The infrared and Raman spectra of 2,4-and 2,5-dimethylbenzaldehydes

The vibrational frequencies of 2,4-and 2,5-dimethylbenzaldehydes have been reported from the study of ir absorption and Raman spectra of these molecules recorded in liquid phase. The probable assignments for all the frequencies are presented in terms of the fundamentals, overtones or combinations     

Phosphorescence excitation spectra of methylbenzaldehydes in durene single crystals: Site structure and effects of deuterium substition

The low-temperature (15 K) laser phosphorescene excitation spectra of 2,4,5-trimethylbenzaldehyde-1h₁ and -1d₁, and 2,5-dimethylbenzaldehyde-1h₁ and -1d₁, each isolated in durene single crystals are presented. Spectra of molecules in each of two different durene crystal sites were obtained. The red shift of the T₁(0.0) bands upon deuteration, is consistent with these bands arising from φφ1 excitations. The red shift of the TMB T₂(0.0) and the blue shift of the DMB T₂(0.0) may reflect the differing electronic configurations of these states (φφ1 or nφ1 respectively) or may be determined by the details of the coupling of T₂ to the underlying T₁ vibronic levels. A broad continuum-like features was observed in the region of T₂. Changes in this feature brought about by deuteration or a change in crystal environment suggest the involvement of both vibronic and crystal-field-induced coupling of T₂ to the underlying T₁ vibronic levels     

The near ultraviolet absorption spectra of 2,3-, 2,4- and 2,5-dimethylbenzonitriles

A new technique—photoacoustic spectroscopy (PAS—was applied for the study of the electronic transitions in three isomers, 2,3-, 2,4- and 2,5-dimethylbenzonitriles. The PAS spectra were compared with solution and vapour phase spectra. A probable detection of S → T absorption in the molecules is considered to be of significance in the present investigation. It is also shown that two π → π* transitions analogous to the benzene strong 200 nm and weak and forbidden 260 nm transitions could be identified in all three molecules. An interesting feature is regarding the identification of a few combination bands in the excited state of an electronic transition which could be fairly comparable with such combinations observed in the near i.r. PAS spectra of the molecules, in the ground state.     

PMR spectra ard structure of 3,4- and 2,5-disubstituted sulfolanes

Conclusions Based on studies of the PMR spectra of 3,4- and 2,5-disubstituted sulfolanes it has been shown that the difference in the chemical shifts of the geminal protons and the values of the chemical shifts of the protons associated with substituents are reliable criteria for establishing the configuration of the disubstituted five-membered cyclic compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1029–1033, May, 1980.The authors would like to thank U. M. Dzhemilev for donating the 2,5-divinylsulfolanes for investigation.     

Laser Raman and i.r. spectra of 2,4-, 3,4- and 3,5-dichlorobenzoylchlorides

The laser Raman and i.r. spectra of 2,4-, 3,4- and 3,5-dichlorobenzoylchlorides have been recorded and analysed. A complete assignment of vibrational frequencies has been proposed assuming C_s point group symmetry for these molecules. Complex carbonyl absorption has been noticed in the case of 2,4-dichlorobenzoylchloride and the same has been explained in terms of Fermi resonance.     

Guest and host deuterium effects of the large ZFS parameters D of dimethylbenzaldehydes in durene single crystals

The ZFS parameters D of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h₁ and -1d₁ guests in perhydrogenated and perdeuterated durene single crystals are determined by comparing the experimental and calculated resonance curves. It is found that the deuterium substitution of the guest aldehydic group in a given host leads to the decrease of the D values and to the increase of the energy gaps ΔE_T between the zero-point levels of the ³nπ* and ³ππ* states of the guests. On the other hand, the perdeuteration of the host results in the decrease of ΔE_T with a corresponding increase of the D value of a given guest. The D value of 1 cm⁻¹ determined for 2,5-dimethylbenzaldehyde-1h₁ in perdeuterated durene is the lategest ever found for an aromatic carbonyl compound. Correlations between D and ΔE_T indicate that the ZFS parameters D of the guests are determined by contributions from both spin-spin and spin-orbit interactions between the ³nπ* and ³ππ* states. The large guest and host deuterium effects observed on the D values are attributed to the changes of the gaps ΔE_T of the guests.     

