Theoretical Studies of the Mechanism of Carbamoylation of Nucleobases by Isocyanates

Isocyanates with the -N═C═O functional group are highly reactive compounds. They are used in various industrial applications and have been found as possible metabolites of hydroxamic acids. Isocyanates interact with biopolymers and are notorious mutagens. Mutagenic effects of isocyanates are caused by the formation of covalent adducts with nucleobases of DNA, primarily cytosines, through carbamoylation of NH₂ groups to give the corresponding urea. The mechanism of carbamoylation of nucleobases by aryl isocyanates is studied by high-level density functional theory calculations. Three possible pathways are analyzed. It is demonstrated that the reaction follows the stepwise pathway, which starts with the formation of a π-complex followed by a rate-determining C-N covalent bond formation via the reactive tautomeric imine forms of the nucleobases. The reaction proceeds further through two consecutive proton transfers mediated by water molecules to give the final adduct. The predicted activation free energies of the rate-determining step in water agree with experimental data. In line with experiments, the reactivity of isocyanates toward nucleobases decreases in the order cytosine > adenine > guanine, and we rationalize this order of reactivity by the fall of their basicity and destabilization of the imine forms. Activation barriers of the alternative concerted pathways are higher than that of the preferred stepwise mechanism, and the match to experiment is poor. The kinetic effect of adding electron-withdrawing or electron-donating groups to the aryl group of aryl isocyanate is minute, which suggests that mutagenicity of isocyanates is determined exclusively by the reactivity of the -N═C═O group and as such cannot be removed by structural alterations of the adjacent aryl.

Characterization of N-(2,6-dimethylphenyl)hydroxylamine adducts of 2'-deoxyguanosine under weakly basic conditions

Aromatic amines are a class of chemical carcinogens that are activated by cytochrome P450 enzymes to form arylhydroxylamines that are conjugated to form N-acetoxyarylamines or N-sulfonyloxyarylamines. These conjugates undergo N-O bond cleavage to become reactive nitrenium ions that may form DNA adducts. Numerous studies in the past using N-acetoxyarylamines to investigate DNA adduct formation were conducted, however, less is known in regard to DNA adduct formation directly from arylhydroxylamines - especially under conditions that mimic the physiological conditions of cells such as weakly basic conditions. In this study, 2'-deoxyguanosine (dG) was exposed to N-(2,6-dimethylphenyl)hydroxylamine (2,6-DMPHA) and N-phenylhydroxylamine (PHA) at pH 7.4 without enzymes and analyzed by liquid chromatography high resolution mass spectrometry (LC-HRMS). 2,6-DMPHA exposure resulted in the production of relatively low amounts of adducts however the identities of at least six different adducts that were formed through reactions with carbon, nitrogen and oxygen of 2'-deoxyguanosine were proposed based upon different analytical approaches including HRMS CID fragmentation and NMR analyses. Contrastively, PHA exposure under identical conditions resulted in one adduct at the C8 position. It was concluded from these results and results of theoretical calculations that nitrenium ions produced from 2,6-DMPHA were relatively more stable resulting in longer nitrenium ion lifetimes which ultimately led to greater potential for 2,6-DMPHA nitrenium ions to react with multiple sites on dG.

Generation of N,N-Di(4-bromophenyl)nitrenium Ion under Acidic Conditions: Search for a Nitrenium Dication

The behavior of the N,N-di(4-bromophenyl)nitrenium ion under acidic aqueous conditions was examined via laser flash photolysis experiments. A long-lived species forms and can be assigned as the cation radical or the dication. This species is unreactive toward nucleophiles and reactive toward strong electron donors, consistent with a cation radical. Mechanistic analysis indicates that its formation is through a separate pathway compared to that of the nitrenium ion, suggestive of a triplet mechanism.

Time-Resolved Spectroscopic Observation of Diphenylnitrenium Ion Reactions with Guanosine

Arylnitrenium ions have gained attention for their high reactivity toward guanosine, which in some cases has been linked to carcinogenesis. Although many studies have examined covalent addition reactions between arylnitrenium ions and guanosine, there is still some uncertainty regarding the attack position of nitrenium ions on guanosine and its derivatives. In this paper, we employ nanosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopy to investigate the reaction between the N,N-di(4-bromophenyl) nitrenium ion (2) and guanosine. Our time-resolved spectroscopic results and photochemical product analysis results show that the reaction of guanosine with 2 generates an N7 intermediate that subsequently undergoes rearrangement and deprotonation to produce a C8 adduct. Comparing these results to our previous study between the 2-fluorenylnitrenium ion and guanosine indicates that the structure and properties of arylnitrenium ions are able to influence the reaction pathways and intermediate structures.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Acidic Aqueous Solution

Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR³) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4′–dibromodiphenylamino)–2,4,6–trimethylpyridinium BF₄⁻ salt (1) in an acidic aqueous solution. Efficient cleavage of the N–N bond (4 ps) to form the N,N–di(4–bromophenyl)nitrenium ion (DN) was also observed in the acidic aqueous solution. As a result, the dication intermediate 4 appears more likely to be produced after abstracting a proton for the nitrenium ion DN in the acid solution first, followed by an electron abstraction to form the radical cation intermediate 3. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions

Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved resonance Raman (ns-TR³) experiments were employed to examine the photochemical pathways of N-(4,4'-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF₄- (salt (DN) from just absorption of a photon of light to the production of the important N,N-di(4-bromophenyl)nitrenium ion 2. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion 2 was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion 2 with H₂O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate 3 shows that there is an efficient and exclusive reaction pathway for 2 with H₂O. The results shown in this paper give new characterization of 2, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies.

Direct time-resolved spectroscopic observation of arylnitrenium ion reactions with guanine-containing DNA oligomers

The metabolic activation of a number of aromatic amine compounds to arylnitrenium ions that can react with DNA to form covalent adducts has been linked to carcinogenesis. Guanine in DNA has been shown to be the main target of N-containing carcinogens, and many monomeric guanine derivatives have been utilized as models for product analysis and spectroscopic investigations to attempt to better understand the reaction mechanisms of DNA with arylnitrenium ions. However, there are still important unresolved issues regarding how arylnitrenium ions attack guanine residues in DNA oligomers. In this article, we employed ns-TA and ns-TR(3) spectroscopies to directly observe the reaction of the 2-fluorenylnitrenium ion with selected DNA oligomers, and we detected an intermediate possessing a similar C8 structure as the intermediates produced from the reaction of monomeric guanosine derivatives with arylnitrenium ions. Our results suggest that the oligomeric structure can lead to a faster reaction rate of arylnitrenium ions with guanine residues in DNA oligomers and the reaction of arylnitrenium ions take place in a manner similar to reactions with monomeric guanosine derivatives.

Identification of a reduction product of aristolochic acid: implications for the metabolic activation of carcinogenic aristolochic acid

Aristolochic acids are nephrotoxic and carcinogenic natural products that have been implicated both in endemic nephropathy in the Balkan region and in ailments caused by ingestion of herbal remedies. Aristolochic acids are metabolized to active intermediates that bind to DNA. In this study, reduction of aristolochic acid I with zinc in acetic acid afforded a new product that was characterized as 9-methoxy-7-methyl-2H-1,3-oxazolo[5',4'-10,9]phenanthro[3,4-d]-1,3-dioxolane-5-carboxylic acid, designated as aristoxazole, along with the expected aristolactam I. This new compound is a condensation product of aristolochic acid and acetic acid that may be related to the aristolochic acid-DNA adducts. The proposed mechanism of formation of aristoxazole involves nucleophilic attack of acetic acid on the nitrenium ion of aristolochic acid I. On the basis of these studies, a route to the metabolic activation of aristolochic acids and formation of adducts with DNA in in vitro systems is proposed and discussed.

Structural identification of DNA adducts derived from 3-nitrobenzanthrone, a potent carcinogen present in the atmosphere

3-Nitrobenzanthrone is a powerful bacterial mutagen and carcinogen to mammals. To obtain precise information on DNA-adduct formation by 3-nitrobenzanthrone, a number of DNA adducts, including N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone (13 a), 2-(2'-deoxyguanosin-N2-yl)-3-aminobenzanthrone (14 a), N-(2'-deoxyadenosin-8-yl)-3-aminobenzanthrone (15 a), 2-(2'-deoxyadenosin-N6-yl)-3-aminobenzanthrone (16 a), and their N-acetylated counterparts 13 b, 14 b, 15 b, and 16 b were synthesized by palladium-catalyzed aryl amination of the corresponding nucleoside and bromobenzanthrone derivatives. Among these DNA adducts, DNA adducts 13 a, 13 b, 14 a, 14 b, and 16 a were identified in the reaction mixture of nucleosides (2'-deoxyguanosine, 2'-deoxyadenosine, or DNA) with N-acetoxy-3-aminobenzanthrone or N-acetyl-N-acetoxy-3-aminobenzanthrone, both of which are recognized as activated metabolites of 3-nitrobenzanthrone. The formation of these multiple DNA adducts may help explain the potent mutacarcinogenicity of 3-nitrobenzanthrone.

