Nitrenium ions calculations

The triplet nitrenium ion, however, lacks symmetry but, unlike the excited triplet states of other azaaromatic systems such as pyridine, is surprisingly planar.283 Theoretical dipole moments for cyclopent[c]- and cyclopent[rf]azepine, and the molecular geometry of cyclopent[rf]-azepine, have been calculated using semiempirical ab initio methods.3... 

Calculations of the singlet-triplet energy gaps of a series of nitrenium ions [X(H)N ]+ in the gas phase and in solution indicate that the gap decreases in the order X = H > CN > F as a result of stabilization of the singlet state by r-donation from the substituent. For strong tt-donors (Cl, F), the singlet state is more stable than the triplet. ... 

Calculations on the isoelectronic series Me(Ph)B , Me(Ph)C , and [Me(Ph)N ]+ show that the singlet-state geometries are different, reflecting differences in the orbital interactions between the hypovalent atom and the 7r-system. The high calculated barrier (21.5 kcal moP ) for [1,2]-H shift in the nitrenium ion is the result of migration using the orbital which is conjugated with the tt-system. 

The nitrenium ion +NH2 has been the subject of a detailed, comprehensive calculation. Calculations on (48) with 15 different X substituents reveal a large substituent sensitivity, and also that aqueous solvation preferentially stabilizes the singlet state. This substiment sensitivity agrees with the results of a time-resolved IR study of the diphenylnitrenium ion (49), which shows that resonance contributors such as (50) and (51) are very important to the overall structure. Substituted 4-biphenyl nitrenium ions... 

Gonzales and co-workers extended their work on simple nitrenium ions, examining the reactions of several small, monosubstituted nitrenium ions (XNH+, where X = H, F, Cl, CN, and Me) with water. These computational studies (QCISD(T)/6-311-1—1- G ) treated only the singlet states of the nitrenium ion. According to these calculations, each singlet nitrenium ion adds to water to form an O-protonated hydroxylamine intermediate (R2N—OHj). No activation barrier was detected for... 

In order to increase the experimental benchmarks available for comparison with theoretical calculation, Mcllroy et al., determined the singlet triplet energy splitting in a diaminosubstituted nitrenium ion, 1,3-dimethylbenzotriazolium ion 37 (Fig. 13.22). This species can be viewed a nitrenium ion by virtue of mesomer 37a, which places a positive charge on the central nitrogen. Because this species... 

The parent nitrenium ion (NH2) is firmly established as a ground-state triplet both extensive ab initio calculations as well as PES experiments all agree that the singlet-triplet energy gap is 30 kcal/mol. There have been several investigations on its behavior in solution. Takeuchi et al. " showed that this species could be generated by photolysis of l-amino-(2,4,6-triphenylpyridinium) ion. These photolyses were carried out in the presence of various aromatic compounds. It was found that the triplet state abstracted hydrogen atoms from traps such as toluene... 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

The DFT calculations (BPW91/cc-pVDZ) on the same singlet nitrenium ions give excellent agreement with these measurements. For calculated and experimental C=C stretches, deviation between experiment and calculation is generally <5 cm On the other hand, similar calculations carried out for the triplet states showed significantly greater deviation from the measured values. 

More recently, time-resolved vibrational spectroscopy has been applied to nitrenium ions. These techniques hold much promise for future research as they can provide more detailed structural information about nitrenium ions. In addition, the vibrational frequencies measured in this way can be used to assess the accuracy of theoretical calculations. 

Reactions with carbon, nitrogen and sulfur nucleophiles 214 Direct observation of nitrenium ions acid-base chemistry and singlet-triplet chemistry 227 Heteroarylnitrenium ions 238 Calculations 244... 

