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.