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Trans Conformers (trans + conformer)
Selected AbstractsConformational analysis, Part 41.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2006-unsaturated carbonyl compounds, A modelling, LIS/NMR investigation of the conformations of Abstract A novel Lanthanide Induced Shift Analysis (LISA) is presented. In this analysis both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately in contrast to previous techniques in which only the paramagnetic shifts were normalised. This procedure is used together with molecular mechanics (MMFF94) and ab initio (RHF/6-31G, RHF/6-311G** and B3LYP/6-311G**) calculations to investigate s - cis/s - trans isomerism in some ,,, -unsaturated aldehydes, ketones and esters. In tiglic aldehyde 1 and trans -cinnamaldehyde 4 the s - trans conformer predominates with energy differences ,E (s - cis,s - trans) of 1.64 and 1.76,kcal/mol. In methyl vinyl ketone 2 and trans -cinnamyl methyl ketone 5 the populations of the s - cis and s - trans isomers are almost equal (,E 0.24 and 0.0,kcal/mol) and in methyl crotonate 3 and methyl trans -cinnamate 6 the s - cis conformer is more stable (,E ,0.72 and ,0.41,kcal/mol). These results are in agreement with both the MMFF94 and ab initio calculated energies for the compounds except tiglic aldehyde 1 in which all the calculated values are too large and cinnamyl ketone 5. In this compound the ab initio calculations predict the s - cis form to be more stable than the s - trans in contrast to both the MM calculations and the observed result which give both forms of equal energy. Also in both the MM and ab initio calculations phenyl substitution in the ketone (2 vs. 5) considerably stabilises the s - cis form. This is not observed in practise. In phenyl acetate 7 the B3LYP calculations give two equally stable structures, one planar one non-planar. The MMFF94 and MP2 calculations and the LIS analysis support the existence of only the non -planar conformer in solution, which is also the conformation of phenyl acetate in the crystal. Copyright © 2006 John Wiley & Sons, Ltd. [source] The Low-temperature Photochemistry of s - cis Acyclic 1,3-Dienes,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2002Michael Squillacote ABSTRACT In this article we have examined the very low-temperature photochemistry of three acyclic 1,3-dienes. We have used high-temperature deposition techniques combined with matrix isolation to create samples enriched with the thermally meta-stable s - cis form. This technique has allowed us to examine the separate photochemistry of the s - cis and s - trans conformers. Our results suggest the presence and the absence of barriers on the excited-state surface. In particular, we have found that the electrocyclic closure and s - cis,s - trans photochemical isomerization stops at 15 K for 2,3-dimethyl-1,3-butadiene-d10. The closure occurs at higher temperatures in solution but is slowed by a deuterium isotope effect. The s - trans conformer of EE-2,4-hexadiene shows almost no photoreactivity in a matrix under 254 nm irradiation, but the s - cis conformer is rapidly converted to ZE-2,4-hexadiene (ZE-HXD). The photoreactivity of ZE-HXD is similar in that there is a relatively quick conversion of the s - cis conformer under these conditions, with only a very slow conversion of the s - trans to photoproducts. [source] Influence of substituents on the infrared stretching frequencies of carbonyl group in esters of benzoic acidJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2006Vilve Nummert Abstract Infrared spectra of 25 substituted phenyl esters of benzoic acid C6H5CO2C6H4 -X (XH, 3-Cl, 3-F, 3-CN, 3-NO2, 3-CH3, 3-OH, 3-NH2, 4-Cl, 4-F, 4-NO2, 4-CN, 4-OCH3, 4-CH3, 4-NH2, 2-Cl, 2-F, 2-I, 2-NO2, 2-CF3, 2-CN, 2-CH3, 2-OCH3, 2-N(CH3)2, 2-C(CH3)3), 8 alkyl benzoates C6H5CO2R (XCH3, CH2CH3, CH2Cl, CH2CN, CH2CCH, CH2CH2Cl, CH2CH2OCH3, CH2C6H5), and 22 phenyl esters of substituted benzoic acids X-C6H4CO2C6H5 (X3-Cl, 3-NO2, 3-CH3, 3-N(CH3)2, 4-F, 4-Cl, 4-Br, 4-NO2, 4-CH3, 4-C(CH3)3, 4-OCH3, 4-NH2, 2-Cl, 2-F, 2-Br, 2-I, 2-NO2, 2-CN, 2-CF3, 2-CH3, 2-OCH3, 2-NH2) were recorded in tetrachloromethane in the region of 400,4000,cm,1. Carbonyl stretching frequencies ,CO for meta - and para -substituted phenyl esters of benzoic acid and phenyl esters of meta -substituted benzoic acids were shown to correlate with the substituent constants ,o. The influence of the through resonance effect on ,CO was found to be important in the case of +Rpara substituents in the benzoyl part of phenyl benzoates as well. The carbonyl stretching frequencies of