V. Busetti

Conformational flexibility of the dehydroalanine derivatives: molecular and electronic structure of (Z)-N-acetyldehydrophenylalanine

Abstract The molecular structure of (Z)-N-acetyldehydrophenylalanine (1-phenyl-2-acetylaminoprop-2-enoic acid) has been determined from three-dimensional X-ray data. C11H11NO3·2H2O is monoclinic, space group P21/a, with Z = 4 in a cell of dimensions a = 18.260(5), b = 6.050(2), c = 11.411(4) A, β = 105.96(3)°. The molecules exhibit a non-planar conformation as far as the ϕ torsion angle is concerned and they are linked by hydrogen bonds through two water molecules. Conformational flexibility is discussed on the basis of theoretical conformational analysis by taking into account the possibility of deviation from the planarity within the side chain. Theoretical results lead to exclusion any planar conformation for the title molecule. The vapour-phase He(I) photoelectron spectrum is also presented. The assignments are proposed by comparison with simpler related molecular systems and supported by INDO/S calculations. The effects of non-planarity on the electronic structure are discussed.

Conformational flexibility of dehydroalanine derivatives. Crystal and molecular structure of 2-N-acetyldehydrophenylalanyl-l-proline

Abstract The molecular structure of the title compound has been determined from three-dimensional X-ray data. C16H18N2O4· 1 2 H2O is monoclinic, space group C2, with Z=4 in a cell of dimensions: a = 18.508(3), b = 9.517(4), c = 10.538(4) A, β = 120.00(3)°. One water molecule coordinates four molecules in a tetrahedral arrangement. The conformation of this molecular system is discussed also on the basis of the known structure of related compounds and previously reported theoretical results.

Molecular and electronic structure of the dehydroalanine derivatives : The cyclic dipeptide

Abstract The molecular structure of the cyclic dipeptide of dehydroalanine(3,6-dimethylene-piperazine-2,5-dione) has been determined from three-dimensional X-ray data. C6H6N2O2 is monoclinic, space group P21, with Z = 2 in a cell of dimensions a = 5.261(3), b = 13.617(4), c = 4.570(3) A, β = 104.58(5)°. Molecules are planar and they are linked in ribbons by two hydrogen bonds through the amido-groups. The vapour-phase He(I) and He(II) photoelectron spectra are also presented. The assignments are proposed by comparison with simpler related molecules and supported by ab initio calculations. Analogies with respect to the photoelectron results of the corresponding acyclic compound N-acetyldehydroalanine are discussed.

Molecular and electronic structure of the dehydroalanine derivatives: The cyclic dipeptide of dehydrophenylalanine

Abstract The molecular structure of the cyclic dipeptide of dehydrophenylalanine [3,6-bis(phenylmethylene)-piperazine-2,5-dione] has been determined from three dimensional X-ray data. C18H14 N2O2 is monoclinic, space group C2/ c , with Z = 12 in a cell of dimensions a =4O.774(1), b =6.237(2), c =17.731(3) A, β=107.76(5)°. Molecules are approximately planar as far as the piperazinedione ring is concerned, and they are linked in two series of hydrogen-bonded ribbons. The vapour phase He(I) and He(II) photoelectron spectra are also presented. Their assignment is proposed by comparison with related molecules and supported by semiempirical quantum mechanical calculations. Analogies and differences with respect to the photoelectron results of the cyclic dipeptide of dehydroalanine and corresponding acyclic compounds are discussed.

Structural investigation of (Z)-N-acetyl-β(3′-pyridyl)-dehydroalanine methyl ester

The crystal and molecular structure of the methyl ester of (Z)-N-acetyl-β(3′-pyridyl)-dehydroalanine [methylZ-2-acetamido-3(3′-pyridyl) propenoate] have been determined from three-dimensional X-ray data. Crystals are monoclinic, space groupP21]. The molecular [φ=−51.5(5),ψ=157.0(4)°] conformation is discussed making reference to previous experimental results of analogous amino acid derivatives (in particular, ethyl (Z)-N-acetyldehydrophenylalaninate) and theoretical predictions. On the basis of He(I) and He(II) photoelectron spectra, a similar conformation is proposed for the title molecule in the vapor phase also.

