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Context. Knowledge of the hyperfine structure of molecular lines is useful for estimating reliable column densities from observed emission, and essential for the derivation of kinematic information from line profiles.Aims. Deuterium bearing molecules are especially useful in this regard, because they are good probes of the physical and chemical structure of molecular cloud cores on the verge of star formation. However, the necessary spectroscopic data are often missing, especially for molecules which are too unstable for laboratory study.Methods. We have observed the ground-state (J = 1-0) rotational transitions of DCO+, HN13C and DNC with the IRAM 30?m telescope toward the dark cloud LDN 1512 which has exceptionally narrow lines permitting hyperfine splitting to be resolved in part. The measured splittings of 50?300?kHz are used to derive nuclear quadrupole and spin-rotation parameters for these species. The measurements are supplemented by high-level quantum-chemical calculations using coupled-cluster techniques and large atomic-orbital basis sets. Results. We find eQq = + 151.12?(400)?kHz and CI= -1.12?(43)?kHz for DCO+, eQq = 272.5?(51)?kHz for HN13C, and eQq(D) =265.9?(83)?kHz and eQq(N) = 288.2?(71)?kHz for DNC. The numbers for DNC are consistent with previous laboratory data, while our constants for DCO+are somewhat smaller than previous results based on astronomical data. For both DCO+and DNC, our results are more accurate than previous determinations. Our results are in good agreement with the corresponding best theoretical estimates, which amount to eQq = 156.0?kHz and CI= -0.69?kHz for DCO+, eQq = 279.5?kHz for HN13C, and eQq(D) = 257.6?kHz and eQq(N) = 309.6?kHz for DNC. We also derive updated rotational constants for HN13C: B = 43?545.6000?(47)?MHz and D = 93.7?(20)?kHz.Conclusions. The hyperfine splittings of the DCO+, DNC and HN13C J = 1-0 lines range over 0.47?1.28?km s-1, which is comparable to typical line widths in pre-stellar cores and to systematic gas motions on ~1000?AU scales in protostellar cores. We present tabular information to allow inclusion of the hyperfine splitting in astronomical data interpretation. The large differences in the 14N quadrupole parameters of DNC and HN13C have been traced to differences in the vibrational corrections caused by significant non-rigidity of these molecules, particularly along the bending coordinate.

A new Karplus equation for pseudorotating five-membered rings is derived by expanding calculated NMR spin−spin coupling constants (SSCCs) J as a function J(q,φ) of the puckering amplitude q and the pseudorotational phase angle φ. The approach was tested for cyclopentane but is equally applicable to ribose sugars and other biochemically interesting five-membered rings. It is based on the calculation of the conformational potential V(q,φ), which in the case of cyclopentane was determined at the MBPT(2)/cc-pVTZ level of theory. Cyclopentane is a free pseudorotor (barriers ΔE and ΔH ≤ 0.01 kcal/mol) with a puckering amplitude q = 0.43 Å and a barrier to inversion ΔH = 5.1 kcal/mol, in perfect agreement with experimental data. The SSCCs of cyclopentane were calculated at MBPT(2)/cc-pVTZ geometries by use of coupled perturbed density functional theory (CP-DFT) with the B3LYP functional and a (9s,5p,1d/5s,1p)[6s,4p,1d/3s,1p] basis set. In addition, coupled-cluster singles and doubles (CCSD) calculations were carried out to verify the CP-DFT results. All geometrical parameters and the 10 SSCCs of cyclopentane are determined as functions of the phase angle φ and averaged to give 〈ⁿJ〉 values that can be compared with experimental data. The following SSCCs (in hertz) were obtained at CP-DFT/B3LYP/[6s,4p,1d/3s,1p]:  〈¹J(CC)〉 = 34.0; 〈¹J(CH)〉 = 127.6, exp 128.2; 〈²J(CCC)〉 = 2.3, exp (+)2.8; 〈²J(CCH)〉 = −2.6, exp (−)3.0; 〈²J(HCH)〉 = −12.4, exp (−)12.4; 〈³J(CCCH)〉 = 3.9; 〈³J(HCCH, cis)〉 = 7.7, exp 7.7; 〈³J(HCCH, trans)〉 = 5.6, exp 6.3; 〈⁴J(HCCCH, cis)〉 = 0.1; 〈⁴J(HCCCH, trans)〉 = −0.6. Magnitude and trends in calculated SSCCs are dominated by the Fermi contact term [with the exception of ¹J(CC)]. A new way of determining puckering amplitude and pseudorotational angle by a combination of measured and calculated SSCCs is presented.

The r_e structure of allene is revised on the basis of high-level quantum chemical calculations (CCSD(T)/cc-pVQZ and CCSD(T)/cc-pCVQZ) as well as via analysis of experimental rotational constants for C₃H₄, C₃D₄ and H₂C₃D₂ employing vibrational corrections obtained from quantum chemical calculations (SDQ-MBPT(4)/cc-pVTZ). For the equilibrium geometry a least-squares fit to the corrected rotational constants yields r(C–H)=1.081 Å, r(C–C)=1.307 Å, α(HCH)=118.35° in satisfactory agreement with a previous, though less precise r_e structure based on gas-phase electron diffraction data. In addition, computed harmonic and fundamental frequencies are reported and compared to experimental values.