ortho derivatives in phenoxy part were shown to correlate with the inductive substituent constant ,I only. In the benzoyl part of the esters the carbonyl stretching frequencies of cis and trans conformers (relative to the carbonyl group) of ortho derivatives were nicely described by dual parameter equations: (,CO)cis,=,(,CO)o,+,c1,I,+,c3,and (,CO)trans,=,(,CO)o,+,c1,p+,+,c3, (R,=,0.99). The trans isomers of phenyl esters of ortho -substituted benzoic acids showed direct resonance similar to that for para derivatives. The positive steric term found for both the cis and trans conformers could be considered as measure of the steric inhibition of resonance between the phenyl ring and the carboxy-group caused by bulky ortho substituents. The existence of cis/trans conformations was supported by frequency calculations with Density Functional Theory (DFT) method at B3LYP/6-311+G** level for the ortho -substituted benzoates. In the case of alkyl benzoates good correlations of ,CO values were obtained when both the Taft ,* and the steric constants were used. For meta - and para -substituted phenyl benzoates s - trans conformation where the plane of the benzene ring in the benzoyl part of the ester is coplanar with the carbonyl bond plane and the plane of the benzene ring in the phenoxy part is twisted nearly perpendicular relative to the carbonyl bond plane was supported. Copyright © 2006 John Wiley & Sons, Ltd. [source] Structures of monolayers formed from different HS,(CH2)2,X thiols on gold, silver and copper: comparitive studies by surface-enhanced Raman scatteringJOURNAL OF RAMAN SPECTROSCOPY, Issue 11 2003Andrzej Kudelski Abstract Molecules of 2-aminoethanethiol (cysteamine, CYS), 2-mercaptoethanol (MET), 3-mercaptopropionic acid (MPA), sodium 2-mercaptoethanesulphonate (MES) and 1-propanethiol (PTH) were spontaneously chemisorbed on electrochemically activated silver, copper and gold surfaces. The structure of monolayers formed was studied with surface-enhanced Raman scattering (SERS). In CYS and MPA monolayers the relative surface concentration of gauche conformers was higher than in MET, PTH and MES monolayers. This is probably due to double bonding to the metal surface by a fraction of adsorbed MPA and CYS molecules (via the sulphur moiety and the terminal carboxylic or amino group). In CYS, MET, MPA and PTH monolayers the surface concentration of trans conformers is significantly higher on Ag than on Au or Cu. The structures of MES monolayers on Ag, Au and Cu are similar, with very high surface concentrations of trans conformers. Statistically, the wavenumbers of ,(C,S) bands of both gauche and trans conformers are the highest on Au, slightly lower on Ag and the lowest for Cu. However the positions of ,(C,S) bands are also sensitive to the other parameters (e.g. C,C,S,metal torsion angle, the overall ordering of the monolayer). Therefore, the wavenumbers of ,(C,S) bands are not good indicators of differences in the interaction between chemisorbed thiols and the metal substrates. Desorption measurements suggest that part of the adsorbed molecules is bonded significantly weaker than the rest. Thiolate monolayers on Cu decomposed relatively easily, forming a layer of copper sulphide. The strength of adsorbed MPA acid is similar on all substrates. Copyright © 2003 John Wiley & Sons, Ltd. [source] The Low-temperature Photochemistry of s - cis Acyclic 1,3-Dienes,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2002Michael Squillacote ABSTRACT In this article we have examined the very low-temperature photochemistry of three acyclic 1,3-dienes. We have used high-temperature deposition techniques combined with matrix isolation to create samples enriched with the thermally meta-stable s - cis form. This technique has allowed us to examine the separate photochemistry of the s - cis and s - trans conformers. Our results suggest the presence and the absence of barriers on the excited-state surface. In particular, we have found that the electrocyclic closure and s - cis,s - trans photochemical isomerization stops at 15 K for 2,3-dimethyl-1,3-butadiene-d10. The closure occurs at higher temperatures in solution but is slowed by a deuterium isotope effect. The s - trans conformer of EE-2,4-hexadiene shows almost no photoreactivity in a matrix under 254 nm irradiation, but the s - cis conformer is rapidly converted to ZE-2,4-hexadiene (ZE-HXD). The photoreactivity of ZE-HXD is similar in that there is a relatively quick conversion of the s - cis conformer under these conditions, with only a very slow conversion of the s - trans to photoproducts. [source] | ||||||||||