Bohlmann Bands Research Articles

Dimethylpiperazine on Air-Water Interface: Vibrational Sum frequency Generation study of a cyclic tertiary amine

Dimethyl piperazine (DMP) is a weakly basic, cyclic, tertiary diamine. Being a saturated 6-membered ring, it may attain a number of energetically close conformational orientations in the ground state. In a particular environment, its conformational distribution depends on the stabilising factors present in the surroundings. Though soluble in water, it is expected to have substantial surface propensity due to presence of multiple –CH2 and –CH3 bonds. Detection of Bohlmann bands in the C–H stretch region of the Vibrational Sum Frequency Generation (VSFG) spectrum revealed that comparatively higher energy chair conformer with one axial and one equatorial –CH3 bond is present on the pure liquid DMP-air interface. At the air-aqueous solution interface, those bands were found to disappear as the lone electron pair on N-atom is engaged in hydrogen bonding with water molecule. Due to this hydrogen bonding, the peak of “dangling free OH” bonds disappeared from VSFG spectrum of water molecules at air–water interface. Alongwith this hydrogen bonding, the hydrophobic nature of the –CH3 bonds of DMP molecules cause disorganisation of native hydrogen bonded network of water molecules in the air–water interface. Structure and orientation of DMP is expected to be pH-responsive since the chemical conditions of the surrounding medium directly affect the extent of protonation of the amine group. Upon addition of HCl to the bulk, protonated DMP ions, too, could be detected on the air-solution interface by their VSFG spectrum which were considerably different from unprotonated DMP. As the bulk pH of solution changed towards more acidic, spectral features of VSFG spectra of DMP on the surface showed significant change, implying variation in the surface pH, as structure and orientation changes at different degrees of protonation. Since the VSFG spectral features of DMP was found to be pH-responsive, it can be utilised to ascertain the pH of any surface that has DMP absorbed on it. pH-sensitivity of the DMP molecules on air–water interface also suggest that orientation of headgroups will have some effect on the surface orientation of cationic surfactants with amine based headgroups.

Molecular mechanics (MM4) study of amines

The MM4 force field has been extended to include aliphatic amines. About 20 amines have been examined to obtain a set of useful molecular mechanics parameters for this class. The vibrational spectra of seven amines (172 frequencies) calculated by MM4 have an overall rms error of 27 cm(-1), compared with corresponding MM4 value of 24 cm(-1) for alkanes. The rms and signed average errors of the moments of inertia of nine simple amines compared with the experimental data were 0.18% and -0.004%, respectively. The heats of formation of 30 amines were also studied. The MM4 weighted standard deviation is 0.41 kcal/mol, compared with experiment. Electronegativity effects occur in the hydrocarbon portion of an amine from the nitrogen, and are accounted for by including electronegativity induced changes in bond lengths and angles, and induced dipole-dipole interactions in the molecule. Negative hyperconjugation results from the presence of the lone pair of electrons on nitrogen, and leads to the Bohlmann bands in the infrared, and also to strong and unusual geometric changes in the molecules (Bohlmann effect), all of which are fairly well accounted for. The conformational energies in amines appear to be less straightforward than those for most other classes of molecules, apparently because of the Bohlmann effect, and these are probably not yet completely understood. In general, the agreement between the MM4 calculated results and the available data is reasonably good.

Venom Chemistry of the Ant Myrmicaria melanogaster from Brunei

Analysis of the extracts of the ant Myrmicaria melanogaster from Brunei in the Indonesian archipelago by GC-MS and GC-IR revealed the presence of five new alkaloids, identified as (9Z)-3-propylindolizidine (1), cis- and trans-2-butyl-5-propylpyrrolidine (2 and 3, respectively), (10E)-3-butyllehmizidine (7), and (5Z,8Z,9Z)-3-butyl-5-propyl-8-hydroxyindolizidine (10a), whose structures were established by comparison with synthetic samples. In addition the monoterpene hydrocarbons beta-pinene, myrcene, and limonene were detected along with all four isomers of 3-butyl-5-methylindolizidine (4a-d), cis- and trans-2-butyl-5-(4-pentenyl)pyrrolidine (5a and 5b), trans-2-butyl-5-pentylpyrrolidine (6), (5Z,9Z)-3-butyl-5-propylindolizidine (8), and (5Z,9E)-3-butyl-5-propylindolizidine (9), alkaloids well known from ants and frogs, whose structures were established on the basis of published spectra or comparison with authentic samples. This study utilized vapor-phase infrared analysis for the assignment of stereochemistry using Bohlmann bands for the bicyclic alkaloids and, in the case of 10a, the detection of an intramolecular hydrogen bond. A biogenetic relationship between the mono- and bicyclic ring systems is proposed.

