values.

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

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.

Molecular Geometry of Benzaldehyde and Salicylaldehyde: A Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Investigation

The molecular geometries of benzaldehyde and salicylaldehyde have been determined by gas-phase electron diffraction and ab initio molecular orbital calculations at the MP2(FC)/6-31G* level. Several...

Is dicyclopropylacetylene freely rotating? A combined ab initio and gas-phase electron diffraction investigation of the molecular structure and conformations of dicyclopropylacetylene

The molecular structure of dicylopropylacetylene (DCPA) has been studied by gas-phase electron diffraction and ab initio calculations using the basis sets 3–21G∗, 4–21G∗, 6–31G∗, 631-G∗∗, and MP2 6–31 G∗ . The diffraction data are consistent with the gauche rotamer ( C 2 symmetry) which has the bisectors of the cyclopropyl groups rotated 91.7(7.5)° away from one another. However, a model with a low torsional barrier in which the cyclopropyl rings are considered to be essentially freely rotating cannot be ruled out. Ab initio calculations utilizing the RHF 6–31 G∗ (and MP2 6–31 G∗ ) basis sets are consistent with the gauche rotamer lying 383.8 cal mol −1 (362.8 cal mol −1) and 418.5 cal mol −1 (376.4 cal mol −1) lower in energy than the trans and cis rotamers, respectively. Large amplitude treatment has been undertaken applying a dynamical model consisting of a mixture of numerous pseudo-conformers. The force field provided by the ab initio calculations was utilized to calculate the vibrational mean amplitudes and to determine the r α structure. The major bond distances ( r a ) obtained from least-squares analysis (uncertainties in parentheses are 3σ) are: r( CH) = 1.091(3) A ̊ , r( CC>) = 1.197(3) A ̊ , r( CC) = 1.438(4) A ̊ , r( C 1C 2) = 1.525(2) A ̊ , and r( C 2C 3) = 1.511 A ̊ (assumed). The torsional angle around the CCCC axis τ = 91.7(7.5)°. The noticeable shortening of the carbon-carbon single bonds between the electron-rich systems, the triple bond and the cyclopropyl rings is discussed invoking the concept of π-electronic charge distribution of the carbon-carbon triple bond and other approaches. In order to obtain more pertinent details about the origin of the potential barrier in DCPA, the structures and the potential barriers for internal rotation of the dihalogenated derivatives of DCPA, bis-(1-fluorocyclopropyl)acetylene, (1-fluorocyclopropyl)(1-chlorocyclopropyl)acetylene, and bis-(1-chlorocyclopropyl)acetylene have been determined by ab initio calculations using the basis sets 4–21G∗ and 6–31G∗. The values of the most interesting parameter in this context, the dihedral angle τ, were found to be 98.65°, 102.11°, and 107.08°, respectively. The structural results of DCPA and its dihalogenated derivatives are compared and discussed.

Molecular structure of 3,3-dimethyl-3-silatetrahydrofuran from gas-phase electron diffraction analysis and ab initio calculations

The molecular structure of 3,3-dimethyl-3-silatetrahydrofuran has been studied by means of gas-phase electron diffraction (GED) and ab initio calculations at the HF/6-31G∗ level. Six different conformers of the title compound have been refined in the course of the GED analysis, allowing the vibrational amplitudes to vary over a wide range. Several bond length differences were taken from the results of the ab initio geometry refinement. Almost the same value of the R-factor, about 3%, has been obtained for all six models, whereas the ab initio calculation resulted in one single conformer (“O-envelope”). Based upon the combined information from the ab initio calculations and the GED work, the “O-envelope” conformer is chosen as the final structure. Geometrical parameters obtained (GED) are as follows (bond lengths in Ångströms, angles in degrees; three times standard deviation estimates in parentheses): r(SiC cyc) = 1.891(3) and 1.904; r(SiC) Me) = 1.877 and 1.879; r(CC) = 1.556(7); r(CO) = 1.431(3) and 1.445; r(C cycH) = 1.11(2); r(C MeH) = 1.10(2); ф( O C C Si) = 33.0(14) ; ∠C cycSiC cyc = 92.2(5); ∠C MeSiC Me = 107.2(8); ∠SiCC = 102.6(6); ∠CCO = 109.9(11); ∠HC cycH = 110.5(47); ∠SiC MeH = 111.6(14).

