NMR structure determination of (11E)-trinervita-1(14),2,11-triene, a new diterpene from sexual glands of termites

Female alates of Nasutitermes ephratae termites from Guadeloupe and Nasutitermes sp. from Brazil produce a diterpene hydrocarbon of the molecular formula C₂₀H₃₀ as the main component of their tergal gland secretion. Analysis of NMR, IR, and mass spectra of the diterpene led to a structure of (11E)-trinervita-1(14),2,11-triene. Based on a comparison with the published oxygenated trinervitane skeleton from termites we prefer the enantiomer with absolute configurations (4R,7S,8R,15S,16S). The suggested structure is supported by ab initio quantum chemical calculation of ¹H and ¹³C chemical shifts for the optimized geometry of the molecule.     

Mass spectrometry of γ-Complexes of transition metals : XXII. Fragmentations of ferrocenyl-substituted pyrazolines-2 under electron impact

The mass spectra of 1-N-acetyl- and 1-N-phenyl-3-ferrocenyl-5-arylpyrazolines-2, their 5-ferrocenyl-3-aryl isomers and their 3,5-diferrocenyl analogues have been studied. The “pyrazoline” type of the molecular ion fragmentation involves various processes of heterocyclic nucleus destruction and elimination of the substituents or their fragments. The directions and intensifies of the processes observed are interpreted in terms of preferred positive charge localization on the transition metal atom. Interactions between the acetyl and ferrocenyl groups manifest themselves by the appearance of intense [P  C₅H₅]⁺ ions (ferrocenyl type fragmentation products) in the mass spectra of 1-N-acetyl-3-aryl-5-ferrocenylpyrazolines-2 only. The fragmentation mechanism leading from [P  C₅H₅]⁺ to C₇H₇OFe⁺ is discussed.     

Fast-atom bombardment mass spectrometry of brassinosteroid analogs

Positive ion fast-atom bombardment mass spectra of brassinosteroid analogs have been systematically obtained for the first time. The spectra of six brassinosteroid analogs and their corresponding 22S,23S isomers included the protonated molecule in medium to high relative intensity. The fragmentation pattern is dominated by side chain cleavage. There is a marked preferential loss of acetic acid from the [M + H]⁺ ion of 5α-hydrogen-3β-acetylated derivatives compared to the 5α-hydroxy-3β-acetylated analogs.     

The mass spectra of some pyrazole compounds

The mass spectra of ten pyrazole compounds have been determined. Fragmentation schemes have been derived by means of the metastable defocusing method. The predominant process is cleavage of the nitrogen-nitrogen bond resulting in expulsion of HCN. The process second in prominence is the loss of a nitrogen molecule after initial removal of a hydrogen radical or a substituent, giving the species [C₃H₂R]⁺, probably a cyclopropenyl ion. In general, the fragmentation pattern is strongly influenced by the substituent.     

Fragmentation patterns in multiphoton ionization: a statistical interpretation

The simplest statistical theory wherein the mass spectral fragmentation pattern is governed by the mean energy is derived and compared with the observed results for resonance-enhanced multiphoton ionization of benzene. By varying the mean energy absorbed it is possible to span the range of different patterns up to the dominance of C⁺₁ions above 37 eV/molecule.     

Natural abundance C REDOR coupled to a singly N-labeled nucleus: simultaneous determination of interatomic distances in crystalline ammonium [N] -glutamate monohydrate

REDOR technique was applied to natural abundance ¹³C nuclei coupled to a singly labeled ¹⁵N nucleus to determine the ¹³C, ¹⁵N interatomic distances simultaneously in crystalline ammonium [¹⁵N] -glutamate monohydrate (1). Consequently, the interatomic C–N distances between ¹⁵N and ¹³C=O, ¹³C_α, ¹³C_β, ¹³C_γ, and ¹³C_δ carbon nuclei for 1 were determined with a precision of ±0.15 Å, after the experimental conditions such as the location of samples in the rotor, length of π pulse etc. were carefully optimized. ¹³C-REDOR factors for three spin system, (ΔS/S₀)_CN1N2, and the sum of two isolated 2-spin system, (ΔS/S₀)^*=(ΔS/S₀)_CN1+(ΔS/S₀)_CN2, were further evaluated by the REDOR measurements on isotopically diluted 1 in a controlled manner. Subsequently, the intra- and intermolecular C–N distances were separated by searching the minima in the contour map of root mean square deviation (RMSD) between the theoretically and experimentally obtained (ΔS/S₀)^* values against two interatomic distances, r_C–N1 and r_C–N2. When the intramolecular C–N distance (r_C–N1) of the particular carbon nucleus is substantially shorter than the intermolecular one (r_C–N2), C–N distances within a single molecule were obtained with an accuracy of ±0.06 Å as in the cases of C=O, C_α and C_β carbon nuclei. C–N distances between the molecule in question and the nearest neighboring molecules can be also obtained, although accuracy was lower. On the contrary, it was difficult to determine the interatomic distances in the same molecule when the intermolecular dipolar contribution is larger than the intramolecular one as in the case of C_δ carbon nucleus.     