Ammonia chemical ionization tandem mass spectrometry in structural determination of alkaloids. II. Tetraponerines from pseudomyrmecine ants.

Chemical ionization tandem mass spectrometry (CI-MS/MS) of alkaloids with ammonia reagent gas and collision-activated dissociation as well as EI-MS/MS were applied to the tetraponerine alkaloids in extracts from six pseudomyrmecine ants of the genus Tetraponera. The MS/MS techniques along with gas chromatography Fourier transform infrared (GC/FTIR) spectra allowed identification in two extracts of seven of the eight known tetraponerines. The EI-MS/MS fragmentations proved diagnostic for the ring system and the CI-MS/MS patterns for the C-8 or C-9 substitution, while the Bohlmann bands in FTIR spectra were diagnostic for the C-8 or C-9 configurations. An Indian ant (T. allaborans) had T-2, T-4 and T-8, while a Chinese ant (T. binghami) had T-5, T-6, T-7 and T-8. Four other ants, T. rufonigra (India), T. penzigi (Africa), T. clypeata (Africa) and T. sp. cf. emeryi (Africa), had no tetraponerines.

Determination of cis and trans configurations in pentahydrobenzopyranopyrroles by FT-IR spectroscopy

A method of distinguishing cis and trans isomers in polyheterocycles containing β,β′-disubstituted pyrrolidines is described. It is based on the relative absorbance of the Bohlmann bands and the normal symmetric CH stretching bands. The method is applied using FT-IR spectroscopy in solution and coupled with gas chromatography. An interpretation is given, with the help of molecular computing.

1H and 13C NMR and FT-IR studies of the interaction between 1,8-bis(dimethylamino)naphthalene and 3,5-dichlorophenol

The 1H and 13C NMR spectra of the complex between 1,8-bis(dimethylamino)naphthalene and 3,5-dichlorophenol have been studied at temperatures between 30 °C and –70 °C in CD2Cl2 and at different acid to base concentration ratios. At a temperature of –60 °C a splitting of the 1H and 13C signals of DMAN and DMANH+ is observed. The protonation degree deduced from the NMR data and the intensity of the Bohlmann bands are in very good agreement. At a temperature below 8 °C, a splitting (2.6 Hz) of the methyl protons caused by spin–spin coupling with the (NHN)+ bridge protons is observed. Comparison with literature data and with the coupling constant determined in this work for protonated N,N-dimethylaniline suggest that the potential surface of the NHN+ cation includes two symmetrically situated wells between which fast proton migration takes place.

Cis- and trans-configurations of α,α′-disubstituted piperidines and pyrrolidines by GC-FTIR; application to decahydroquinoline stereochemistry

GC-FTIR spectroscopy provides a convenient, rapid method to distinguish cis- and trans-α,α′-disubstituted piperidines and pyrrolidines; the pyrrolidines usually first require N-methylation. Cis-isomers exhibit significant Bohlmann bands, trans-isomers only very weak bands. The method is also applicable to alkaloids containing a piperidine ring. For example, a 2-propyl-5-methyldecahydroquinoline, named trans- 195A, recently detected in skin extracts of a dendrobatid frog is shown by GC-FTIR to have C(2) and C(8a) in a cis-orientation.

Protonation of 1,8-bis(dimethylamino)naphthalene by various acids in acetonitrile

FTIR studies on a number of systems composed of 1,8-bis(dimethylamino)naphthalene (DMAN) and various organic acids (phenols, benzoic acid derivatives and dicarboxylic acids) were carried out. The degree of protonation was estimated based on the intensity of the Bohlmann bands. In the ionic equilibria the homoconjugated anions play an essential role so that the degree of protonation depends on the excess of acid. The 1:1 complexes of phenols and benzoic acids in acetonitrile form a mixture of protonated DMAN, free DMAN, homoconjugated anions and more complicated species. The broad absorption extending over the whole IR region results from the combination of bands ascribed to ν(NHN+) of protonated DMAN and ν(OHO–) of homoconjugated anions.

