Alan D. Richardson

Conformational analysis. 23: A gas-phase electron-diffraction and ab initio molecular orbital investigation of 3-fluoropropan-1-ol. Is there significant internal hydrogen bonding? ☆

Abstract There are 14 possible rotational conformers of 3-fluoropropan-1-ol, based on the assumption of three potential minima for rotation about single bonds. Of these, only the g G − G form ( g ,G= gauche , a ,A= anti ) where the sequence indicates rotation around the C–O, the C O –C, and the C–C F bonds, with the minus sign indicating clock-wise rotation has the hydrogen and fluorine atoms in position for formation of a hydrogen bond. Analysis of gas-phase electron-diffraction data obtained at a sample temperature of 20°C indicates the dominant form to be a GG (41 (2σ=19)% of the sample). The hydrogen-bonded form g G − G comprises 2 (19)% of the sample. These values reflect the assumption that the amounts of the remaining components of the system exist in proportions given by the results of ab initio (B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)) calculations. Weighted average parameter values ( r g /A, ∠ g /deg) for all forms are r (C–C)=1.528(3), r (C 1 –C 2 )− r (C 2 –C 3 )=0.066(12), r (C–H)=1.115(4), r (C–O)=1.380(8), r (C–F)=1.450(6), ∠(COH)=105.9(assumed), ∠(CCF)=109.7(8), 〈∠(CCC), ∠(CCO)〉=111.8(6), ∠(COH)−∠(COH)=3.1(assumed), ∠(CCH)=109.4(7). Torsion-angle parameters could not be refined. Weighted average theoretical torsion angles for conformers with G or G − orientations are ∠(OCCC)=63.2(3), and ∠(CCCF)=61.0(3). Ab initio calculations at several levels of theory with several basis sets predicted substantially different amounts of the g G − G form. Best agreement with experiment was given by the B3LYP/6311++G(3d,2p)//B3LYP/6-31G(d) calculations.

Conformational analysis 20. A gas-phase electron-diffraction and ab initio molecular orbital investigation of 3-chloro-propan-1-ol

Abstract The composition of gaseous 3-chloropropan-1-ol at 115°C and 367°C, and the molecular structures of the rotational conformers have been investigated by electron diffraction aided by results from ab initio ( HF 6-31 G ∗ ) molecular orbital calculations. The mole fractions of the two conformers capable in principle of forming intramolecular OH⋯Cl hydrogen bonds was found to be only 0.04 (14) and 0.05 (24) at the lower and higher temperatures, in good agreement with the theoretical prediction of 0.017 at 0 K, but substantially less than that estimated from an earlier electron diffraction study. Our result stands in contrast to that for 2-chloroethanol where the gauche conformer dominates the vapor-phase composition, presumably in large part because of intramolecular hydrogen bonding. The parameter values of 3-chloropropan-1-ol are unexceptional. Some of the more important ones ( r g A , ∠ α deg ) at 115°C with estimates of 2σ uncertainties are 〈r(CC〉 = 1.527 (5), r(COC) = 1.547 (12), r(CCCl) = 1.497 (12), 〈r(CH〉 = 1.099 (9), r(CO) = 1.424 (6), r(CCl) = 1.803 (4), ∠(COH) = 107.3 (assumed), ∠(CCCl) = 111.6 (11), [∠( CCC ) + ∠( CCO )] 2 = 110.3 (11) , ∠(CCC) − ∠(CCO) = 3.7 (assumed); assumed values are from the theoretical calculations.

A Reinvestigation of the Structure and Torsional Potential of N2O5 by Gas-Phase Electron Diffraction Augmented byAb Initio Theoretical Calculations

Gaseous N2O5 consists of two NO2 groups bonded to a bridging O-atom to form a nonlinear N−O−N moiety. The NO2 groups undergo slightly hindered internal rotation around the bonds to the bridge so that instantaneous composition of the gaseous system is characterized by molecules with all combinations of torsion angles. In an earlier investigation, an attempt was made to determine the coefficients for an empirical form of the double-rotor torsional potential, and the bond lengths and bond angles measured subject to assumptions that the structure of the O−NO2 groups was invariant to torsion angle and that these groups had C2v symmetry. The system has now been reinvestigated in terms of a more realistic model in which this symmetry restriction was relaxed, account was taken of structural changes in the NO2 groups with torsion angle as predicted by ab initio theory at the B3LYP/6-311+G* level, and a more convenient form of the torsional potential was assumed. The most stable conformation has C2 symmetry with torsion angles τ1 (defined as ∢(N−O−N=O4)) equal to τ2 (defined as ∢(N−O−N=O6)) equal to 33.7°; because of the broad potential minimum in this region, the uncertainty in these angles is difficult to estimate, but is probably 3 – 4°. The results for the bond lengths and bond angles for the most stable conformation are rg(N−O)=1.505(4) A, rg(N=O)=1.188(2) A, ∢α(N−O−N)=112.3(17)°, ∢α(O=N=O)=134.2(4)°, 〈∢α(O−N=O)〉=112.8(2)°. The difference between the symmetry-nonequivalent O−N=O angles is estimated to be ca. 6.7° with the larger angle positioning the two N=O bonds on different NO2 groups nearest each other. These average values are similar to those obtained in the original study. The main difference is found in the shape of the torsional potential, which at τ1/τ2=0/0 has a saddle point in the present work and a substantial peak in the earlier. The implication of the torsion-angle findings for electron-diffraction investigations of this type is discussed.