Molecular structure and conformational composition of 1-Chlorobutane, 1-Bromobutane, and 1-Iodobutane as determined by gas-phase electron diffraction and ab initio calculations

Gas-phase electron diffraction (ED), together with ab initio molecular orbital calculations, have been used to determine the structure and conformational composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane. These molecules may in principle exist as mixtures of five different conformers, but only three or four of these were observed in gas phase at temperatures of the ED experiments, 18‡C, 18‡C, and 23‡C, respectively. The observed conformational compositions (1-chlorobutane, 1-bromobutane, and 1-iodobutane) were AA (13 ± 12%, 21 ± 14%, 19 ± 17%), GA (60±13%, 33±32%, 17±31%), AG (12±16%, 8±12%, <1%), and GG (12 ±16%, 38± 34%, 64±31%). A and G denotesanti andgauche positions for the X-C1-C2-C3 (X=Cl, Br, I), and the C1-C2-C3-C4 torsion angles. The results for the most important distances (rg) and angles (∠α) from the combined ED/ab initio study for the GA conformer of 1-chlorobutane, with estimated 2σ uncertainties, arer(C1-C2)=1.519(3)a,r (C2-C3)=1.530(3) a,r (C3-C4)=1.543(3) a,r (C1-Cl)=1.800(4) a, <C1C2C3=114.3(6)ℴ, <C2C3C4=112.0(6)ℴ, <CCCl=112.3(5)ℴ. The results for the GA conformer of 1-bromobutane arer (C1-C2)=1.513(4) a,r (C2-C3)=1.526(4) a,r (C3-C4)=1.540(4) a,r(C1-Br)=1.959(8) a, <C1C2C3=115.3(11)ℴ, <C2C3C4=112.8(11)ℴ,<CCBr=112.1(14)ℴ. The results for 1-chlorobutane and 1-bromobutane are compared with those from earlier electron diffraction investigations. The results for the GA conformer of 1-iodobutane arer (C1-C2)=1.506(5) a,r (C2-C3)=1.518(5) a,r (C3-C4)=1.535(5) a,r (C1-I)=2.133(11) a, <C1C2C3=116.8(15)ℴ, <C2C3C4=115.3(15)ℴ, <CCI=110.2(14)ℴ. Differences in length between the different C-H bonds in each molecule, between the different C-C bonds, between the different CCH angles, and between the different CCC angles were kept constant at the values obtained from the ab initio calculations.

A combined ab initio and gas-phase electron diffraction investigation of the molecular structure and conformation of (silylmethyl)cyclopropane

The molecular structure and conformational composition of (silylmethyl)cyclopropane (SMCP), C 3H 5CH 2SiH 3, with respect to the C ringCH 2 and the CH 2Si axes have been studied by gas-phase electron diffraction and ab initio calculations using a 6-31G∗ basis set. The conformational analysis, with respect to the C ringC bond, has shown that the gauche (skew) form is the predominant conformation (> 98%) in the gas phase. A reasonable explanation for the preference of the gauche conformation is provided by the σ–π hyperconjugation effect. Repulsive through-space interactions are most probably responsible for the cis conformer, being higher in energy and thus less favorable. The major bond distances (r a, Å −1) and angles (°) for the skew conformer obtained from the least-squares refinements with uncertainties estimated at 3σ are r(CH) = 1.120(4), r(C 1C 2) ring = 1.510(10), r(C 2C 3) ring = 1.490(12), r(C ringC) = 1.540(10), r(SiC) = 1.876(2); ∠HSiC = 111.0(2.0), ∠SiCC = 112.1(1.0), ∠CC(ring plane) = 122.7(0.5), and the dihedral torsional angle τ SiCCX = 119.0(2.0) (X is the center of the ring).

An electron diffraction, ab initio and vibrational spectroscopic study of 1,2-di- tert-butyldisilane

The molecular structure of 1,2-di- tert-butyldisilane has been accurately determined by gas-phase electron diffraction (GED) and ab initio calculations. These techniques show that the large majority of molecules at room temperature have the anti conformation with overall symmetry C 2, and vibrational spectra confirm this conclusion. Infrared spectra of the gas and liquid phases, and Raman spectra of the liquid and solid phases, have been recorded for (CH 3) 3CSiH 2SiH 2C(CH 3) 3 and (CH 3) 3CSiD 2SiD 2C(CH 3) 3. The most striking feature of this structure ( r a) is a relatively large deviation of the SiSiC angle from the parent tetrahedral angle 109.5° (113.7(3)°, GED; 114.4°, SCF 6-31 G∗ as calculated for the anti form). That the SiSi bond length does not show any substantial deviation from its usual value (234.8(3) pm, GED; 236.8 pm, SCF 6-31 G∗ computed for the anti form) is also substantiated by the value of the SiSi valence force constant (169 N m −1) given by normal coordinate analysis. The t-butyl groups are tilted so that the SiC bonds (GED ( SCF 6-31 G∗ ): 190.1(1) (191.9) pm) do not coincide with the local C 3 axes of the C(CH 3) 3 groups in which the CC bond length is 154.1(1) (GED); 154.0 ( SCF 6-31 G∗ ) pm. The conformations along all the single bonds are more or less staggered.