Protonation of 1,8-bis(dimethylamino)naphthalene by N–H proton donors in acetonitrile

FTIR studies were carried out on a number of systems composed of 1,8-bis(dimethylamino)naphthalene (DMAN) and various N–H proton donors in acetonitrile. The degree of protonation was estimated based on the intensity of the Bohlmann bands. As in the case of systems containing OH acids homoconjugated anions play an important role in ionic equilibria. In the case of moderately strong proton donors the species present in solution are protonated DMAN, free DMAN, NHN– homoconjugated anions and more complicated self-solvated species. The homoconjugated NHN– anions are characterized by a broad doublet with submaxima at ca. 1950 and 2500 cm–1.

The Perlin Effect: bond lengths, bond strengths, and the origins of stereoelectronic effects upon one-bond C–H coupling constants

The magnitude of a one-bond C–H coupling constant depends upon the chemical environment of the hydrogen atom and, especially, upon its stereochemical relationship to vicinal lone electron pairs. However, a lone electron pair is not essential for the observation of a stereoelectronic effect, since even cyclohexane exhibits different axial and equatorial C–H coupling constants. We propose the name "Perlin Effect" to describe such observations. An analysis of the extensive experimental data regarding the Perlin Effect reveals that, in cyclohexane and in six-membered rings having one or more heteroatoms of the first row attached to the carbon of interest, 1JC–H is always larger for an equatorial hydrogen than for an axial hydrogen. The magnitude of the Perlin Effect is reduced when the carbon carrying the hydrogen of interest is attached to first row and second row atoms or heteroatoms, and it reverses when the carbon atom carries two heteroatoms from below the first row.The existence of the Perlin Effect in nuclear magnetic resonance spectra is reminiscent of an infrared effect known as the Bohlmann bands, whose origin has previously been explained by quantitative perturbational molecular orbital (PMO) theory in terms of the effects of lone electron pairs upon the lengths and strengths and, therefore, the chemical reactivities of vicinal C—H bonds. Since the magnitude of a one-bond C–H coupling constant is expected to vary inversely with bond length, the origins of the Perlin Effect and of the Bohlmann bands would seem to be the same, i.e., the longer (weaker) C—H bond has the smaller one-bond coupling constant. This expectation has now been confirmed: for 25 molecules, representing a total of 35 different kinds of C—H bonds, the bond lengths, stretching force constants, and charge distributions have been determined from fully optimized 6-31G* molecular orbital calculations. In nine of ten cases for which experimental data exist for pairs of diastereomeric or diastereotopic hydrogens, the shorter C—H bond of the pair has the larger coupling constant; in the tenth case, the experimental difference is only 1–2 Hz. Moreover, a global analysis of the data in terms of the equation J = A + BqCqH + C/r, where J is an experimental coupling constant, q is a total atomic charge, and r is a C—H bond length, correlates 23 different types of C—H bonds linearly with a correlation coefficient of 0.915. The C parameter is the leading term of the correlation. Among the systems studied theoretically are eight molecules of the type CH3CHXY (Y = OH, SH; X = F, Cl, OH, SH), which are representative of systems containing both endocyclic and exocyclic first row and second row anomeric effects. The exocyclic effect is found to be very similar for first row and second row substituents, but the endocyclic effect is larger for the first row substituent. Both findings agree with experimental data in solution. Finally, quantitative PMO analysis has been employed to analyse the origins of the different C—H bond lengths in the various molecules of the study. Keywords: anomeric effect, PMO analysis, NMR, stereochemistry, molecular orbital calculations.

A b i n i t i o computation of force constants. IV. The theoretical anharmonic force fields and vibrational frequencies of methylamine, methanol, and methanethiol

The anharmonic force fields of methylamine, methanol, and methanethiol have been computed ab initio. The harmonic force constants agree fairly well with experiment. Better agreement is found between the experimental frequencies and the theoretical anharmonic frequencies. The stretching modes are predicted especially well. The calculated ν13 and ν14 for CH3NH2 agree with the solid phase but not the gas phase experimental results. Methylamine, methanol, and methanethiol exhibit Bohlmann bands, i.e., a lowering of the stretching frequency of a C–H bond antiperiplanar to a lone pair. The computations reveal that these bands are linked to a smaller force constant for the anti CH bond and to Fermi resonance between the symmetric C–H stretch and CH bending overtones.

