The potential surface of the HCN–HCCH dimer has two minima corresponding to one molecule or the other being the proton donor, and some interesting comparisons of the two forms have come from rotationally resolved infrared spectra for the clusters in helium nanodroplets. Because of the sizable dipole of HCN, the most favorable approach for the quadrupolar acetylene molecule acting as a proton donor is linear at the nitrogen end of HCN. On the other hand, with HCN as the proton donor, the favorable arrangement is T-shaped. The results for the determination of these structures and their energetics are given in Table 1. Augmentation of the cc-pVTZ basis produces changes in the separation
distances of less than 0.03 AA and stability changes of less than 9%. The comparison with the results from the MMC model potential shows that it
provides separation distances and stabilities that
are within the same range of differences between
the cc-pVTZ/MP2 and aug-cc-pVTZ/MP2 values;
i.e., the error is of the size of what may be the
errors in cc-pVTZ/MP2 values from lingering basis
set deficiency. However, the MMC stability differences
are in the other direction (undervaluing).
The effect of counterpoise correction [23], isolated
in values collected in Table 1, remains important
even with the aug-cc-pVTZ basis. The results of
the DFT calculations are about midway between
cc-pVDZ/MP2 and cc-pVTZ/MP2 results.