共查询到20条相似文献,搜索用时 15 毫秒
1.
F. Benoit 《Journal of mass spectrometry : JMS》1973,7(3):295-303
The effect of substituents on the electron-impact-induced fragmentation of the molecular ions of para and meta substituted benzoic acids has been examined. The substituent is observed to exert an effect on the ionisation potential of the molecular ion, on the appearance potentials of the primary daughter ions and on the amount of H/D scrambling in the molecular ion of the carboxyl-d1 analogues prior to the loss of hydroxyl therefrom. The energy of activation for the loss of hydroxyl from the molecular ion is in general dependent upon the nature but not the position of the substituent, while the amount of H/D scrambling in the molecular ion of the carboxyl-d1 derivative is dependent upon both the nature and the position of the substituent. No correlation of the relative ion abundances with σ+ constants was observed. The results are consistent with the molecular ions of each compound having a dissimilar energy distribution, which could arise either by different energy transfers from the electron beam to the molecule or by the participation of different isolated electronic excited states (or similar states but to varying extents) in the dissociation of the molecular ions. 相似文献
2.
Frank M. Benoit 《Journal of mass spectrometry : JMS》1974,9(6):626-627
Three empirical equation which have been used to calculate ionisation potentials of para and meta disubstituted benzenes are evalyated according to their ability to predict the IP's of para and meta YC6H4COX, (where x ? OH, NH2, CL). 相似文献
3.
Daniel J. V. A. dos Santos Ana S. Newton Raul Bernardino Rita C. Guedes 《International journal of quantum chemistry》2008,108(4):754-761
The O–H and S–H homolytic bond dissociation enthalpies of a set of disubstituted phenols and thiophenols (NH2, OH, CH3, Cl, CF3, and NO2) have been computed by a density functional theory procedure with the 6‐311++G(d,p) basis set. A very good agreement between our results and available experimental ones is observed. The effect of substituents on structure, charges and BDEs are investigated and their correlation with Hammett parameters is studied. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
4.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1988,44(8):823-827
The results of an ESR study of two homologous series of phenothiazine derivatives, in polycrystalline form and in liquid and frozen solutions, are analyzed in order to determine the influence of the R2 and R10 substituents on the g tensor. In solution the g-values are mainly affected by R2 through electronic effects, however in polycrystalline form both substituents can modify the g-values through packing effect. A highly electronegative R2 substituent can induce a different molecular stacking. 相似文献
5.
Loss of HCN from the molecular ion of benzaldoximes occurs via two distinct involving a 4-membered and a 5-membered intermediate, respectively. Both low energy high energy molecular ion decomposition were investigated by means of metastable peak shapes and the energetics of further decomposition of ions, respectively. Substituent effects previously held responsible for differences in the behaviour of m- and p-methoxybenzaldoximes are shown to be negligible; the differences are due to the differense in internal energy contents. 相似文献
6.
Linear plots may be obtained for the fragmentations of aryl p-cyanobenzoates using the McLafferty approach, but the correlations are not as satisfactory as those observed1 for aryl nitrobenzoates. Some satisfactory correlations of substituent effects are noted in the negative ion spectra of substituted benzyl-p-nitrobenzoates and substituted phenyl-(p-nitrophenyl)acetates. Pronounced rearrangement peaks are observed in the spectra of substituted phenyl-(p-nitrophenyl)acetates. Appearance potential studies suggest that the similarities in ρ values obtained1 for the fragmentations of aryl m- and p-nitrobenzoate molecular anions are probably due to the correspondence between Hammett σ values and appearance potentials in the two series. 相似文献
7.
A brief discussion is given of some factors that merit consideration in the analysis of peptides by mass spectrometry, with emphasis on sequence determination. The first systematic study of simple peptides by field ionization (FI) is described, and comparison made with the corresponding electron-impact (EI) ionization spectra, both at low resolution (approx. 1500). The substances examined were either benzyloxycarbonyl or t-butyloxycarbonyl derivatives of di-through pentapeptide methyl esters, containing the amino acids glycine, alanine, leucine, serine, threonine, proline and tyrosine. The incidences of sequence-characteristic cleavage peaks and rearrangement ion peaks were both slightly lower in the FI than the EI spectra, although the peak relative intensities generally were higher under FI conditions. 相似文献
8.
F. Benoit 《Journal of mass spectrometry : JMS》1972,6(12):1289-1292
It is demonstrated that the ionisation potentials of mono- and di-substituted benzenes are related to the sum of Brown's σp+ of the substituents for both the meta and para isomers. From this it is concluded that the charge, in substituted arenes, lies predominantly on the aromatic ring rather than on either of the substituents. An empirical equation relating the IP's of mono- and disubstituted benzenes to the sum of σp+ is presented. 相似文献
9.
