Conformational studies of 1,2,3-trichloro- and 1,2,3-tribromopropane by nuclear magnetic resonance spectroscopy: attractive parallel (1:3) halogen-hydr

Analysis of the NMR spectra of 1,2,3-trichloropropane and 1,2,3-tribromopropane in various media shows the most stable conformer to be AG_-. The populations of several conformera have been estimated by using pure trans and gauche coupling constants obtained from closely similar molecules. The calculated populations found in non-polar solvents agree well with those obtained by electron diffraction studies in the gas phase. It is suggested that the AG_- form is stabilised relative to AG₊ by the former having two parallel (1:3) halogen-hydrogen attractions against one in the AG₊ form. Comparison is made to related molecules where the most stable conformers also have the greatest number of parallel (1:3) halogen-hydrogen interactions.     

A comparison of phototautomerism in different sites of free-base porphin (H2P) in n-alkane crystals

Selective laser-induced phototautomerism of free-base porphin in different n-alkanes (n-hexane, n-heptane, n-octane, nnonane, n-decane, n-undecane and n-dodecane) has been used to identify tautomer splittings in a number of sites in polycrystalline samples at 2 K. Pairs or lines arising from the two molecular orientations in each site can be reversibly transformed into each other. In the odd n-alkanes the occupation of different sites was not dependent on the rate at which the samples were frozen, but in the even n-alkanes metastable sites were only occupied following rapid quenching.     

Photochemistry of β, γ-unsaturated ketones-V: The direct irradiation of some γ-phenyl β, γ-enones

The photochemistry of some members of the two series of γ-phenyl substituted acyclic β, γ-unsaturated ketones 1 and 2 upon direct irradiation with γ 310nm has been investigated, viz 1c–1h and 2b+2c.The alkyl substituted (E)-5-phenyl-4-penten-2-ones 1c–1h yield the corresponding 1,3-acyl shift products and (Z)-isomers, and 1g and 1h in addition two decarbonylated products. 2b only yields the (Z)-isomer and some benzaldehyde, but 2c yields the 1,3-acyl shift product, the ODPM product, three hydrocarbons formed by disproportionation of the allyl radical, and some benzaldehyde. The β-phenyl β, γ-UK 3a proved to be photostable. The 1,3-acyl shift products of 1c–1h result mainly from the singlet excited state in a cage radical process. The exclusive formation of the (E)-configuration of the 1,3-acyl shift product is explained in terms of conformational preference of the intermediate allyl radical. It is proposed that the formation of the (Z)-isomer proceeds from ¹T(π -π^*) which is populated according to

. Evidence is presented which supports the proposed mechanism.The β,γ-UK 2b containing a benzoyl moiety leads to a higher degree of (E)-(Z) isomerization than the corresponding 1d which has an acetyl moiety.The triplet energies of (E)- and (Z)-1h are 56 and ca 70 kcal/mol respectively.     

Properties of strong hydrogen-bonded systems: The formation of hydrogen-bonded ion pair in amine·HCL systems

The hydrogen-bonded complexes between CH₃NH₂ and (CH₃)₂NH with HCl have been studied by the ab initio molecular orbital method using the 4–31G basis set. Calculations show that the proton potential curve has a single minimum near to the nitrogen atom in both complexes. This means that the proton has been transferred from HCl to the amine. ΔE and the dipole moment of the complexes studied are as follows: ?18.2 kcal mol^?1, 10.3 D for methylamine ·HCl, and ?21.7 kcal mol^?1 11.1 D for the corresponding dimethylamine complex. Other properties of the hydrogen-bonded ion pairs are discussed.     

Kinetics of the phosphorescent triplet states of tetracyanobenzene complexes as studied by microwave induced delayed phosphorescence

Using microwave induced delayed phosphorescence techniques we determined the populating and depopulating rate constants for the individual spin-sublevels of the phosphorescent states of tetracyanobenzene (TCNB) and its charge-transfer complexes with benzene and hexamethylbenzene (HMB) in n-hexane at 1.25 K. It was shown that the non-radiative decay process from the shortest lifetime sublevel is most responsible for the previously observed shortening in the phosphorescence lifetime of the TCNBHMB complex.     

Microcomputers and pattern recognition

Microcomputer software for chemists is becoming available at an ever increasing rate. Following Professor Forina's general pattern recognition program, published in last month's Computer Corner, we present a specific technique, the linear learning machine. This is also available in Professor Forina's PARVUS but the software presented here has additional possibilities. It is proposed by Dr D.C. Leegwater from the Central Institute of Food and Nutrition Research of the Dutch organization of Applied Scientific Research (CIVO-TNO) at Zeist and by his son, who is studying physics and mathematics at the State University of Utrecht.     

Chemical characterization of yeasts and yeast-like fungi by factor analysis of their pyrolysis—mass spectra

A factor analysis technique is described for determining differences in the pyrolysis—mass spectra of complex biological material in terms of chemical components. The technique is based on factor analysis with subsequent graphical rotation in order to find axes, describing changes in chemical components. Three yeast species could be clearly distinguished on the basis of four classes of chemical building blocks, viz. N-acetylamino-sugar, protein, hexose and pentose.     

Preparation and aluminum trichloride-induced cationic rearrangements of bicyclo[2.2.0]hexene carboxylic esters

Reaction between the tetramethyl[4]annulene aluminum trichloride σ-complex and α, β-alkenic esters gave the corresponding 1,4,5,6-tetramethylbicyclo[2.2.0]hex-5-one-2-endo-carboxylic esters. AlCl₃-induced cationic rearrangements of these bicyclic esters yielded a number of bicyclo[3.1.0]hexene carboxylic esters and related lactones which were isolated and identified. The isomerization seems to proceed via stereospecific endo protonation and subsequent rearrangement to 2-methoxycarbonyl-1,4,5,6-exo-tetramethylbicyclo(3.1.0]hex-4-yl carbocations.     

Characterization of noise in inductively-coupled plasma emission spectrometry

The nebulizing process in inductively-coupled plasma emission spectrometry is a major source of the instrumental variability; an attempt has been made to isolate this source so that its influence on the final result could be evaluated. Direct measurements of the time-dependent variations of the sample supply to the plasma were achieved by using a device based on the light-scattering property of nebulized test solutions. The noise from the nebulizer and of the emission signal was characterized by autocovariance functions and power spectral densities. The types of noise observed showed f^{-$\text{1}\text{2}$} and f^{-$\text{1}\text{4}$} character, depending on the concentration of the test elements employed. Cross-covariance revealed a strong correlation between the noise sources considered. The applicability and limitations of possible correction and electronic filtering procedures are indicated.     

Limitations on mndo and mndo/ci computations of activation barriers

The ability of MNDO and MNDO/CI for predicting activation barriers of unimolecular reactions is tested for decompositions of methane, ethane, nitromethane, and methyl nitrite. MNDO is not useful for this purpose; MNDO/CI gives energy barriers in agreement with experiment to about ±10 $\text{kcal}\text{mole}$ for the types of reactions considered.