Intramolecular Hydrogen Bonding and Molecular Geometry of 2-Nitrophenol from a Joint Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Investigation

The molecular geometry of 2-nitrophenol has been determined by a joint investigation of gas-phase electron diffraction and ab initio molecular orbital calculations. RHF/6-31G * , RHF/6-31G ** , and MP2/6-31G * optimizations were used to determine small parameter differences, such as Δ(N=O), Δ(C-C), and Δ(C-N=O), which in turn were utilized as constraints in an electron diffraction structure analysis. The present experimental and calculated geometries are consistent regarding (i) the planarity of the molecule and (ii) all the structural features including strong hydrogen bonding between the nitro group oxygen and hydroxy hydrogen and the structural changes in the rest of the molecule as compared with phenol and nitrobenzene on the one hand and 2-nitroresorcinol on the other

Molecular structures and conformational compositions of 1,2-dichloro-1,1,2,3,3,3-hexafluoropropane and 1,2-dibromo-1,1,2,3,3,3-hexafluoropropane, as determined by gas-phase electron diffraction and ab initio calculations

The molecular structures and conformational compositions of 1,2-dichloro-1,1,2,3,3,3-hexafluoropropane (CFP) and 1,2-dibromo-1,1,2,3,3,3-hexafluoropropane (BFP) have been studied by gas-phase electron diffraction at 23 and 25°C, respectively. Both compounds form a mixture of two conformers, a more stable anti form (with the C1X bond anti to the C2C3 bond) and a less stable gauche+ form. For CFP and BFP the mole fractions of the anti forms, with uncertainties estimated at 2σ, were found to be 0.67(13) and 0.82(11), respectively, corresponding to energy differences of Δ E CFP = 0.53 and Δ E BFP = 1.0(5) kcal mol −1. To obtain a more detailed investigation of the conformational geometries, results from ab initio molecular orbital calculations have been included in the analysis.

Solvent effects on the barrier to isomerization for a tertiary amide from ab initio and Monte Carlo calculations

The effects of solvation on the free energies of activation for rotation about the carbonyl C-N bond in N,N-dimethylacetamide (DMA) have been examined through a combination of gas-phase ab initio calculations and solution-phase statistical mechanics simulations. The geometries of the planar ground state and the transition states with the nitrogen lone anti and syn to the oxygen were optimized with the 6-31G(d) basis set. Subsequent calculations of the vibrational frequencies and correlation energies (MP4(fc)SDTQ/6-31G(d)//6-31G(d)) predict ΔGs of 14.6 and 18.7 kcal/mol for the anti and syn transition states at 298 K

Theoretical calculation of electrode potentials: Electron‐withdrawing compounds

AbstractThe electrode potential of 2,3‐dicyanobenzoquinone in aqueous solution has been calculated relative to parabenzoquinone using a thermodynamic cycle approach that includes accurate gasphase ab initio calculations and calculation of differences in free energies of hydration using the free‐energy perturbation method. The discrepancy between the calculated and experimental electrode potential is disappointingly large (99 mV) compared to previous studies using this approach. This, along with the experimental evidence, suggests that the experimental value itself is too large and that theoretical approaches may indeed be as reliable as experimental ones for determining redox properties of molecules such as 2,3‐dicyanobenzoquinone. In the light of this discrepancy we have examined the variation of the results with the basis set, inclusion of electron correlation and changes in the parameters used in the molecular dynamics free‐energy simulations. The results are shown to be dependent upon the torsional parameters and especially dependent upon the basis set or semiempirical method used to obtain the electrostatic potential‐derived charges. The best charge set was determined using the ab initio criteria of completeness—as far as it can be applied to large molecules—and also by studying the effect of hydration on these charges. This was done by allowing the solvent to perturb the wave function prior to the electrostatic potential determination. Thus, 3‐21G and 6‐31G* basis sets were found to give satisfactory results. Similar results were obtained using semiempirical and ab initio geometries.

Determination of the molecular structure of ethylphosphonothioic dichloride by gas-phase electron diffraction and ab initio calculations

The molecular structure of ethylphosphonothioic dichloride, CH 3CH 2P (S)Cl 2, in the gas phase has been determined from an electron diffraction study. The gas consists of an equilibrium mixture of two conformers, in which the methyl group is trans or gauche with respect to the sulfur atom, but it is not possible to measure the proportions of the two forms because of the similarity of the sulfur and chlorine scattering powers, and of the PS and PCl bond lengths. Ab initio calculations at the 3-21G* level, were therefore performed, and these indicate that the conformation should be 82% gauche and 18% trans. These proportions were then used in the electron diffraction refinements, and the calculated differences between geometric parameters, most notably in the PCC angles, are also incorporated. Refined parameters ( r a) for the major conformer include r (PCl) 203.0(1), r(PS) 189. ∠ClPC (mean) 103.1 (5), ∠CPS 116.1 (12) and ∠ClPCl 102.0(4)°.

The CH 2Cl torsional barrier of benzyl chloride studied by gas-phase absorption spectroscopy and ab initio calculations

The high-resolution gas-phase 269 nm absorption spectrum of benzyl chloride in the 0-0 region is reported. A long sequence on the high wavenumber side of the 0-0 band, assigned to the torsional mode, points to an internal rotation barrier larger in the excited state than in the ground state. This can be explained by a stronger interaction between the π-electron system and the chlorine atom. Ab initio calculations at the 6-31G level yield an S 0 CH 2Cl torsional barrier of 11.1 kJ mol −1 in benzyl chloride. A barrier of 22.6 kJ mol −1 is estimated in S 1 from the gas-phase absorption spectrum.