We have measured the intermolecular dissociation energies D 0 of supersonically cooled 1-naphthol (1NpOH) complexes with the solvents S=furan, thiophene, 2,5-dimethylfuran and tetrahydrofuran. The naphthol OH forms non-classical H-bonds to the aromatic π-electrons of furan, thiophene and 2,5dimethylfuran, and a classical H-bond to the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization (SEP-R2PI) method, the ground-state D 0 (S 0 ) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory (DFT-D) methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set) and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (Edge) isomers are more stable than the Face isomers bound by dispersion; experimentally, we only observe Edge isomers. We compare the calculated and experimental D 0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent,ᾱ(S). In contrast, the D 0 values of the classically H-bonded complexes are larger, increase more rapidly at lowᾱ(S) but saturate for largeᾱ(S). The calculated D 0 (S 0 ) values for the cyclopropane, benzene, furan and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol, those of thiophene and 2,5-dimethylfuran are ∼ 3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D 0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD=1.7 kJ/mol), the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively.

The S0 → S1 vibronic spectrum and S1 state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. The 0(0)(0) rotational band contour is polarized in-plane, implying that the electronic transition is (1)ππ*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the (1)ππ* (S1) transition of 5FCyt. The spectral region from 0 to 300 cm(–1) is dominated by overtone and combination bands of the out-of-plane ν1′ (boat), ν2′ (butterfly), and ν3′ (HN–C6H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the (1)ππ* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm(–1); this is attributed to the (1)ππ* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν1′, ν2′, and ν3′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm(–1), rapid nonradiative relaxation (k(nr) ≥ 10(12) s(–1)) sets in, which we interpret as the height of the (1)ππ* state barrier in front of the lowest S1/S0 conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν1′, 2ν2′, 2ν1′ + 2ν2′, 4ν2′, and 2ν1′ + 4ν2′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν1′ + 4ν2′ level at +234 cm(–1). These gas-phase lifetimes are twice those of S1 state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its S1 ((1)ππ*) state is essentially controlled by the same ~1200 cm(–1) barrier and conical intersection both in the gas phase and in solution.

The gas-phase rotational motion of hexafluorobenzene has been measured in real time using femtosecond (fs) time-resolved rotational Raman coherence spectroscopy (RR-RCS) at T = 100 and 295 K. This four-wave mixing method allows to probe the rotation of non-polar gas-phase molecules with fs time resolution over times up to ∼5 ns. The ground state rotational constant of hexafluorobenzene is determined as B0 = 1029.740(28) MHz (2σ uncertainty) from RR-RCS transients measured in a pulsed seeded supersonic jet, where essentially only the v = 0 state is populated. Using this B0 value, RR-RCS measurements in a room temperature gas cell give the rotational constants Bv of the five lowest-lying thermally populated vibrationally excited states ν7/8, ν9, ν11/12, ν13, and ν14/15. Their Bv constants differ from B0 by between -1.02 MHz and +2.23 MHz. Combining the B0 with the results of all-electron coupled-cluster CCSD(T) calculations of Demaison et al. [Mol. Phys. 111, 1539 (2013)] and of our own allow to determine the C-C and C-F semi-experimental equilibrium bond lengths re(C-C) = 1.3866(3) Å and re(C-F) = 1.3244(4) Å. These agree with the CCSD(T)/wCVQZ re bond lengths calculated by Demaison et al. within ±0.0005 Å. We also calculate the semi-experimental thermally averaged bond lengths rg(C-C)=1.3907(3) Å and rg(C-F)=1.3250(4) Å. These are at least ten times more accurate than two sets of experimental gas-phase electron diffraction rg bond lengths measured in the 1960s.

Articles you may be interested inAccurate rotational constant and bond lengths of hexafluorobenzene by femtosecond rotational Raman coherence spectroscopy and ab initio calculations J. Chem. Phys. 141, 194303 (2014) Femtosecond Raman rotational coherence spectroscopy (RCS) detected by degenerate four-wave mixing is a background-free method that allows to determine accurate gas-phase rotational constants of non-polar molecules. Raman RCS has so far mostly been applied to the regular coherence patterns of symmetric-top molecules, while its application to nonpolar asymmetric tops has been hampered by the large number of RCS transient types, the resulting variability of the RCS patterns, and the 10 3 -10 4 times larger computational effort to simulate and fit rotational Raman RCS transients. We present the rotational Raman RCS spectra of the nonpolar asymmetric top 1,4-difluorobenzene (para-difluorobenzene, p-DFB) measured in a pulsed Ar supersonic jet and in a gas cell over delay times up to ∼2.5 ns. p-DFB exhibits rotational Raman transitions with ∆J = 0, 1, 2 and ∆K = 0, 2, leading to the observation of J−, K−, A−, and C-type transients, as well as a novel transient (S-type) that has not been characterized so far. The jet and gas cell RCS measurements were fully analyzed and yield the ground-state (v = 0) rotational constants A 0 = 5637.68(20) MHz, B 0 = 1428.23(37) MHz, and C 0 = 1138.90(48) MHz (1σ uncertainties). Combining the A 0 , B 0 , and C 0 constants with coupled-cluster with single-, double-and perturbatively corrected triple-excitation calculations using large basis sets allows to determine the semi-experimental equilibrium bond lengths r e (C 1 -C 2 ) = 1.3849(4) Å, r e (C 2 -C 3 ) = 1.3917(4) Å, r e (C-F) = 1.3422(3) Å, and r e (C 2 -H 2 ) = 1.0791(5) Å. C 2015 AIP Publishing LLC. [http://dx

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-h and -d 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-h is strongly asymmetric, leading to a large number of asymmetry transients in its RCS spectrum. In contrast, the d-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-h to an asymmetric-top model yields the ground-state rotational constants A = 5084.4(5) MHz, B = 4684(1) MHz, C = 2744.7(8) MHz, and (A + B)/2 = 4884.5(7) MHz (±1σ). The analogous values for p-dioxane-d are A = 4083(2) MHz, B = 3925(4) MHz, C = 2347.1(6) MHz, and (A + B)/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 A(X),B(X),C(X) for increasing basis-set size X with non-linear extrapolation to the experimental constants A,B,C allows to determine the equilibrium ground state structure of p-dioxane. For instance, the equilibrium C-C and C-O bond lengths are r(CC) = 1.5135(3) Å and r(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.