Time-resolved resonance Raman investigation of the 2-fluorenylnitrenium ion reactions with C8 guanosine derivatives

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the reactions of the 2-fluorenylnitrenium ion with several C8-substituted guanosine derivatives is reported. The TR3 spectra show that the 2-fluorenylnitrenium ion reacts with the C8-substituted guanosine derivatives (C8-methylguanosine and C8-bromoguanosine) to produce C8 intermediates with the methyl and bromine moieties still attached to the intermediate species at the C8 position. The C8-bromoguanosine species was observed to be less reactive toward the 2-fluorenylnitrenium ion compared to the guanosine and C8-methylguanosine species. Comparison of the TR3 spectra to the results obtained from density functional theory calculations was used to characterize the C8 intermediates observed to learn more about their structure and properties. The implications of these results for the chemical reactivity of arylnitrenium ions toward substituted guanosine derivatives are briefly discussed.

N,N-Di(4-halophenyl)nitrenium ions: nucleophilic trapping, aromatic substitution, and hydrogen atom transfer

The reactive intermediates N,N-di(4-chlorophenyl)nitrenium ion and N,N-di(4-bromophenyl)nitrenium ion were generated through photolysis of the corresponding N-amino(2,4,6,-collidinium) ions. The behavior of these diarylnitrenium ions was characterized by laser flash photolysis, analysis of the stable photoproducts, and ab initio calculations with density functional theory. The latter predict these species to have singlet ground states. The halogenated diarylnitrenium ions are significantly longer lived than the unsubstituted diphenylnitrenium ion. Specifically, cyclization to form carbazole derivatives occurs negligibly, if at all, with the halogenated derivatives. They do, however, carry out most of the characteristic reactions of singlet arylnitrenium ions, including combining with nucleophiles on the aryl rings, adding to arenes, and accepting electrons from readily oxidized traps. Interestingly these species also abstract H atoms from 1,4-cyclohexadiene and various phenol derivatives. The implication of the latter process in relation to the computed singlet-triplet energy gaps of ca. -12.5 kcal/mol is discussed.

Time-resolved resonance Raman study of the reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide

A time-resolved resonance Raman investigation of the reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in a mixed aqueous solvent is presented. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in the mixed aqueous solution generates two new species on the microsecond time scale. One of these species is identified as 2,2'-azobisfluorene, and the other species is tentatively assigned to a 1,4-bis-(2,2'-fluorenyl)-tetrazadiene cation intermediate. The structure and properties of these two species are briefly discussed. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide is also briefly compared to that of the 2-fluorenylnitrenium ion reactions with guanosine and water.

Computational studies of the properties of phenyloxenium ions — A comparison with phenylnitrenium and phenylcarbenium ions

Properties of phenyloxenium ion 13a, phenylnitrenium ion 14a, and their 4-methyl and 4-phenyl analogues have been studied at the HF/6-31G* and pBP/DN*//HF/6-31G* levels to explain differences in their relative ease of formation and their stabilities. The phenyloxenium ions 13 are ground-state singlets but S0–T1 gaps are smaller than those of the corresponding nitrenium ions. The S0 states are stabilized by donor methyl and phenyl substituents in both classes of ions, but phenyloxenium ion has much greater charge localization on the ring, primarily at the 4 position. Evidence for this difference stems from ground-state HF/6-31G*geometries, dipole moments, and vibrational frequencies. Nitrenium ions exhibit some quinoidal character, but the calculated C—N bond lengths are longer than those of their 4-hydroxy-2,5-cyclohexadienone imine hydration products 17 and the symmetric C-N stretching frequencies are ca. 60–100 cm–1 less than those of 17. However, the C—O bond lengths and stretching frequencies of the phenyloxenium ions are slightly shorter and greater, respectively, than those of their 4-hydroxy-2,5-cyclohexadienone hydration products (16). The oxenium ions are best described by their 4-oxo-2,5-cyclohexadienyl carbenium resonance structures. Accordingly, a 4-phenyl group stabilizes the phenyloxenium ion more than the phenylnitrenium ion leading to a planar geometry and considerably more charge in the distal ring, thus accounting for regioselectivities of azide reactions. Isodesmic comparisons of the energy difference between phenyloxenium and phenylnitrenium ions and their neutral hydration products explains their relative stabilities under aqueous conditions; whereas 4-biphenylyloxenium ion 13c has a lifetime in water of 12 ns as opposed to the corresponding nitrenium ion 14c (300 ns), the 4-methylphenyloxenium ion 13b is less stable to hydration by 18.7 kcal mol–1 (1 cal = 4.184 J) and cannot be observed under the conditions used to generate 13c.Key words: oxenium ions, nitrenium ions, computational chemistry, nucleophilic addition, singlet state properties.