Elektronenstossinduzierte Fragmentierung von Acetylenverbindungen—V: Vergleich der Isomeren Naphthyl- und Phenyl-Pentinen-Alkohole

The differences found in the fragmentation pattern of the isomeric naphthyl- and phenyl-pentynen-alcohols have been elucidated by the investigation of ²H- and ¹³C-labelled derivatives. As shown by high resolution mass spectrometry, in the case of the acetylenic compound, the ion [C₉H₇]^⊕ is formed mainly by ketene elimination from the [M – 1]⁺-ion and from the naphthalenes by elimination of acetylene from the benztropylium-ion. The proposed mechanism of CO- and CHO-elimination given for benzylalcohol cannot be transferred to the compounds investigated.     

Heats of immersion of amorphous As2S3, As2S5 and As2Se3 in organic liquids

The heats of immersion of hydrophobic, amorphous arsenic chalcogenides have been measured in several organic liquids. For hexane, butanol, butylchloride and nitropropane, the heats of immersion with As₂S₃, As₂S₅ and As₂Se₃ showed linear dependences on the dipole moment of the wetting liquid molecule. From the results the average values of the electrostatic field strength were calculated to be 0.29 × 10⁵, 0.31 × 10⁵, and 0.57 × 10⁵ e.s.u. cm^?2. The heats of immersional wetting of As₂S₃ and As₂Se₃ in n-alkanols linearly increased with an increase of n, the number of carbon atoms in C_nH_2n+1OH. The contributions due to polarization of the liquid molecule by the electrostatic field of the solid surface, due to the dispersion force and due to the interaction between the dipole moment of the liquid with the electrostatic field of the solid were calculated by applying the additivity of intermolecular forces. The result showed that the dispersion force was the dominant contribution to the interaction in As chalcogenides-n-alkanol systems.     

A mass spectral study of some symmetric and unsymmetric sulfides,disulfanes, and trisulfanes

The low-resolution mass spectra of 13 trifluoromethyl and/or methyl sulfur-containing compounds (CF₃S_nCF₃, CF₃S_nCH₃, CH₃S_nCH₃, CF₃S_mCl, and CH₃S_mCl; n = 1, 2, 3; m = 1, 2) are reported and discussed. There is a considerable similarity between the fragmentation pattern of compounds with the same terminal groups. However, with a fluorinated and a non-fluorinated methyl group, the major fragmentation products produced are similar to those for the dimethyl compounds.     

Specific and random fragmentations in the mass spectra of some neopentyl compounds

The mass spectra of neopentyl alcohol, bromide and chloride and some ¹³C and ²H labelled analogues have been studied. Most fragmentations of the molecular ions of these compounds occur by simple bond cleavages and do not involve rearrangement before fragmentation. We propose that in the [M ? CH₃]⁺ fragment ions, seven of the eight hydrogen atoms and all four carbon atoms are involved in randomisation when an ethylene molecule is ejected. The eighth hydrogen atom (which comes from a methyl group) is probably associated with the heteroatom. The neopentylcation, observed only in the mass spectrum of the bromide, fragments mainly by loss of an ethylene molecule, also containing randomly selected hydrogen and carbon atoms. The [C₄H₇]⁺ ion also was observed to undergo complete atom scrambling.     

Fourier transform spectra of carbon disulfide in the 2800–3000 cm−1 region

Fourier transform spectra were recorded for carbon disulfide in the region between 2800 and 3000 cm⁻¹. A total of 20 bands for the carbon disulfide isotopologue ¹²C³²S₂, 2 for ¹²C³²S³³S, 8 for ¹²C³²S³⁴S, 1 for ¹²C³⁴S₂ and 5 for ¹³C³²S₂ have been investigated. Two bands 13¹1–01¹0 of ¹²C³²S₂ and 12⁰1–00⁰0 of ¹²C³²S³⁴S present local perturbations.Vibrational frequencies and rotational constants have been determined for the bands.     

