Conformational Properties of cis- and trans-N-Cyclopropylformamide Studied by Microwave Spectroscopy and Quantum Chemical Calculations

Abstract

The microwave spectra of cis- and trans-N-cyclopropylformamide, C3H5NHC(═O)H, have been investigated in the 31-123 GHz spectral region at room temperature. Rotational isomerism about the Cring-N bond is possible for both cis and trans. MP2/cc-pVTZ and CCSD/cc-pVTZ calculations indicate that there are two conformers in the case of cis, called Cis I and Cis II, while only one rotamer, denoted Trans, exists for trans-N-cyclopropylformamide. The quantum chemical methods predict that Cis I has an electronic energy that is 8-9 kJ/mol higher than the energy of Cis II. The CCSD H-Cring-N-H dihedral angle is 0.0° in Cis I, 93.0° in Cis II and 79.9° in Trans. The CCSD and MP2 calculations predict a slightly nonplanar structure for the amide moiety in both Trans and Cis II, whereas Cis I is computed to have a planar amide group bisecting the cyclopropyl ring. Surprisingly, the MP2 and CCSD methods predict practically the same energy for Trans and Cis II. The spectra of Cis II in the ground state and in two vibrationally excited states were assigned, while the spectrum of Cis I was not found presumably because of a low Boltzmann population due to a relatively large energy difference (8-9 kJ/mol). The spectra of the ground vibrational state and seven vibrationally excited states of Trans, were assigned. Vibrational frequencies of several of the excited state of both Cis II and Trans were determined by relative intensity measurements. The experimental and CCSD rotational constants are in satisfactory agreement. The MP2 values of the quartic centrifugal distortion constants of both species are in relatively poor agreement with their experimental counterparts. The MP2 vibration-rotation constants and sextic centrifugal distortion constants have little resemblance with the corresponding experimental values.

Development of the Multicomponent Coupled-Cluster Theory for Investigation of Multiexcitonic Interactions

“…Using the mcCC method, we calculated total correlation energy and biexciton binding energy for a set of eight model systems. Total correlation energy of the biexciton systems was calculated by subtracting the vacuum energy from total energy, ∆E mcCC = E mcCC − E ref (64) and biexciton binding energy E BBE was defined as the difference between the energies of the free excitons (E X ) and the total biexciton energy E X 2 .…”

“…Coupled-cluster (CC) theory has been used successfully for studying electron-electron correlation in many-electron systems. [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] In the context of multicomponent systems, the CC ansatz provides a balanced framework for a size-consistent and size-extensive treatment of many-particle correlation. Application of CC to excitons has been demonstrated earlier by Sundholm et al 36 and Vänskä et al 37 for a two-band effective mass approximation Hamiltonian model.…”

“…The overarching goal of this work is to present an approximate solution of the following multicomponent Schrödinger equation using a multicomponent coupled-cluster (mcCC) ansatz for the many-particle wave function. Coupled-cluster (CC) theory has been used successfully for studying electron–electron correlation in many-electron systems. − In the context of multicomponent systems, the CC ansatz provides a balanced framework for a size-consistent and size-extensive treatment of many-particle correlation. The general theory of multicomponent CC for the molecular Hamiltonian has been described earlier by Monkhorst .…”

Microwave and Quantum Chemical Study of the Hydrazino Group as Proton Donor in Intramolecular Hydrogen Bonding of (2-Fluoroethyl)hydrazine (FCH2CH2NHNH2)

“…Much of our work has focused on weak proton donor groups. Recently, we reported gas-phase studies of alcohols, − carboxylic acids, thiols, − thiolcarboxylic acids, selenols, − amines, − amides, , and phosphines, − where the said groups are proton donors in internal H bonds with acceptors such as the fluorine atom, ,,, the chlorine atom, ,,, π-electrons of triple bonds, − ,,,, π-electrons of double bonds, ,,,, and Walsh pseudo-π electrons. ,, References of many of our earlier works are found in the cited literature as well as in reviews. − …”

“…Recently, we reported gas-phase studies of alcohols, 1−5 carboxylic acids, 6 thiols, 7−10 thiolcarboxylic acids, 11 selenols, 12−15 amines, 16−18 amides, 19,20 and phosphines, 21−24 where the said groups are proton donors in internal H bonds with acceptors such as the fluorine atom, 1,6,9,11 the chlorine atom, 17,19,23,24 π-electrons of triple bonds, [3][4][5]7,13,15,18 π-electrons of double bonds, 2,8,10,14,21 and Walsh 25 pseudo-π electrons. 12,20,22 References of many of our earlier works are found in the cited literature as well as in reviews. 26−29 No gas-phase studies of hydrazines (R−NHNH 2 ), where the hydrazino group (−NHNH 2 ) acts as a proton donor, appear to have been reported.…”