Photochemical reactions of charge-transfer complexes. III. Molecular orbital studies of the charge-transfer complexes between 1,2-dimethoxyethylenes and 1,2-dicyanoethylenes

Abstract

. Can. J. Chem. 59,974 (1981). Ab initio and PCILO molecular orbital (MO) calculations for the four n-n* charge-transfer complexes formed between maleonitrile (MN) and fumaronitrile (FN), and cis (CS) and trans-l,2-dimethoxyethylene (TR) are reported. The donor and the acceptor molecules are stacked in parallel planes. Trans and cis-1,2-dihydroxyethylene are used as the model compounds for TR and CS in the determination of the optimum geometries of the complexes. Both types of MO calculations indicate that the eclipsed geometry (with the two central double bonds parallel and the centres of the double bonds centred over one another) is relatively unstable. With limited geometry searchin we find that the slipped geometry (with the two central double bonds parallel but separated at a projected distance of 2 . 2 f and the oxygen atoms above the cyanocarbon atoms) is the preferred structure. According to the ab initio calculations the relative order of the calculated heats of formation (AHe's) for these complexes is: Introductionwith those of the acceptor molecule in the complex In spite of the fact that the binding energy of a in solution. A basic understanding of the orientacharge-transfer (ct) complex in solution is of the tion effects would help US to predict the geometry order of a few kcal mol-~ (11, it has been very of the ct complex which in turn will be useful in (I) difficult to predict experimentally or theoretically the the 3-D structure the exact orientation of the donor and the acceptor biological macromolecules where ct interactions in the complex. X-ray crystallographic structures are important ( 1~1 , and (2) the selection of suitable provide the only reliable experimental information and for the study photochemfor ct complexes. H~~~~~~, caution must be taken ical isomerization of olefins through the irradiation in applying these solid phase results to the solution the ct The latter is a potentially phase since there is uncertainty about the impor-useful synthetic method for preparing stereospecific tance of crystal packing forces for the individual olefins (3). Selective irradiation of the ct complex complex (2). Orientation effects in ct complexes formed by one olefin isomer with a Proper donor (or have not been fully investigated by theoretical acceptor) would lead to enrichment of the alternamethods. Here, we define orientation effects as the tive olefin isomer. A successful example has been effects of the orientation of the substituents on the demonstrated in the irradiation of the ct complex geometry and stability of the ct complex. Several between trans-stilbene (t-ST) and f~mar-onitrile theoretical studies of the most stable geometries of (FN) where cis-stilbene (c-ST) can be obtained in ct complexes have been reported (see the refer-UP to 86% yield (3)-Of course in this particular ences in later sections). However, the effects of the example, the difference in the absorption spectra substituents of the subsystems on the geometry of and the stability of the trans and cis-stilbene

Relative thermodynamic stabilities of isomeric 1,2-dialkoxyethenes

“…Besides the experimental study [4], the structures and/or energies of the 1,2-dimethoxyethenes have been treated in three theoretical studies [6][7][8]. According to the ab initio calculations of Ref.…”

Stereochemical controls on exciplex reactions. Excited state proton transfer

“…The assumptions are further supported by a number of bimolecular photochemical reactions initiated by energy or charge (electron) transfer interactions wherein the excitation of the CT band of ground-state complexes generally leads to successful photoreactions. [24][25][26][27][28][29] In spite of these reports on energy and charge (electron) transfer interactions, the present observation is puzzling in that the excitation of Xs absorption at >400 nm causes no self-nitrosation of 1-NpOH. Since an exciplex is a loosely bound complex possessing an enthalpy of dissociation of <10 kcal/mol,30™32 cases of the reorientation of an exciplex faster than its reaction processes are known.20™29 However, in H abstraction from alkylbenzenes by the , * triplet states of aromatic ketones (by CT mechanism), two distinct and irreversible exciplexes have been identified to explain observed H-abstraction selectivity.…”

Methoxyethenes: Structures and Conformations

“…We report structure determinations by GED and ab initio calculations for 1,1-dimethoxyethene (1,1-DME), ( Z )-1,2-dimethoxyethene ( Z -1,2-DME), and tetramethoxyethene (TME). The conformational properties of 1,1-DME and Z -1,2-DME have been studied previously by various theoretical methods. − Molecular mechanics and ab initio methods (STO 3G and HF/3-21G) result in different predictions for the most stable conformers of these two compounds. Vibrational spectra for the Z -1,2-DME were interpreted in terms of a mixture of two conformers, one of which possesses a planar structure .…”