Etude structurale en serie dihydro iso-indole par spectrographie : De l'utilisation des bandes de Bohlmann en ir

Reduction of substituted 6,11-diphenyldibenzo[b,f] diazocines [1,4] with diborane gave two non-fluorescent products identified as cis or trans dihydroisoindole 3. This structure was based on IR Bohlmann bands, and on mass-spectrometric data. An isoindole structure was attributed to fluorescent products.

On the Origin of the Bohlmann Bands

A perturbational molecular orbital analysis has been performed of the strengths of the CH bonds of methylamine and methanol in their staggered conformations. This analysis leads to the prediction that a CH bond anti-coplanar to a directed lone pair is stronger than a gauche CH bond, and is in disagreement with experimental observation. The origin of the disagreement is to be found in the underestimation of the role of the nuclear–nuclear contribution to the bond strengths. Abinitio computation of the gauche and anti stretching force constants of methylamine provides quantitative theoretical support for the view that these differ because of a nuclear-dominated effect. It is suggested that effects, analogous to those observed in the Bohlmann bands, may be seen even in the absence of nonbonded electron pairs.

Criteria for the determination of the stereochemistry of steroidal indolizidines

E/F-Indolizidine steroids, which represent a structural unit of Solanum alkaloids, have been synthesized and used to evaluate criteria (IR—Bohlmann bands, 1a-c —hydrogenbond, 2a-e NMR, 3a-c mass spectra, 4 kinetic studies of quaternization with methyl iodide, 5a,b oxidation with mercuric acetate 6) for the determination of the stereochemistry of these systems. It was found that the most reliable method is the measurement of the rate constants of quaternization of the bridgehead nitrogen with methyliodide.

ChemInform Abstract: BOHLMANN BANDS, A REASSESSMENT

Chemischer InformationsdienstVolume 4, Issue 47 Physical Organic Chemistry ChemInform Abstract: BOHLMANN BANDS, A REASSESSMENT EDGAR E. ERNSTBRUNNER, EDGAR E. ERNSTBRUNNERSearch for more papers by this authorJOHN HUDEC, JOHN HUDECSearch for more papers by this author EDGAR E. ERNSTBRUNNER, EDGAR E. ERNSTBRUNNERSearch for more papers by this authorJOHN HUDEC, JOHN HUDECSearch for more papers by this author First published: November 20, 1973 https://doi.org/10.1002/chin.197347066Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume4, Issue47November 20, 1973 RelatedInformation

Bohlmann bands - a reassessment

Raman spectra of several alicyclic amines in the regions 3000-2500 cm −1 and 1500-1000 cm −1 have been obtained. The appearance of additional bands below 2800 cm −1, in these compounds (Bohlmann bands), is ascribed to the combined effects of weakened CH bonds trans to a nitrogen lone pair and of Fermi resonance. A reassignment for trimethylamine is proposed.

Conformational requirements for the existence of Bohlmann bands in the infrared spectra of indolo [2,3-a]quinolizidines. I. Cis- and trans-2-tert-Butyl derivatives

Conformational requirements for the existence of Bohlmann bands in the infrared spectra of indolo [2,3-a]quinolizidines. I. Cis- and trans-2-tert-Butyl derivatives

A study of some pyridopyrazines and pyridopyrimidines

AbstractThe d‐ and l‐forms of 2H‐octahydropyrido[1,2‐a]pyrazine (I) have been prepared and their absolute configurations established. These configurations were confirmed by an optical rotary dispersion study. The infrared spectra of 2H‐octahydropyrido[1,2‐a]pyrazine (I), lH‐octahydropyrido[ 1,2‐a]pyrimidine (11) and 2H‐octahydropyrido[ 1,2‐c]pyrimidine (111) were compared with the infrared spectrum of quinolizidine. All of these spectra showed the Bohlmann bands characteristic of a trans ring structure. A few derivatives of these ring systems have been prepared.

Infrared-spectral Studies on the Orientation of the Lone Pairs in Piperidine Derivatives

Abstract The infrared spectra of stereoisomers of several piperidines and their N-methyl derivatives in carbon tetrachloride have been measured in the region of 3020 to 2500 cm−1. The estimated intensities (A) of their Bohlmann bands have been analyzed, provided that Formula (1) concerning the intensity defined in the text holds for the compounds examined. The result indicates that piperidine exists mainly, but not exclusively, in the conformation with the lone pair axial.