Analytic first and second derivatives of the energy are implemented for closed-shell systems described by the CCSDT-n (n=1, 1b, 2 and 3) and CC3 electron correlation models. A detailed discussion of the computational requirements of these calculations is given, along with diagrammatic formulas for all relevant quantities. The method is applied to calculate the nuclear magnetic shielding of H₂O, CO and N₂O and the structure and properties of propadienylidene.

A scheme for the calculation of NMR chemical shifts using local second-order Møller–Plesset (LMP2) perturbation theory together with gauge-including atomic orbitals (GIAOs) is presented. Test calculations on the basis of a preliminary implementation within a conventional GIAO-MP2 code show that the deviations between GIAO-LMP2 and GIAO-MP2 are small, e.g., for ¹³C typically less than 1 ppm, and that the GIAO-LMP2 approach holds great promise for application to larger molecules.

The re structure of benzene is revised on the basis of high-level quantum chemical calculations at the CCSD(T)/cc-pVQZ level as well a reanalysis of the experimental rotational constants using computed vibrational corrections. A least-squares fit to empirically determined Be constants yields re(CC) = 1.3914 ± 0.0010 Å and re(CH) = 1.0802 ± 0.0020 Å; the latter distance is significantly shorter than the best previous estimate based on experimental data. Comparison of computed rg and rz distances with experiment as well as considerations of bond lengthening due to anharmonicity are consistent with the estimated re distance, indicating that the recommended structural parameters are very accurate.

A scheme for the calculation of spin-orbit coupling constants using coupled-cluster (CC) electronic structure methods is described based on response-theory expressions for transition properties. An implementation is reported for singlet–triplet transitions within the coupled-cluster singles and doubles (CCSD) approximation. An atomic mean-field representation of the spin-orbit interaction is used to simplify the calculation of spin-orbit coupling constants. Sample calculations are presented for spin-orbit couplings for the 1¹Σ⁺→1³Π transitions for BH and AlH and for the 1¹A'→1³A" and the 1³A"→1¹A" transitions for the silylenes HSiX, X=F, Cl, Br, and are compared to results obtained from full configuration interaction (FCI) calculations with the full Breit-Pauli spin-orbit operator.

A long-standing controversy regarding the re structure of cyclopropane is resolved by performing high-level quantum chemical calculations and analyzing the experimental rotational constants for C3H6 and C3H4D2 augmented by calculated vibrational corrections. For the latter, a least-squares fit yields the following set of parameters:  re (CC) = 1.5030(10) Å, re(CH) = 1.0786(10) Å, and αe(HCH) = 114.97(10)°, which compare favorably with both the pure computational result obtained at the CCSD(T)/cc-pVQZ level as well as an earlier estimate of the re structure of cyclopropane based on analysis of gas-phase electron diffraction data. Our results are in rather poor agreement with a structure based on a previous analysis of the rotational constants that used empirically estimated vibrational corrections.

Context. The astrophysical relevance of the fluoromethylidynium ion and its importance for the interstellar chemistry of fluorine motivated the present laboratory spectroscopic investigation of both 12CF+and the corresponding 13C-containing isotopologue, 13CF+.Aims. This investigation has been carried out to provide accurate rest frequencies for future (radioastronomical) observations, to improve the accuracy of the values for the spectroscopic parameters available in the literature for CF+, and to provide them for the first time for 13CF+.Methods. Rotational spectra of CF+and 13CF+were recorded in the millimeter- and submillimeter-wave frequency ranges. Their investigation was supplemented by high-level quantum-chemical computations using state-of-the-art coupled-cluster techniques.Results. We report the most accurate ground-state rotational parameters available so far for CF+and 13CF+. Conclusions. The ground-state rotational parameters as well as the rest frequencies of 13CF+will be useful for future observational purposes with the aim of improving the knowledge of fluorine interstellar chemistry.

Context. The astrophysical relevance of the fluoromethylidynium ion and its importance for the interstellar chemistry of fluorine motivated the present laboratory spectroscopic investigation of both 12CF+and the corresponding 13C-containing isotopologue, 13CF+.Aims. This investigation has been carried out to provide accurate rest frequencies for future (radioastronomical) observations, to improve the accuracy of the values for the spectroscopic parameters available in the literature for CF+, and to provide them for the first time for 13CF+.Methods. Rotational spectra of CF+and 13CF+were recorded in the millimeter- and submillimeter-wave frequency ranges. Their investigation was supplemented by high-level quantum-chemical computations using state-of-the-art coupled-cluster techniques.Results. We report the most accurate ground-state rotational parameters available so far for CF+and 13CF+. Conclusions. The ground-state rotational parameters as well as the rest frequencies of 13CF+will be useful for future observational purposes with the aim of improving the knowledge of fluorine interstellar chemistry.