Characterization and crystal structures of dirubidium and dicesium salts of 1,2-carbodicyanocyclobuten-3,4-dione

Abstract The synthesis and the crystal structure determination by X-ray diffractometry of dirubidium and dicesium salts of 1,2-carbodicyanocyclobuten-3,4-dione are reported. The results of calorimetric measurement and the IR spectra are also briefly discussed.

Crystal structure of phenyl-substituted cyclopentenes

The crystal structures of two stereoisomers of tetraphenyl- and pentaphenyl-substituted cyclopentenes 1 and 2 have been determined by X-ray analysis. An envelope conformation 1E has been ascertained for the cis isomer 1a, whereas the cis, cis isomer 2a, which crystallizes in two different space groups, P1¯ and P21/n, displays a twisted 2T1 conformation. The phenyl substituents are all tilted with respect to the cyclopentene ring in both structures. Compound 1a crystallizes in the space group P21/a with a = 18.553(3), b = 6.006(2), c = 19.355(5), β = 102.67(4)°, and V = 2104.2(g) Å3 for Z = 4; compound 2aI crystallizes in P21/n with a = 10.064(2), b = 20.756(5), c = 12.245(3) Å, β = 95.21(2)°, and V = 2547(1) Å3 for Z = 4; compound 2aII crystallizes in P1¯ with a = 10.117(3), b = 11.750(2), c = 12.359(2) Å, α = 111.25(2), β = 94.84(2), γ = 108.78(2)°, and V = 1262.3(6) Å3 for Z = 2.

Structural investigation of compounds related to α,β-dehydroaminoacids: (E)-methyl-α-cyanocinnamate

The crystal and molecular structure of the methyl ester ofα-cyanocinnamic acid [(E)-methyl-α-cyanocinnamate] has been determined from three-dimensional X-ray data. C11H9NO2 is monoclinic, space groupC2/c,Z=8,a=12.386(2),b=8.237(1),c=19.320(2) Å,β=98.49(5)°. The He(I) and He(II) photoelectron (PE) spectra of (E)-methyl-α-cyanocinnamate are presented and discussed in comparison with those of previously investigated molecules, within a comprehensive investigation of the electronic structure ofα,β-dehydroamino acid derivatives and related compounds.

Conformational and electronic effects in dehydroaminoacid derivatives

Abstract The electronic structure of the cyclic dipeptide of dehydrophenylalanine has been investigated by vapour-phase He(I) and He(II) UV photoelectron spectroscopy. Assignments are proposed by comparison with related molecules and supported by INDO/S calculations. Analogies and differences with respect to the photoelectron results of the cyclic dipeptide of dehydroalanine and corresponding acyclic compounds are discussed.

The vibrational spectrum of quinones. Part II. The infrared spectrum of 1,4-napthaquinone and 1,4-[2H6]naphthaquinone

The absorption spectra of solution and powder samples of 1,4-naphthaquinone have been measured from 4000 to 33 cm.–1; in the spectral region 4000–400 cm.–1 the polarized spectra of the (100), (010), and (001) crystal planes have also been obtained. The dichroic behaviour of the absorption bands observed for the three different crystal planes is consistently interpretable by applying the oriented gas model. The validity of this assumption is further supported by the observed vapour-phase band envelope when it presents a clearly interpretable pattern and by the slightness of the crystal field effects. In this way it is possible to suggest a well supported vibrational assignment.The investigation was extended to vapour, solution, and the (100) crystal plane of 1,4-[2H6]naphthaquinone of high isotopic purity. The successful assignment achieved in this case adds substantial information to put the complete rationalization of the vibrational behaviour of this molecule on a firmer experimental base.