Effect of meta Electron-Donating Groups on the Electronic Structure of Substituted Phenyl Nitrenium Ions

Density functional theory (UB3LYP/6-31G(d,p)) was used to determine substituent effects on the singlet-triplet-state energy gap for 21 meta-substituted phenylnitrenium ions. It was found that strongly electron-donating substituents stabilize the triplet state relative to the singlet state. With sufficiently strong meta electron donors (e.g., m,m'-diaminophenylnitrenium ion) the triplet is predicted to be the ground state. Analysis of equilibrium geometries, Kohn-Sham orbital distributions, and Mulliken spin densities for the triplet states of this series of nitrenium ions leads to the conclusion that there are two spatially distinct types of low-energy triplet states. Simple arylnitrenium ions such as phenylnitrenium ions as well as those having electron-withdrawing or weakly donating meta substituents have lowest-energy triplet states that are n,pi in nature. That is, one singly occupied molecular orbital is orthogonal to the plane of the phenyl ring and one is coplanar. These n,pi triplets are generally characterized by large ArNH bond angles (ca. 130-132 degrees ) and an NH bond that is perpendicular to the plane of the phenyl ring. In contrast, meta donor arylnitrenium ions have a lowest-energy triplet state best described as pi,pi. That is, both singly occupied molecular orbitals are orthogonal to the aromatic ring. Such pi,pi states are characterized by NH bonds that are coplanar with the phenyl ring and have ArNH bond angles that are more acute (ca. 110-111 degrees ). These triplet nitrenium ions have electronic structures analogous to those of meta-benzoquinodimethane derivatives.

Generation and Trapping of the 4-Biphenylyloxenium Ion by Water and Azide: Comparisons with the 4-Biphenylylnitrenium Ion

Generation of the 4-biphenylyloxenium ion, 1a, from hydrolysis of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, is demonstrated by common ion rate depression and azide trapping. The ion is less selective with a shorter lifetime (12 ns at 30 degrees C) in aqueous solution than the corresponding nitrenium ion, 6a (ca 0.3 mus at 30 degrees C). Its lifetime is considerably longer than those of structurally related carbenium ions. Unlike 6a, 1a reacts with N3-, in part, at the distal ring to generate 4'-azido-4-hydroxybiphenyl, 5. These results are rationalized by calculations at the HF/6-31G* and pBP/DN*//HF/6-31G* levels that show that 1a is planar but is less stable than 6a relative to their hydration products by ca. 12 kcal/mol. The p-tolyloxenium ion, 1b, has not been unequivocally demonstrated to be formed in H2O, but kinetic data show that it must be at least 104-fold less stable than 1a relative to their respective 4-acetoxy derivatives. It is also calculated to be ca. 19 kcal/mol less stable than the corresponding nitrenium ion, 6b, relative to their hydration products.

Transient resonance Raman and density functional theory investigation of the 4-acetamidophenylnitrenium ion

This paper reports a transient resonance Raman and density functional theory study of the 4-acetamidophenylnitrenium ion in a mostly aqueous solvent. The transient Raman bands combined with results from density functional theory calculations indicate that the spectrum should be assigned to the singlet state of the 4-acetamidophenylnitrenium ion. The 4-acetamidophenylnitrenium ion was found to have a substantial iminocyclohexadienyl character comparable to previously studied para-phenyl-substituted phenylnitrenium ions and noticeable charge on both the acetamido and nitrenium moieties. The structure and properties of the 4-acetamidophenylnitrenium ion are compared to those of other arylnitrenium ions. We briefly discuss the chemical reactivity and selectivity of the para-acetamido-substituted phenylnitrenium ions compared to para-phenyl- or para-alkoxy-substituted phenylnitrenium ions.

