Accurate gas-phase structure of para-dioxane by fs Raman rotational coherence spectroscopy and ab initio calculations

p-Dioxane is non-polar, hence its rotational constants cannot be determined by microwave rotational coherence spectroscopy (RCS). We perform high-resolution gas-phase rotational spectroscopy of para-dioxane-h₈ and -d₈ using femtosecond time-resolved Raman RCS in a gas cell at T = 293 K and in a pulsed supersonic jet at T∼130 K. The inertial tensor of p-dioxane-h₈ is strongly asymmetric, leading to a large number of asymmetry transients in its RCS spectrum. In contrast, the d₈-isotopomer is a near-oblate symmetric top that exhibits a much more regular RCS spectrum with few asymmetry transients. Fitting the fs Raman RCS transients of p-dioxane-h₈ to an asymmetric-top model yields the ground-state rotational constants A₀ = 5084.4(5) MHz, B₀ = 4684(1) MHz, C₀ = 2744.7(8) MHz, and (A₀ + B₀)/2 = 4884.5(7) MHz (±1σ). The analogous values for p-dioxane-d₈ are A₀ = 4083(2) MHz, B₀ = 3925(4) MHz, C₀ = 2347.1(6) MHz, and (A₀ + B₀)/2 = 4002.4(6) MHz. We determine the molecular structure with a semi-experimental approach involving the highly correlated coupled-cluster singles, doubles and iterated triples method and the cc-pCVXZ basis set series from double- to quadruple-zeta (X = D, T, Q). Combining the calculated vibrationally averaged rotational constants A₀^calc(X),B₀^calc(X),C₀^calc(X) for increasing basis-set size X with non-linear extrapolation to the experimental constants A₀^exp,B₀^exp,C₀^exp allows to determine the equilibrium ground state structure of p-dioxane. For instance, the equilibrium C-C and C-O bond lengths are r_e(CC) = 1.5135(3) Å and r_e(CO) = 1.4168(4) Å, and the four axial C-H bond lengths are 0.008 Å longer than the four equatorial ones. The latter is ascribed to the trans-effect (anomeric effect), i.e., the partial delocalization of the electron lone-pairs on the O atoms that are oriented trans, relative to the axial CH bonds.

Accurate determination of the structure of cyclohexane by femtosecond rotational coherence spectroscopy and ab initio calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar molecules cyclohexane (C(6)H(12)) and cyclohexane-d(12) (C(6)D(12)). We measured the rotational B(0) and centrifugal distortion constants D(J), D(JK) of the v = 0 states of C(6)H(12) and C(6)D(12) to high accuracy, for example, B(0)(C(6)H(12)) = 4306.08(5) MHz, as well as B(v) for the vibrationally excited states ν(32), ν(6), ν(16) and ν(24) of C(6)H(12) and additionally ν(15) for C(6)D(12). To successfully reproduce the experimental RCS transient, the overtone and combination levels 2ν(32), 3ν(32), ν(32) + ν(6), and ν(32) + ν(16) had to be included in the RCS model calculations. The experimental rotational constants are compared to those obtained at the second-order Møller-Plesset (MP2) level. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows determination of accurate semiexperimental equilibrium structure parameters, for example, r(e)(C-C) = 1.526 ± 0.001 Å, r(e)(C-H(axial)) = 1.098 ± 0.001 Å, and r(e)(C-H(equatorial)) = 1.093 ± 0.001 Å. The equilibrium C-C bond length of C(6)H(12) is only 0.004 Å longer than that of ethane. The effect of ring strain due to the unfavorable gauche interactions is mainly manifested as small deviations from the C-C-C, C-C-H(axial), and C-C-H(equatorial) angles from the tetrahedral value.

Solvation Dynamics by Computer Simulation: Coumarin C153 in 1,4-Dioxane

Computer simulation results are presented for an atomistic pair potential model of 1,4-dioxane which takes into account molecular flexibility. The model has been conceived to be applied to the study of solvatochromism and solvation dynamics in the presence of the polar probe coumarin C153. Computer simulations on the pure liquid have produced thermodynamical, structural, and dynamical data in good agreement with available experimental measures. This constitutes a valuable test of the 1,4-dioxane all-atom model employed. The study of solute-solvent interactions for C153 in 1,4-dioxane has been motivated by the aim of casting light, through simulations, on the interesting experimental findings according to which such a solvent behaves as a "polar" solvent with respect to dynamic solvation properties. Molecular dynamics is particularly suitable to model the process and provides an interpretation of the so-called "dioxane anomaly". An investigation of the structure of the solvation shell and of the dynamics of solvation is presented and discussed. In particular, the satisfactory accordance between simulated and experimental solvation response implies that the simulations give a reliable description of both solute and solvent at a molecular level and reinforces the idea that the explicit inclusion of discrete solvent molecules is needed for a realistic treatment of solvation phenomena in which the local structure of the liquid plays a key role.