Mass Spectrometric Decomposition Processes in Labelled 1-Heptenes

The 70 eV mass spectra of a number of ¹³C- and D-labelled analogs of 1-heptene have been measured, as well as the metastable transitions in the non-labelled compound. Isotopic distributions in the major fragment ions have been calculated from the high and low resolution data. The results show that considerable skeletal rearrangement must take place before formation of most of the fragment ions. Loss of methyl and ethyl radicals occurs mainly from the two ends of the molecule. Ethylene fragments come primarily from the unsaturated end of the molecule, but show evidence of significant prior skeletal rearrangement. The predicted McLafferty rearrangement accounts for only 2/3 of the C₄H₈⁺ ions formed, less for the C₃H₆⁺ ions. At least 80% of C₄H₉⁺ ions appear to be formed by allylic cleavage, as expected, but this mechanism can only account for a maximum of 20% of the formation of the complementary ion C₃H₅⁺. Both, this latter ion and C₃H₆⁺, are probably generated by loss of hydrogen from C₃H₇⁺. Figures obtained for label retention in 1-[¹³C]- and 1-D-labelled analogs were nearly identical for most fragment ions, probably indicating that the hydrogen atoms in position 1 remain on C(1) even following skeletal rearrangement. A similar result was found for the 7-[¹³C]- and 7-D-labelled compounds. The main exceptions in the case of the products labelled in position 1 (C₄H₇⁺, C₃H₃⁺) seem to be due to initial loss of an hydrogen atom from this position followed by further fragmentation.     

Raman,infrared and NMR spectra and structure of divinylmethylborane

The Rarnan spectra of gaseous, liquid and solid divinylmethylborane have been recorded from 20–3500 cm^?1 and the IR spectra of gaseous and solid divinylmethylborane recorded over the range 30–3500 cm^?1. A variable temperature study of the Raman spectrum of the liquid phase has been carried out. A complete vibrational assignment is presented. In the solid phase the molecule appears to have a planar heavy atom skeleton (C_s symmetry). From analysis of the spectra of the fluid phases, the presence of a second isomer, in which one or both of the vinyl groups are twisted slightly out of the BC₃ plane (C₁ symmetry), is proposed. Variable temperature ¹³C NMR studies have been carried out. A comparison of the ¹³C chemical shift of C_β of the vinyl group with the corresponding value in other vinylboranes indicates that relatively little delocalization of the π-electron density occurs in this molecule. Low temperature (?115°C) ¹³C NMR data are consistent with a low barrier to rotation about the boron-vinyl carbon bond.     

The nickel-group IV molecules NiC,NiSi, and NiGe

All electron ab initio calculations have been applied to elucidate the electronic states and the nature of the chemical bonds in the molecules NiC, NiSi, and NiGe. The calculations have revealed that the ground states of all three molecules are¹Σ⁺, but due to the open 3d shell of the Ni atom the molecules have many low-lying electronic states. The NiC molecule is strongly polar, and the low-lying electronic states have been identified as those arising when the angular momenta of the³F_g Ni⁺ ion are coupled to the angular momenta of the⁴S_uC^? anion. The chemical bond in the NiC molecule has triple bond character due to the valence bond couplings between the Ni 4s and 3dπ electrons and theC 2p electrons. The chemical bonds in the molecules NiSi and NiGe are very much alike; they are double bonds composed of oneσ and oneπ bond. Theσ bond is due to the doubly occupied delocalized molecular orbital composed of the Ni 4s orbital and the Si 3pσ or the Ge 4pσ orbital. Theπ bond originates from the valence bond coupling between the localized hole in the Ni 3dπ orbital and the valencepπ electron of Si or Ge.     

Synthesis of 13c-labelled [s]-proline and its conformational analysis by nuclear magnetic resonance

In order to analyze completely the ring conformation of [S]-Proline (l-thiazolidine-4-carboxylic acid) this compound has been synthesized with its C_δ atom enriched at 90% in ¹³C. The ¹H-¹H, ¹³C-¹H, ¹³C-¹³C vicinal coupling constants measured at several pH values describe well the geometry of the thiazolidine ring. The results suggest that the ring has an average planar conformation only in a narrow range of pH close to the pK of the amine group. Above and below this pH the ring tends to adopt preferentially the S_γendo and S_γexo puckered conformations respectively. It is concluded that a good correlation exists between the value of the dihedral angle ø and the character S_γendo or S_γexo of the ring.     

