Paul G. Jasien

Electronic effects in a prototype push-pull ethylene: a study of rotational barriers in C4H4N4 isomers

Quantum mechanical calculations of the rotational barriers in the prototype push-pull ethylene 1,1-diamino-2,2-dicyanoethylene (DADCE) and its non-push-pull analogues diaminofumaronitrile (DAFN) and diaminomaleonitrile (DAMN) have been used quantitatively to assess the influence of the push-pull effect. Results for DADCE indicate an in vacuo barrier for CC rotation of ≈ 28 kcal mol−1 which decreases in solvents of increasing dielectric strength. The barrier for CN rotation in DADCE is found to be ≈ 8 kcal mol−1 and increases with solvent dielectric strength. The comparable CC barrier connecting DAMN and DAFN is calculated to be ∗> 20 kcal mol−1 higher than the DADCE barrier. Analysis of the conformational energies of DADCE, DAMN, and DAFN gives further evidence for the increased dipolar character and nitrogen lone pair conjugation in DADCE as compared with DAMN and DAFN.

A CIS study of solvent effects on the electronic absorption spectrum of Reichardt's dye

Abstract A configuration interaction singles wavefunction, in conjunction with a self-consistent reaction field and a simple Onsager spherical cavity model, has been used to calculate the solvent induced shifts in Reichardts dye (RD). The results from this simple method are in good agreement with experiment for aprotic solvents, but large discrepancies exist for protic solvents. This discrepancy can be partially compensated for by including one explicit solvent molecule H-bonded to RD. Geometry optimization of the S 1 state in RD predicts a pyramidal sp 3 hybridized N atom and an overall structure that differs significantly from that in the S 0 state. Analysis of the calculated dipole moments and selected molecular orbitals are in accord with previous analyses of the S 0 →S 1 charge transfer transition in RD. However, the nature of the S 0 →S 2 transition appears to change with solvent dielectric strength ( ϵ ). At low ϵ this transition seems to have significant charge transfer character that decreases as ϵ increases.

Rotational Barriers in Push-Pull Ethylenes: An Advanced Physical-Organic Project Including 2D EXSY and Computational Chemistry

An integrated upper-division physical-organic experiment for chemistry majors has been developed. It involves the determination and mechanistic interpretation of the C=C and C-N rotational barriers in a push-pull ethylene. In the course of the experiment students will synthesize an organic compound, acquire variable temperature 1D and 2D NMR spectra, and use computational quantum chemistry to gain a deeper understanding of the unique electronic features of the molecule. Low temperature 2D EXchange SpectroscopY (EXSY) is used to quantitate the rotational barriers in a series of solvents. The quantum mechanical calculations provide a means to compare the properties of the push-pull ethylene with a similar non-push-pull system. Analysis of the experimental and theoretical results leads to a nearly complete picture of how substituent effects can influence bond lengths, rotational barriers, and the electronic distribution in these ethylenes.

A CIS study of the solvent effects on the electronic absorption spectra of push-pull ethylenes

Abstract The lowest energy electronic transitions of two push-pull compounds, 1,1-bisdimethylamino-2-cyano-2-para-fluorophenylethene (PP-F) and 1,1-bisdimethylamino-2-cyano-2-para-nitrophenylethene (PP-NO 2 ) were investigated as a function of solvent polarity using experimental and quantum mechanical techniques. Ab initio calculations at the unrestricted Hartree-Fock (UHF) level were employed for structure optimization with the solvent effects simulated via a self-consistent reaction field (SCRF) using the Onsager model. The configuration interaction singles (CIS) method was used to calculate the transition energies using various solute-solvent interaction models. Experimentally, while the principal absorption for both compounds exhibited a red shift with increasing solvent polarity, PP-NO 2 demonstrated a more pronounced shift. The predicted shifts from the CIS calculations were in good agreement with the experimental data. The different behavior for the two systems with solvent was attributed to qualitative differences in the nature of their electronic transitions. While the transition of PP-F was attributed to a π (CC) to π ∗ (CC) transition, that of PP-NO 2 was identified as an intramolecular charge transfer transition from π(CC) to π ∗ (-para-Ph-NO 2 ).

The photochemical isomerization of DAMN to ACI: stability of potential intermediates

Abstract Correlated calculations with a 6-31G ∗∗ basis set indicate that, although 4-amino-5-cyanoimidazole (ACI) is substantially more stable (≈ 20 kcal mol −1 ) than diaminomaleonitrile (DAMN) or diaminofumaronitrile (DAFN) on the S 0 potential energy surface, ACI is less stable (≈ 7 kcal mol −1 ) on the T 1 surface. Excited state calculations predict that the S 1 state of ACI is only slightly more stable than the S 2 state of DAMN and is probably less stable than the S 1 state of DAMN. This decreased stability of ACI may be due to the loss of aromaticity in the T 1 and S 1 states. No excited states of DAMN were found to lead to direct photochemical rearrangement to ACI. The present results also predict that azetine (AZE), which has been proposed as an intermediate in the interconversion of DAMN to ACI, is thermodynamically unstable with respect to ACI for all electronic states studied. The difference in energies for AZE and ACI is least (≈ 14 kcal mol −1 ) in the S 1 state. This energy difference, however, may still be too large to allow AZE to be an important intermediate in the photochemical reaction mechanism.