Mass spectra of the four stereoisomeric 1-methyldecalins and the four 2-methyldecalins have been measured. For each set of stereoisomers, the order of relative intensity of the parent-less-CH3 ion, but not of the parent ion, parallels the order of relative stability of the molecules. The correlation suggests that loss of the methyl group from the members of each set of stereoisomers leads to a common product, with the differing conformational energies converting into differences in vibrational energy, which in turn lead to differing extents of further decomposition. 相似文献
10.
The dianions of two isomeric bis(hydroxystyryl)benzenes show dramatically different photophysical properties. 相似文献
11.
The 70 eV negative ion mass spectra of a series of phosphoranes exhibit significant peaks for ions resulting from skeletal rearrangements. The primary fragmentation reactions of the molecular anions, which involve reduction of the phosphorus from the penta- to the ter-valent state, have been rationalised in terms of the relative stabilities of the products formed. The processes occurring are in some cases analogous to and in other cases complementary to, those previously observed in the positive ion spectra of similar compounds. 相似文献
12.
Stalin T Devi RA Rajendiran N 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(11-12):2495-2504
Spectral characteristics of ortho, meta and para dihydroxy benzenes (DHB's) have been studied in different solvents, pH and beta-cyclodextrin. Solvent study shows that: (i) the interaction of OH group with the aromatic ring is less than that of amino group both in the ground and excited states, (ii) in absorption, the charge transfer interaction of OH group in para position is larger than ortho and meta positions. pH studies reveals that DHB's are more acidic than phenol. The higher pK(a) value of oDHB (monoanion-dianion) indicates that the formed monoanion is more stabilized by intramolecular hydrogen bonding. DHB's forms a 1:1 inclusion complex with beta-CD. In beta-CD medium, absorption spectra of DHB's mono and dianions shows unusual blue shifts, whereas in the excited state, the spectral characteristics of DHB's follow the same trend in both aqueous and beta-CD medium. 相似文献
13.
14.
Peter Brown 《Journal of mass spectrometry : JMS》1970,3(5):639-646
The [M]+˙ → [M ? Cl]+ reaction in a series of m- and p-X substituted chlorobenzenes has been studied, utilizing a simple kinetic approach, comparison of metastable ion relative abundances, and by measurement of ionization and appearance potentials. All evidence obtained is consistent with rearrangement prior to cleavage in the molecular ions, in which substituent position becomes effectively randomized. These findings are related to known hydrogen randomization reactions occurring in either the molecular ion or [M ? Cl] ion of chlorobenzenes. Mechanisms involving carbon scrambling via such species as ionized benzvalenes or prismanes, or ring-opening to isomeric acyclic molecular ions in which hydrogen randomization might occur can be entertained, but mechanisms involving simple hydrogen shifts in the intact benzene ring appear less likely. 相似文献
15.
Pratt DA DiLabio GA Valgimigli L Pedulli GF Ingold KU 《Journal of the American Chemical Society》2002,124(37):11085-11092
Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively. 相似文献
16.
Gary Kemister Addy Pross Leo Radom Robert W. Taft 《Journal of computational chemistry》1981,2(4):470-477
Ab initio molecular orbital theory with the STO -3G basis set is used to examine both charge and energy interactions in a series of meta- and para-substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH3 is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH3. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO ) theory. 相似文献
17.
Sixty MHz NMR proton spectra of 15 2,4-substituted benzaldehydes and 12 methylbenzenes dissolved in different solvents have been accurately analyzed. Substituent effects on the coupling constants have been determined and have been found to be additive in those trisubstituted compounds where there are neither internal hydrogen bonds or strong interacting groups present. 相似文献
18.
The negative-ion mass spectra of a variety of substituted nitroaryl benzoates and aryl nitrobenzoates have been studied. Pronounced proximity effects are observed in the spectra of ortho-substituted compounds. The meta and para pairs fragment in a similar manner. Linear plots are observed for many cleavages using either the McLafferty or Harrison methods. Evidence is provided which suggests that the fragmentations of meta and para isomers may proceed by the same mechanistic pathway. The magnitude of ρ does not appear to provide precise information concerning the nature of the transition state for a particular reaction of a negative ion. 相似文献
19.
The mass spectra of enol derivatives of β-diketones, such as enol ethers and enaminies are discussed. Their behaviour under electron-impact is in accordance with the fragmentation we suggested for an acyclic β-diketone in the enol form. Rearrangement of the enol functional group is observed in the spectra. This process is a general one, as it show not only migration of oxygen and nitrogen in the enol ethers and enamines, but also migeration of sulphur and chlorine in the thio-ether and in 4-chloro-3-pentene-2-one. Enol derivatives are suitable compounds for determining the branching of alkyl chains in β-diketones. 相似文献
20.
The kinetic shifts for an ion decomposition involving loss of CH2CO from a series of substituted acetanilide ions have been calculated using the Rice-Ramsperger-Kassel-Marcus theory. The results indicate that in the main the observed changes in appearance potential-ionisation potential for this reaction are due to a change in the number of degrees of freedom on introducing the substituent and not due to a substituent effect on the critical energy of reaction. 相似文献