Stark effects on the phosphorescence spectra of p-benzoquinone

Stark effects on the phosphorescence spectra of p-benzoquinone have been studied in naphthalene crystals at temperatures between 1.7 and 30 K. The experimental data establish that the band splitting of 24 cm^?1 arises from a double minimum potential in the T₁ state.     

Synthesis of 2,4- and 2,5-dialkyltetrahydrothiophenes

Summary Some 2,4- and 2,5-dialkyltetrahydrothiophenes were prepared by conversion of the corresponding tetrahydrofuran homologs into 1,4-dibromoalkanes and cyclization of the latter under the action of sodium sulfide into dialkyltetrahydrothiophenes.     

Synthesis and reduction of 5,5-dimethyl-2,4-bis(3,4-dimethoxyphenyl)-2-chloro-2,5-dihydrofuran

A new, stable 5-membered ring heterocyclic cation was prepared and reduced into the corresponding dihydrofuran, tetrahydrofuran and pentanol derivatives.     

Infrared spectra and assignments of 6Li/7Li substituted single crystals of lithium-2,4-pentanedionate

Polarized i.r. reflection spectra of single crystals of Li-2,4-pentanedionate (Li-acac) were recorded in the region from 200–600 cm⁻¹. Bands due to vibrations of the lithium ions were identified using the ⁶Li/⁷Li isotopic replacement technique. Various LiO distances as well as different degrees of distortion in the four symmetrically unequal LiO₄ coordination polyhedra lead to a specific spectroscopic behaviour for each of these polyhedra. It is therefore possible to suggest an assignment of the observed LiO bands to specific LiO₄ groups. The results of a rough calculation support this assignment. A correlation between LiO distances and LiO stretching force constants is proposed.     

Inter-hydrogen bond dynamical coupling effects in polarized IR spectra of N-methylacetamide single crystals

This article presents the investigation results of polarized IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, ν_N–H and ν_N–D. A similar study was also performed for crystals of the deuterium isotopomers of the compound, D₇-NMA (CD₃CONDCD₃) and D₆-NMA (CD₃CONHCD₃). On the basis of the analysis of the linear dichroic and temperature effects, the two-branch structure of the ν_N–H bands in the spectra was ascribed to centrosymmetric hydrogen bond pairs in the lattice. Each hydrogen bond in such a dimeric system belonged to another chain of the associated molecules. The exciton interactions involving the dimer hydrogen bonds were considered to be responsible for the band shape generation. For the deuterium-bonded crystals the exciton interactions were found to be weaker since the ν_N–D bands were less split. Within an individual hydrogen or deuterium bond chain the in-chain exciton couplings involving hydrogen bonds were estimated as considerably weaker than the inter-chain ones. The exciton dilution retains the two-branch fine structure pattern of the “residual” ν_N–H and ν_N–D bands. This means that the inter-chain couplings involving hydrogen bonds do not change, when the in-chain couplings vanish. These results are the evidence of the influence of non-conventional co-operative interactions occurring in the hydrogen bond systems on the spectra. These co-operative interactions are responsible for the non-random distribution of the hydrogen isotope atoms in the hydrogen bridge lattices, namely for the grouping of identical hydrogen isotope atoms in the dimers. The proposed interpretation of the IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals casts light on the spectra generation mechanisms and gives a new meaning to the traditional nomenclature applied for describing the ν_N–H band structure pattern in IR spectra of amides.     

External electric field effects on absorption, fluorescence, and phosphorescence spectra of diphenylpolyynes in a polymer film

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.     

Experimental investigation of the absolute enthalpies of formation of 2,3-, 2,4-, and 3,4-pyridynes

