Improved virtual orbital multireference Møller–Plesset study of the ground and excited electronic states of protonated acetylene, C2H3+

Abstract

The ground state geometries and associated normal mode frequencies of the classical and nonclassical protonated acetylene ion, i.e., the vinyl cation C(2)H(3) (+), are computed using the complete active space self-consistent field and improved virtual orbital (IVO) complete active space configuration interaction methods. In addition, the minimum-energy reaction path for the classical to nonclassical interconversion is determined (as are excitation energies) using the IVO modification of multireference Moller-Plesset (MRMP) perturbation theory. The IVO-MRMP treatment predicts the nonclassical structure to be 4.8 kcalmol more stable than the classical one, which is consistent with other high level theoretical estimates. The proton affinity of acetylene from the IVO-MRMP treatment (154.8 kcalmol) also agrees well with experiment (153.3 kcalmol) and with earlier CASPT2 calculations (154.8 kcalmol). We further report geometries and vibrational frequencies of low lying excited states of C(2)H(3) (+), which have not been observed and/or studied before. Comparisons with previous highly correlated calculations further demonstrate the computational efficiency of the IVO-MRPT methods.

A photoionization mass spectrometric and threshold photoelectron-photoion coincidence study of vinylbromide (C2H3Br) in the vacuum ultraviolet range of 6–21eV

“…To our knowledge, no spectroscopic data on the electronically excited states of this species are available in the literature. Only recent high-level ab initio calculations applied to C 2 H 3 + have been reported by Chaudhuri and Freed [18], These authors confirmed our earlier prediction [7] that the bridged structure is the most stable isomer of C 2 H 3 + . They further calculated the vibrational wavenumbers associated to both the bridged and Y-shaped structures confirming that the Y-shaped structure is not a transition state.…”

“…7). In agreement with Chaudhuri and Freed [18], we calculate for this state an adiabatic EE of 1.99eV with respect to the ground state bridge-structure. The vertical EE is equal to 6.14eV.…”

“…The vertical electronic excitation energies to singlet and triplet states were also calculated. For the Y-shaped C 2 H 3 + ion [18] the lowest predicted vertical excitation energy (EE) is about 2.5-2.6 eV (depending upon the calculation level) above the ground state and corresponds to the 1 3 A 2 state. Our own calculations at the QCISD/CC-PVTZ level confirm these energy data (Table 4).…”

“…The vinyl cation C 2 H 3 + exists in a bridged ( X 1 A', most stable structure) or a Y-shaped ( X 1 A 1 ) configuration, with a predicted energy difference of 0.2 eV [7,11,18]. The value IE ad (C 2 H 3 ) = 8.468 ± 0.029 eV [11], used for the onset estimation, corresponds to the bridge-structured (or non-classical) ion.…”

Application of improved virtual orbital based multireference methods to N2, LiF, and C4H6 systems

“…Recently, Chaudhuri et al 16,17 proposed a computationally inexpensive version of HNPT in which the first order reference functions are generated from the IVO-CASCI method 15,[18][19][20][21] and are then used in subsequent MRPT calculations. The IVO-CASCI scheme is computationally simpler than CI singles ͑CISs͒ and CASSCF methods.…”