CH···O and CH···F Links Form the Cage Structure of Dioxane−Trifluoromethane

The molecular beam Fourier transform microwave spectrum of 1,4-dioxane-trifluoromethane has been assigned and measured. The two subunits form a cage stabilized by one C-H...O and two C-H...F weak hydrogen bonds. The C-H...O link involves the axial lone pair of one of the two equivalent ring oxygens, while the two C-H...F bridges connect trifluoromethane to the two axial hydrogens in positions 3 and 5. The dissociation energy has been estimated from the D(J) centrifugal distortion parameter to be approximately 6.8 kJ/mol.

Insights into the Structure of Cyclohexane from Femtosecond Degenerate Four-Wave Mixing Spectroscopy and Ab Initio Calculations

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order Møller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.

Single crystal X-ray structures of the two 4-heptadecyl derivatives of (1R,5S)-3,6,8-trioxabicyclo[3.2.1]octane

The single crystal structures of the two diastereomeric 4-heptadecyl derivatives of (1R,5S)-3,6,8-trioxabicyclo[3.2.1]octane have been determined by X-ray diffraction to be (1R,4R,5S)-heptadecyl-3,6,8-trioxabicyclo[3.2.1]octane (I) and (1R,4S,5S)-4-heptadecyl[3,6,8-trioxabicyclo[3.2.1]octane (II), respectively, which have an exo or axial 4-heptadecyl group, and an endo or equatorial 4-heptadecyl group, respectively. The structures of I and II had been suggested by their phase-sensitive 2D NOESY 1H-NMR spectra, but are now established unambiguously. These optically pure non-ionic lipid-like amphipathic molecules (I and II) represent the first 3,6,8-trioxabicyclo[3.2.1]octanes for which single crystal structures have been solved. Crystals of both isomer I and isomer II were orthorhombic with space group P2(1)2(1)2(1), and had unit cell dimensions of a = 9.586, b = 43.14, c = 5.289 A, and a = 7.34, b = 51.8, c = 5.636 A, respectively. The structures of I and II were both solved by using direct methods to R = 0.045 and R = 0.086, respectively. Both I and II pack in stacked bilayers with interdigitating and tilting hydrocarbon chains. The molecular and hydrocarbon cross sections are I: S = 50.70 A2, sigma = 19.00 A2; and II: S = 41.37 A2, sigma = 18.26 A2.

Stereospecific reactions of aryltellurium(IV) trichlorides with 3-cyclohexene-1-methanol and 3-cyclohexene-1,1-dimethanol: the X-ray crystal structure of 2′,4′-dimethoxyphenyl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichloride

Aryltellurium(IV) trichlorides react with 3-cyclohexene-1-methanol and 3-cyclohexene-1,1-dimethanol to give aryl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichlorides and aryl(cis-1-hydroxymethyl-trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichlorides. The structure of 2′,4′-dimethoxyphenyl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichloride has been determined by single crystal X-ray analysis. The compound C15H20Cl2O3Te, Mr = 446.8, crystallizes in the monoclinic space group P21/c with a = 9.626(2) Å, b = 15.768(5) Å, c = 11.176(3) Å, β = 92.08(1)°, V = 1695.2(5) Å3 and D(calc) = 1.751 for Z = 4. Tellurium is bonded axially to two chlorines and equatorially to aryl carbon and carbon from the cyclohexyl portion of oxabicyclo[3.2.1]oct-4-yl in a pseudo-trigonal-bipyramidal arrangement having an equatorial lone pair of electrons. Bond lengths are Te—Cl(1) = 2.492(3) Å, Te—Cl(2) = 2.539(3) Å, Te—C(4) = 2.175(10) Å, and Te—C(9) = 2.090(10) Å. Keywords: organotellurium(IV), 3-cyclohexene-1-methanol, 3-cyclohexene-1, 1-dimethanol.