The methods of configuration interaction with double substitutions to pair natural orbitals (PNO−CI) and of the coupled electron pair approximation (CEPA) proposed by W. Meyer are improved by combination with a new scheme of the calculation of the pair natural orbitals (PNO) and an efficient iterative scheme for the diagonalization of the CI matrix. The relevant matrix elements for the closed shell case are tabulated, the quantities that are pertinent for an analysis of the correlation energy are defined, and the organization of the computer programs is described.

PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. I. Outline of the method for closed‐shell states

Small carbocations are of longstanding interest in mass spectrometry, organic chemistry and astrophysics, but there are few measurements of their spectroscopy in the gas phase. Of existing infrared measurements, few are available across the full range of IR frequencies. In new work described here, a pulsed-discharge supersonic-nozzle ion source produces higher densities of carbocations at low temperatures (20–100 K). Mass-selected photodissociation spectroscopy and the method of rare gas “tagging”, together with new broadly tunable infrared OPO lasers, produce IR spectra for a variety of small carbocations including C2H3+, C2H5+, C3H3+, C3H5+, the tert-butyl cation, protonated benzene, and protonated naphthalene. Spectra in the frequency range 600–4500 cm–1 provide new IR data for these ions and evidence for the presence of coexisting isomeric structures (e.g., C3H3+ is present as both cyclopropenyl and propargyl). Protonated naphthalene has bands at 6.2, 7.7, and 8.6 μm matching prominent features in the...

Infrared Laser Spectroscopy of Mass-Selected Carbocations

The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air‐cooled and water‐cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen‐cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler‐limited, high sensitivity spectroscopy.The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pat...

Infrared spectroscopy of carbo‐ions. V. Classical vs nonclassical structure of protonated acetylene C2H+3

Rund funfundzwanzig Jahre nach der Entdeckung einer sehr umfangreichen neuen Klasse von Reaktionen in der Organischen Chemie, die von Woodward und Hoffmann als „pericyclische Reaktionen” bezeichnet wurden, lassen jetzt quantenmechanische ab-initio-Rechnungen eine detaillierte Analyse der Geometrien, Energien und elektronischen Eigenschaften der Ubergangsstrukturen solcher Reaktionen zu. All diese Reaktionen weisen Gemeinsamkeiten auf, die es ermoglichen, Vorhersagen uber die Geometrien und Energien anderer Ubergangsstrukturen zu machen. Sie zeigen aber auch eine grose Vielfalt an Ubergangsstrukturen, von stark gebundenen, geschlossenschaligen bis hin zu schwach gebundenen, flexiblen mit Diradikalcharakter.

Übergangsstrukturen in pericyclischen Reaktionen von Kohlenwasserstoffen

SCF, IEPA–PNO, CEPA–PNO, and PNO–CI calculations have been performed for the molecules N2 and F2 at their experimental equilibrium distances with two basis sets, a ’’small’’ basis that contains one d set per atom and a ’’standard’’ basis with two d sets and one f set Potential curves of these molecules are calculated with the small basis sets The molecules C2H2 and C2H4 are calculated with the small basis (which contains additionally one p set on H) and with a ’’hydrocarbon’’ basis that is smaller in the s,p part, but includes the same polarization functions For C2H6 in both its staggered and eclipsed forms only the hydrocarbon basis is used The computed correlation energies are analyzed in terms of quantities defined in Part I, in particular, in terms of the IEPA pair correlation energies eμIEPA and the error ΔEIEPA of the IEPA energy A comparison is made between the results in the canonical and the localized representations and a partially localized description in which the σ–π separation is preser

PNO–CI (pair‐natural‐orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. IV. The molecules N2, F2, C2H2, C2H4, and C2H6

Bronisłkaw Żurawski

Papers

Have the ions C2H+3 and C2H+5 classical or non-classical structure?