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

Den, Takuya; Menzi, Samuel; Frey, Hans-Martin; Leutwyler, Samuel (2017). Accurate gas-phase structure of para -dioxane by fs Raman rotational coherence spectroscopy and ab initio calculations. The Journal of Chemical Physics, 147(7), 074306. American Institute of Physics 10.1063/1.4997633

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p-Dioxane is non-polar, hence its rotational constants cannot be determined by microwave rotationalcoherence spectroscopy (RCS). We perform high-resolution gas-phase rotational spectroscopy ofpara-dioxane-h8and -d8using femtosecond time-resolved Raman RCS in a gas cell atT= 293 Kand in a pulsed supersonic jet atT∼130 K. The inertial tensor ofp-dioxane-h8is strongly asym-metric, leading to a large number of asymmetry transients in its RCS spectrum. In contrast, thed8-isotopomer is a near-oblate symmetric top that exhibits a much more regular RCS spectrum withfew asymmetry transients. Fitting the fs Raman RCS transients ofp-dioxane-h8to an asymmetric-top model yields the ground-state rotational constantsA0= 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 forp-dioxane-d8areA0= 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 corre-lated coupled-cluster singles, doubles and iterated triples method and the cc-pCVXZ basis set seriesfrom double- to quadruple-zeta (X = D, T, Q). Combining the calculated vibrationally averaged rotational constants Acalc0(X),Bcalc0(X),Ccalc0(X) for increasing basis-set sizeXwith non-linear extrap-olation to the experimental constantsAexp0,Bexp0,Cexp0allows to determine the equilibrium groundstate structure ofp-dioxane. For instance, the equilibrium C–C and C–O bond lengths arere(CC)= 1.5135(3) Å andre(CO) = 1.4168(4) Å, and the four axial C–H bond lengths are 0.008 Å longerthan the four equatorial ones. The latter is ascribed to thetrans-effect (anomeric effect), i.e., the partialdelocalization of the electron lone-pairs on the O atoms that are orientedtrans, relative to the axialCH bonds.

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