Structure characterization and spectroscopic investigation of ciprofloxacin drug

Ciprofloxacin (CPF, C₁₇H₁₈FN₃O₃) drug is used in the treatment of some bacterial infectious diseases. The drug was investigated using thermal analysis (TA) measurements (TG/DTG) and electron impact mass spectral (EI-MS) fragmentation at 70 eV techniques. Furthermore, the drug was characterized and investigated by other spectroscopic tools as IR, UV–Vis, ¹H-, and ¹³C-NMR. Semi-empirical MO calculation using PM3 procedure has been carried out on neutral molecule and positively charged species. The calculations included, bond length, bond order, bond strain, partial charge distribution, ionization energy, and heat of formation (ΔH _f). The PM3 procedure provides a basis for fine distinction among sites of initial bond cleavage, which is crucial to the rationalization of subsequent fragmentation of the molecule. The mass spectra and thermal analysis fragmentation pathways were proposed and compared to each other to select the most suitable scheme representing the correct fragmentation of this drug. From EI-MS, the main primary cleavage site of the charged molecule is that due to C–COOH bond cleavage with H-rearrangement to skeleton and CO₂ loss which can further decompose by piperazine loss. Thermal analysis of the neutral form of the drug reveals the high response of the drug to the temperature variation with very fast rate. Thermal decomposition has carried out in several sequential steps in the temperature range 40–650 °C. The initial thermal decomposition is similar to that obtained by mass spectrometric fragmentation (C–COOH fragment) but differ in that a rearrangement occurs by OH and CO loss. Therefore, comparison between MS and TA helps in selection the proper pathway representing the fragmentation of this drug. This comparison successfully confirmed by MO calculation. Finally, the effect of fluorine atom on the stability of the drug was discussed.     

Fragmentation in chemical ionization mass spectrometry and the proton affinity of the departing neutral

Chemical ionization mass spectra of six 5,6-dihydro-2-methyl-1,4-oxathiins, and some of the sulfoxides and sulfones derived therefrom, have been determined employing hydrogen, methane and isobutane as reagent gases. The major fragmentation reaction of the protonated molecule, [R′COX·H]⁺, involves loss of the neutral HX molecule. For the sulfides and sulfones, with X ranging from OH to N(CH₃)C₆H₅, it is observed that the importance of this fragmentation is inversely correlated with the proton affinity of the departing HX molecule in both the H₂ and CH₄ chemical ionization. For the sulfoxides no consistent correlation is observed and this is attributed to the interference of competing and/or consecutive fragmentation reactions. In the isobutane chemical ionization mass spectra only the protonated molecule is observed for most of the compounds studied.     

Skeletal and hydrogen rearrangements in the electron impact mass spectrum of propene

The mass spectrum of propene-2-[¹³C] shows 81% retention of C-2 in the [C₂H₃]⁺ fragment ion at 70 eV electron energy, decreasing to 75% C-2 retention at low electron energies. The mass spectra of propene-2-d₁, propene-1,1,3,3,3-d₅, propene-1,1,2-d₃ and propene-3,3,3-d₃ also have been examined at a resolution sufficient to resolve H₂-D doublets. The results at 70 eV electron energy show complete H/D randomization prior to fragmentation to form [C₃(H, D)₅]⁺ but, in agreement with the ¹³C labelling data, incomplete H/D interchange prior to fragmentation to form [C₂(H, D)₃]⁺. The results are interpreted in terms of a reversible isomerization of the propene molecular ion to a cyclopropane structure in competition with fragmentation.     

Photoionization/fragmentation of endohedral fullerenes

Photoionizationlfragmentation of endohedral fullerenes was investigated by use of laser-de sorption time-offlight (LD-TOF) mass spectroscopy. The velocity distribution of the parent ion (LaC ₈₂ ⁺ ) was found to be bimodal, as has previously been shown for laser desorbed C ₆₀ ⁺ . The 0 fragment ions have velocity distributions corresponding predominantly to the fast parent ion distribution. The LD-TOF mass spectra taken with a relatively low laser fluence were independent of the delay time of the extraction pulse, showing only a monotonically decreasing pattern of LaC _2n ⁺ (as n decreased). However, with higher laser fluence, it was shown that the mass distributions drastically changed from the monotonically decreasing pattern to that of C _2n ⁺ and LaC _2n ⁺ with magic numbers. Based on these findings, a plausible photoionization/fragmentation mechanism is presented and discussed.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.