By the late 1980s it was clear that a significant number of thermal and photochemical reactions of arylhydroxylamines and their derivatives, N-chloroanilines, aryl azides, anthranilium salts, and other compounds could be explained in terms of nitrenium ions or transition states that resembled nitrenium ions. Since no monoarylnitrenium ion had been directly observed, and data on the lifetimes and quantitative reactivity/selectivity of these species were not available, it was not possible to assess whether the reactions that had been observed were due to free ions, or ion pairs, or preassociation processes. In many cases Sn2 reactions could not be ruled out because appropriate kinetics experiments had not been performed. Most authors had attributed the presence of reduction products in thermal and photochemical reactions to triplet ions, but calculations suggested that the triplet species may not be accessible in thermal processes. It was clear that singlet ions could be reduced under certain conditions, so the presence of the... 

The substituent effects calculated for A show that aromatic and vinylic jT-donors in the para-position have a stabilizing effect on the nitrenium ions that is much larger than is seen in Op. For example, Op for Me, MeO, and Ph are —0.31, —0.78, and —0.18, respectively, while A for 75y (Ar = 4-tolyl), 75cc (Ar = 4-MeOphenyl) and 75n (Ar = 4-biphenylyl) are 8,1 kcal/mol, 22.7 kcal/mol, and 19.3 kcal/mol, respectively. The calculations and experimental data show that a para-phenyl substituent is about as stabilizing for a nitrenium ion as is a para-methoxy substituent. This unusual stabilization is the major reason that correlations of logS vs. cr are so scattered for nitrenium ions. Substituent effects at N are relatively small. Replacement of NH by NAc destabilizes the ion toward hydration by 4.5 l.Okcal/mol. Based on the correlation line, at 20°C this amounts to a predicted increase in by a factor of 4 to 11 when NH is replaced by NAc. The experimentally observed range of 1.5 to 9.0 (Table 1) is very close to this prediction. These calculated substituent effects on the thermodynamics of hydration and the calculated geometries of nitrenium ions (discussed in another section) indicate that for most nitrenium ions the canonical structure II of Scheme 38 is dominant. ... 

X 10 M Combined with the experimental pK, of 14.8 in the solvent employed in this study (20% CH3CN/H2O), and the requirement that koH /kv, — KJKvj, these rate constants lead to a calculated pK of 75n of 16.0. " Although the parent N-arylnitrenium ion 75a cannot be directly detected in LFP studies, the pH dependence of yields of nitrene and nitrenium-ion-derived products led to the conclusion that the pKg of 75a s=12.4. ... 

Calculations of the properties of nitrenium ions continue to be performed by semi-empirical and ab initio methods, in an effort to provide information concerning ... 

Recently Ford and co-workers performed calculations of the structures and relative stabilities of a series of 20 polycyclic nitrenium ions at the HF/6-31G(d) level.Except for symmetrical cases, all ions existed as distinct syn and anti isomers defined by the configuration of the N—H bond with respect to the unsymmetrical aryl group. The energies of configurational isomers differed by between 0.2 and 3.7 kcal/mol. Inversion barriers through a linear transition state were fairly constant in the range from... 

There have been several attempts to correlate observed chemical, biological, or spectral properties of nitrenium ions with calculated proper-The correlations of azide-solvent selectivities with the... 

Ford and Herman have been able to correlate the calculated AMI stabilities of nitrenium ions vs. those of the corresponding arylmethyl carbo-cations, as defined by the isodesmic reactions of Scheme 69, through a simple relationship shown in equation (6)... 

Attempts to rationalize the regioselectivity of attack of nucleophiles on the aryl rings of nitrenium ions in terms of calculated properties of the ions (LUMO coefficients, localization energies, etc.) have been moderately successful. An adequate explanation of electrophilic reactivity of nitrenium ions at N with certain nucleophiles such as glutathione, C-8 of d-G, and other carbon nucleophiles has not yet appeared. ... 

As shown in Figure 7.10c, because of the lower energy of the nitrenium ion orbitals, interaction of the 2p orbital with the n bonding orbitals of C substituents will be stronger than the interaction with the n orbitals. Substantial stabilization ensues. The interaction raises the separation of the 2p and spn orbitals. Thus, the ground state for C -substituted nitrenium ions is expected to be So, unlike the situation in C -substituted carbenes. This expectation has been supported by ab initio calculations on phenyl nitrenium ion [183, 184]. Because the LUMO remains very low, this type of nitrenium ion will be very electrophilic. 

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