Vinyl dichlorosilane and vinyl dibromosilane (H2CCH–SiHX2, X=Cl, Br): conformational structure and vibrational properties determined by gas-phase electron diffraction, ab initio molecular orbital calculations, and variable-temperature Raman spectroscopy

Abstract The molecular structures, conformations, vibrational spectra, and torsional potentials of vinyl dichlorosilane (VDC) H2C CH–SiHCl2, and vinyl dibromosilane (VDB) H2C CH–SiHBr2, have been studied using gas-phase electron diffraction (GED) data at 23–25°C and variable-temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were handled by a dynamic theoretical model using a cosine Fourier potential function in describing the torsional coordinate. According to the GED refinements, these molecules exist in the gas phase at room temperature as a mixture of two minimum energy conformers, syn (torsional angle φ(CCSiH)=0°) and gauche (torsional angle φ(CCSiH)≈120°). Relevant structural parameters for syn-VDC are as follows: Bond lengths (rg): r( Si–C )=1.847(5) A , r (Si–Cl) =2.042(2) A , r( C C )=1.357(7) A . Bond angles (∠α): ∠CSiCl=110.3(6)°, ∠CCSi=121.8° (calc.). Relevant structural parameters for syn-VDB are as follows: bond lengths (rg): r (Si–C) =1.827(9) A , r (Si–Br) =2.206(2) A , r (C C) =1.366(10) A . Bond angles (∠α): ∠CSiBr=110.1(8)°, ∠CCSi=121.7° (calc.). Uncertainties are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). From the variable-temperature Raman investigation in the liquid phase, the energy differences are: VDC, Δ E° S−G =+0.11± 0.06 kcal mol −1 ; VDB, Δ E ° S−G =+0.23±0.07 kcal mol −1 . The Raman energies are average values obtained from two separate line doublets for each molecule, and they have been used in the GED least-squares refinements as valuable constraints.

Internal hydrogen bonding in gaseous 3-aminoacrolein: an electron-diffraction investigation augmented by ab initio calculations of its molecular structure and conformational composition

Abstract The molecular structure of 3-aminoacrolein has been investigated in the gas phase by electron diffraction, aided by ab initio molecular orbital calculations at the HF 6–31 G ∗ level and normal coordinate analyses. Of the four possible planar or near planar conformers cis s -cis (CSC), trans s -cis (TSC), trans s -trans (TST), and cis s -trans (CST) where the symbols refer respectively to orientations about the CC and CC bonds, CSC is found to be the overwhelmingly dominant conformer in agreement with the MO results. Assuming planarity of the CSC form, which is consistent with the theoretical results to within 0.1° in all torsion angles, the values of its structural parameters ( r g A and ∠ α ) and mole fraction ( x ) with rough estimates of 2σ uncertainties are 〈 r (NH)〉 = 1.002 (26), 〈 r (CH)〉 = 1.086 (20), r (CO) = 1.232 (7), r (CN) = 1.358 (26), r (CC) = 1.363 (31), r (CC) = 1.424 (18), ∠OCC = 127.0 (32), ∠NCC = 124.2 (57), ∠CCC = 120.7 (28), ∠H 6 NC = 119.5 (33), ∠H 7 NC = 122.1 (32), ∠H 1.8 CC = 119.3 (33), ∠H 10 CC = 116.1 (33), ∠CCNH 7 = 180.0, ∠CCNH 6 = 0.0, ∠CCCN = 0.0, ∠CCCO = 0.0, x = 0.98 (41). These values are reasonable, but should be accepted with care because of data contamination caused by sample decomposition during the experiments. However, although the measured composition is subject to considerable experimental uncertainty, the combined theoretical and experimental evidence leaves no doubt that only very small amounts of conformers other than CSC can be present. The relative stability of the CSC form is surely due in large part to the formation of a NH…O internal hydrogen bond.

The Puzzle of Bond Length Variation in Substituted Cyclobutenes. A New Example: Molecular Structure and Conformations of 1,2-Dimethoxy-3,3,4,4-tetrafluorocyclobut-1-ene

The structure and composition of 1,2-dimethoxy-3,3,4,4-tetrafluorocyclobut-1-ene (DMCB) have been measured by electron diffraction from the gas at a temperature of 370 K with the help of auxiliary data from molecular orbital and normal coordinate calculations, the former at several levels of theory and basis-set size, most importantly B3LYP/cc-pVTZ. The compound was found to exist primarily as a rotamer of C(s) symmetry (ca. 98%; 2sigma = 11%) with the remainder one of C(2v) symmetry; theory predicts about 88% C(s). Values for some of the more important parameters (r(g)/A; angle(alpha)/deg) of the C(s) form are r(C=C) = 1.337(21), r(C1-C4) = 1.496(8), r(C2-C3) = 1.501(8), r(C3-C4) = 1.567(12), r(C1-O) = 1.318(12), r(C2-O) = 1.340(12), r(C3-F) = 1.375(4), r(C4-F) = 1.368(4), angle(ave)(C=C-C) = 94.4(4), angle(ave)(C=C-O) = 133.5(12), angle(ave)(C-O-C) = 119.6(13), and angle(ave)(F-C-F) = 104.4(7). Surprisingly, although electron-diffraction values for the fluorinated C3-C4 bond in other cyclobutenes are greater than that for cyclobutene itself, that is not the case for DMCB where it is found to be about the same. Details of the DMCB structure, together with possible reasons for the observed variations in the length of the C3-C4 bond in fluorinated cyclobutene-like molecules, are discussed.