The absolute enthalpies of formation of 3,4-, 2,3-, and/or 2,4-didehydropyridines (3,4-, 2,3- and 2,4-pyridynes) have been determined by using energy-resolved collision-induced dissociation of deprotonated 2- and 3-chloropyridines. Bracketing experiments find the gas-phase acidities of 2- and 3-chloropyridines to be 383 ± 2 and 378 ± 2 kcal/mol, respectively. Whereas deprotonation of 3-chloropyridine leads to formation of a single ion isomer, deprotonation of the 2-chloro isomer results in a nearly 60:40 mixture of regioisomers. The enthalpy of formation of 3,4-pyridyne is measured to be 121 ± 3 kcal/mol by using the chloride dissociation energy for deprotonated 3-chloropyridine. The structure of the product formed upon dissociation of the ion from 2-chloropyridine cannot be unequivocally assigned because of the isomeric mixture of reactant ions and the fact that the potential neutral products (2,3-pyridyne and 2,4-pyridyne) are predicted by high level spin-flip coupled-cluster calculations to be nearly the same in energy. Consequently, the enthalpies of formation for both neutral products are assigned to be 130 ± 3 kcal/mol. Comparison of the enthalpies of dehydrogenation of benzene and pyridine indicates that the nitrogen in the pyridine ring does not have any effect on the stability of the aryne triple bond in 3,4-pyridyne, destabilizes the aryne triple bond in 2,3-pyridyne, and stabilizes the 1,3-interaction in 2,4-pyridyne compared to that in m-benzyne. Natural bond order calculations show that the effects on the 2,3- and 2,4-pyridynes result from polarization of the electrons caused by interaction with the lone pair. The polarization in 2,4-pyridyne is stabilizing because it creates a 1,2-interaction between the nitrogen and dehydrocarbons that is stronger than the 1,3-interaction between the dehydrocarbons.     

Allylic Functionalization of 2,5- and 2,4-Cyclo-Hexadiene-1,3-Dicarboxylates

A series of 2,4- and 2,5-cyclohexadiene-1,3-dicarboxylates were functionalized at the allylic position via oxidation (SeO₂, PDC/t-BuOOH) and halogenation (NBS). The regiochemical outcome for different substrates and reactions was studied and the importance of factors such as reaction mechanism, steric hindrance and reaction intermediates stability was discussed.

     

Stereochemical effects in the mass spectra of 2-hydroxy-, 5-hydroxy- and 2,5-dihydroxyprotoadamantanes

Electron ionization and methane and isobutane chemical ionization mass spectra have been obtained for the four configurationally isomeric 2,5-protoadamantanediols and the corresponding model monoalcohols. Although the electron impact (both 70 and 15 eV) and methane chemical ionization mass spectra of the 2,5-protoadamantanediols are distinct, the variations are not large and in general they are difficult to rationalize in terms of specific structural features. In contrast, the isobutane chemical ionization mass spectra of these diols show significant differences which can easily be correlated with sterochemical orientations of the substituents.     

Thermal and spectral properties of 2,3-, 2,4- and 3,4- dimethoxybenzoates of light lanthanides

The physico-chemical properties and thermal stability in air of light lanthanide 2,3-, 2,4- and 3,4-dimethoxybenzoates were compared and the influence of -OCH₃ substituent on their thermal stability was investigated. The complexes of these series are crystalline, hydrated or anhydrous salts with colours typical of Ln³⁺ ions. The carboxylate group is a bidentate, chelating (2,4- and 3,4-dimethoxybenzoates) or tridentate chelating and bridging ligand (2,3- dimethoxybenzoates). The thermal stability of 2,4- , 3,4- and 2,3- dimethoxybenzoates of light lanthanides was studied in the temperature range 273-1173 K. The positions of methoxy groups in benzene ring influence the thermal properties of the complexes and their decomposition mechanism. This revised version was published online in July 2006 with corrections to the Cover Date.     

Orientational effects on atomic photoelectric cross sections and their relevance to photoelectron spectra from single crystals

We have examined the dependence of the photoelectric cross section from individual atomic orbitals and from groups of orbitals adapted to crystal symmetries in order to ascertain the relevance of this effect to the photoelectron spectra arising from single crystals.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

Synthesis and configuration of diastereomeric 2,4-, 2,5-, and 2,6-piperidinedicarboxylic acids and their dimethyl esters

The reduction in an acidic medium over a platinum catalysts of 2,4-, 2,5-, and 2.6-pyridinedicarboxylic acids gave cis-2,4-, -2,5-, and -2,6-piperidinedicarboxylic acids, heating of which in an alkaline medium led to thermodynamically equilibrium mixtures of diastereomers. Individual trans-2,5-piperidinecarboxylic acid was isolated. The configurations of the 2,4-, 2,5-, and 2,6-piperidinedicarboxylic acids and their methyl esters were established by means of the PMR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 367–371, March, 1985.