The purpose of this paper is to convince practitioners of 1 H NMR spectroscopy to consider simple quantum chemical calculations as a viable option to aid them in the assignment of their spectra. To this end, it is demonstrated, on a test set of 80 conformationally stable molecules of various kinds carrying different functional groups, that, in contrast to what is claimed in the literature, large basis sets are not needed to obtain rather accurate predictions of 1 H NMR chemical shifts by quantum chemical calculations. On the other hand, modeling the solvent by an SCRF-type calculation may improve certain predictions significantly. The best accuracy/cost ratio is provided by GIAO calculations in chloroform as a solvent with the specially parametrized WP04 functional of Cramer et al. using the cc-pVDZ or 6-31G** basis set, closely followed by similar calculations with the ubiquitious B3LYP functional (both predict 1 H chemical shifts with an average deviation of ca. 0.12 ppm, if the results are scaled linearly). A slightly higher accuracy can be attained by adding diffuse functions to the basis set, but going to the triplebasis sets which have invariably been used hitherto in calculations of chemical shifts does not lead to any improvement. The popular increment schemes such as those implemented in the ChemDraw or ACD programs do not do nearly as well and are often incapable of correctly distinguishing stereoisomers.

The current lineup of popular density functional theories, in particular those based on Becke's exchange functionals, fail to predict a correct dissociation behavior in radical ions where charge and spin must be separated (model:  H2 •+) or where both must be localized on one fragment (model:  He2 •+). The repercussions of this on the location of certain transition states on radical ion potential energy surfaces are pointed out.

The performance of 250 different computational protocols (combinations of density functionals, basis sets and methods) was assessed on a set of 165 well-established experimental (1)H-(1)H nuclear coupling constants (J(H-H)) from 65 molecules spanning a wide range of "chemical space". Thereby we found that, if one uses core-augmented basis sets and allows for linear scaling of the raw results, calculations of only the Fermi contact term yield more accurate predictions than calculations where all four terms that contribute to J(H-H) are evaluated. It turns out that B3LYP/6-31G(d,p)u+1s is the best (and, in addition, one of the most economical) of all tested methods, yielding predictions of J(H-H) with a root-mean-square deviation from experiment of less than 0.5 Hz for our test set. Another method that does similarly well, without the need for additional 1s basis functions, is B3LYP/cc-pVTZ, which is, however, ca. 8 times more "expensive" in terms of CPU time. A selection of the better methods was tested on a probe set comprising 61 J(H-H) values from 37 molecules. In this set we also included five molecules where conformational averaging is required. The rms deviations were better than or equal to those with the training set, which indicates that the method we recommend is generally applicable for organic molecules. We give instructions on how to carry out calculations of (1)H chemical shifts and J(H-H) most economically and provide scripts to extract the relevant information from the outputs of calculations with the Gaussian program in clearly arranged form, e.g., to feed them into programs for simulating entire (1)H NMR spectra.

The geometries and force fields of phenylcarbene (PC) and cycloheptatrienylidene (CHT) in their singlet and triplet electronic states as well as of cycloheptatetraene (CHTE) and bicyclo[4.1.0]heptatriene (BCT) and the transition states for the formation and decay of the latter were evaluated by various methods. Relative single point energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G* level. Finally, the effects of extending the basis set to triple-ζ quality were estimated by (R)MP2 calculations and carried over proportionally to CCSD(T). These calculations show that CHTE which has a strongly distorted allenic structure is the most stable species on that part of the C(CH)6 surface which was examined in the present study, followed by planar 3PC. The strained BCT is found to be nearly degenerate in energy with 1PC, but the high activation energy for its formation from 1PC together with the low activation energy for ring-opening to CHTE suggests that this species cannot persist under the experimental conditions employed for production of CHTE. In analogy to the case of cyclopentadienylidene, CHT exists in the form of a closed shell singlet (1A1) and two related pairs of open shell singlet and triplet states (1,3A2 and 1,3B1) which correspond to the Jahn−Teller distorted structures of the cycloheptatrienyl radical. The relative energies and the nature of the different CHT stationary points depend on the method of calculation, but it appears that the decrease in electron repulsion lowers the 1A2 state slightly below the 1A1 state so that the open shell species serves as a planar transition state for enantiomerization of CHTE with an estimated activation energy of ∼20 kcal/mol. The two triplets are very close in energy with the higher lying being either a transition state or a shallow minimum. The 1B1 state is an excited state of the open-shell singlet. The calculated IR spectra of the three most stable isomers were compared to those published previously by Chapman et al. whereby the assignment of the photoproduct of UV photolysis of phenyldiazomethane to CHTE was confirmed. A full study of the force fields of PC and CHTE is under way.