A theoretical systematic study of a series of isocyanopolyynes

Several isocyanopolyynes, HCnNC (n=0, 2,…,16), are analyzed in this theoretical work. We performed calculations at MP2/cc-pVTZ (n=0-16), CCSD/cc-pVDZ (n=0-12) and CCSD/cc-pVTZ (n=0-6) levels. The dipole moments from the best treatment, CCSD/cc-pVTZ, are in much closer agreement with those obtained in CCSD/cc-pVDZ calculations (deviations up to 0.05 Debye) than with the results from MP2/cc-pVTZ (discrepancies that can reach 0.55 Debye). Moreover, the CCSD/cc-pVTZ level yields values in excellent accordance with experimental dipole moments for HNC and HC2NC. Hence, this allows concluding that the correct treatment of electron correlation is more important than increments in basis sets for this electric property in isocyanopolyynes and CCSD/cc-pVDZ values are indicated as the best estimates available for large isocyanopolyynes. The findings from infrared intensities of fundamental vibrational bands are also similar (mean deviations of 3.1 km mol(-1) for CCSD/cc-pVDZ and 12.6 km mol(-1) for MP2/cc-pVTZ with respect to CCSD/cc-pVTZ results). Thus, we decided to use the charge - charge flux - dipole flux (CCFDF) model from multipoles given by the Quantum Theory of Atoms in Molecules (QTAIM), which were obtained at the CCSD/cc-pVDZ level, to investigate the variations in infrared intensities of some selected modes along these systems. The intensity of the band associated with CH stretching shows an increase with the size of these molecules (from 96 to 146 km mol(-1) between n=4 and 12) that is readily explained by variations in the charge flux contribution to this mode, in a similar way as found before for cyanopolyynes. Furthermore, the degenerate CH bending vibrations present almost constant intensities in these isocyanopolyynes (around 37-38 km mol(-1) from n=4 up to n=12) and this pattern is supported by CCFDF/QTAIM contributions. The band mostly assigned to stretching of the NC triple bond, which can also be described as asymmetric stretching mode of all pairs of adjacent triple bonds, shows an increase of intensity up to n=6 and is associated to values around 96-102 km mol(-1) in the remaining larger members. Another important mode detected is a symmetric stretching of NC and CC triple bonds belonging to the isonitrile end (between 66 and 101 km mol(-1) when n=6-12) for which the charge contribution is nearly constant and dynamic contributions change in a more complicated way. The most intense fundamental band in each isocyanopolyyne from n=4 to 12 is assigned to vibrations of triple bonds in both ends of these systems (from 129 to 164 km mol(-1)) and an alternating behavior according with even or odd numbers of CC triple bonds is observed for these values and also for respective CCFDF/QTAIM contributions. Finally, this work also brings values derived from regressions that are indicated as the best estimates of rotational constants and dipole moments to be used for detection of large isocyanopolyynes in the interstellar medium.