Alfons M. F. Hezemans

Screw Sense of Polyisocyanides

From steric considerations as well as from an analysis of CD spectra a (P)-screw configuration could be attributed to the (−)-rotating molecules of poly(t-butyl isocyanide).

Magnetic circular dichroism of group via hexacarbonyls

The magnetic circular dichroism spectra of Cr(CO)6, Mo(CO)6 and W(CO)6 have been measured. The A terms for the two low-lying charge-transfer transitions in these compounds were calculated using the irreducible tensor method. For the second CT transition an alternative assignment is proposed contrary to the one hitherto assumed.

The molecular structure of allenes and ketenes. XVIII. Excited states of the phenylallene chromophore and the role of doubly excited configurations in styrene-type molecules: circular dichroism and absorption spectra of 3-alkyl-1-phenylallenes and 1,3-diphenylallene

Abstract Circular dichroism (CD) and UV absorption spectra of phenylallenes C6H5CHCCHR (R = CH3 (3), R = C(CH3)3 (4), R = C6H5 (5)) have been measured. Through the combined use of CD and UV spectroscopy including studies of solvent and temperature effects nine excited singlet states of the phenylallene chromophore (represented by 3 and 4) with energies less than 50000 cm−1 are detected. Assignments for most electronic bands are achieved empirically. In particular, the CD spectra of 3 and 4 reveal that the strong 250 nm absorption band of phenylallenes is due to two excited states correlating with 1B1u and 1E2g of benzene. This result is also of relevance for the discussion of excited states of styrene (C6H5CHCH2). In the “1E2g excited state” and the two lowest-energy excited states the phenyallenes have distorted geometries. The experimental results for 3 and 4 show that the simple one-electron excitation picture does not suffice for the description of excited states of phenylallenes, as it can explain only six of nine detected excited states. Semi-empirical CNDO/S CI calculations including singly (SE) and doubly excited (DE) configurations are performed for phenylallenes 1(R = H) and 3. For 3 they predict correctly the existence of all the low-energy excited states observed experimentally. The theoretical calculations are also used for the assignments of two electronic bands of 3 for which no analogies with excited states of other aromatics exist. In the CD of 5 thirteen excited states (ṽ ⩽ 52000 cm−1) can be detected and assigned referring to results for phenylallene 1 and its alkyl derivatives 3 and 4. For 5 also ionization energies from its photoelectron spectrum are given.

Screw sense differentiating polymerization of achiral isocyanides

Polymerization of achiral isocyanides p-XC6H4NC (X = MeO, Me, H, Cl)by nickel(II) chloride in the presence of (S)-2-isocyanoisovaleric acid methyl or t-butyl ester gives rise to optically active polymers [p-XC6H4NC]n with an excess of right-handed screw sense.

M.c.d. spectra of Cr(CO)5L (L = piperidine, trimethylamine, pyridine, and pyrazine). Evidence for the nature of the lowest excited ligand field state and for steric and electronic influence of L on the symmetry of the complexes

With the use of m.c.d. spectra the nature of the lowest ligand field (LF) excited state of Cr(CO)5L complexes (L = N-donor ligand) is characterized for the first time, which is of importance for understanding the photochemical behaviour of these complexes.