Concerning the reaction between singlet nitrenium ions and water: a computational investigation on competitive reaction paths

The reaction between singlet nitrenium ions XNH(+) (X = F and Cl) and H(2)O has been investigated by high-level of theory ab initio calculations. The geometries of the involved intermediates, transition structures, and dissociation products have been optimized at the MP2(full)/6-31G(d) level of theory, and accurate total energies have been obtained using the Gaussian-3 (G3) procedure. The reaction commences by the exothermic formation of the F-NH-OH(2) (+) and Cl-NH-OH(2) (+) intermediates, which are in turn able to undergo two distinct low-energy reaction paths, namely, the isomerization to the N-protonated isomers of the hydroxylamines F-NH-OH or Cl-NH-OH, and the eventual extrusion of HF or HCl. The competitive or alternative occurrence of these two processes strictly depends on the nature of the substituent X. In the reaction between FNH(+) and H(2)O, the energy gained in the formation of the complex F-NH-OH(2) (+) from the association between FNH(+) and H(2)O, 52.1 kcal mol(-1), is by far larger than the activation barrier for the loss of HF from F-NH-OH(2) (+), computed as 24.9 kcal mol(-1). In addition, the F-NH-OH(2) (+) intermediate requires 33.0 kcal mol(-1) to overcome the barrier for the isomerization to F-NH(2)-OH(+). Therefore, the reaction between FNH(+) and H(2)O is expected to occur practically exclusively by HF elimination with formation of the HN-OH(+) ionic product. On the other hand, for the reaction between ClNH(+) and H(2)O, it is not possible to get a definitive conclusion on the competitive or alternative occurrence of the two reaction paths. In fact, the transition structure involved in the elimination of HCl from Cl-NH-OH(2) (+) is only 3.4 kcal mol(-1) lower in energy than the transition structure for the isomerization of Cl-NH-OH(2) (+) to Cl-NH(2)-OH(+). In addition, the absolute values of the energy barriers of these two processes, 24.2 and 27.6 kcal mol(-1), respectively, are comparable with the energy gained in the formation of the complex Cl-NH-OH(2) (+) from the association between ClNH(+) and H(2)O, 24.0 kcal mol(-).1 Therefore, the ClNH(+) cation is predicted to react with water significantly slower than FNH(+).

Biomonitoring of arylamines and nitroarenes

Arylamines and nitroarenes are very important intermediates in the industrial manufacture of dyes, pesticides and plastics, and are significant environmental pollutants. The metabolic steps of N-oxidation and nitroreduction to yield N-hydroxyarylamines are crucial for the toxic properties of arylamines and nitroarenes. Nitroarenes are reduced by microorganisms in the gut or by nitroreductases and aldehyde dehydrogenase in hepatocytes to nitrosoarenes and N-hydroxyarylamines. N-Hydroxyarylamines can be further metabolized to N-sulphonyloxyarylamines, N-acetoxyarylamines or N-hydroxyarylamine N-glucuronide. These highly reactive intermediates are responsible for the genotoxic and cytotoxic effects of this class of compounds. N-Hydroxyarylamines can form adducts with DNA, tissue proteins, and the blood proteins albumin and haemoglobin in a dose-dependent manner. DNA and protein adducts have been used to biomonitor humans exposed to such compounds. All these steps are dependent on enzymes, which are present in polymorphic forms. This article reviews the metabolism of arylamines and nitroarenes and the biomonitoring studies performed in animals and humans exposed to these substances.

Reactions of N-methyl-N-(4-biphenylyl)nitrenium ion with electron-rich arenes: laser flash photolysis and product studies

An arylnitrenium ion, N-methyl-N-(4-biphenylyl)nitrenium ion, was generated through photolysis of 1-(N-methyl-N-4-biphenylyl)amino-2,4,6-trimethylpyridinium tetrafluoroborate, and its reactions with various donor-substituted arenes (e.g., 1,3,5-trimethoxybenzene, mesitylene, 1,4-dimethoxybenzene, hexamethylbenzene, etc.) were examined using product analysis and laser flash photolysis. In general, trapping of the short-lived nitrenium ion by the arenes leads to three types of products: (1) the parent amine, N-methyl-N-4-biphenylylamine; (2) an ortho-adduct, where the ring position ortho to the nitrenium ion center is bonded to the arene ring; and (3) an N-adduct, where the nitrenium ion nitrogen is bonded to the trap. Laser flash photolysis studies show that the rates of these trapping reactions vary from 10(4) to 10(9) M(-1) s(-1), depending on the structure of the arene trap. These trapping rate constants do not correlate with the one-electron oxidation potential of the arene, nor with the expected stability of a sigma-complex derived from direct electrophilic aromatic substitution. It is argued that the observed rate constants correspond to initial formation of a pi-complex between the arylnitrenium ion and the arene trap. This complex then forms the observed products.

Quantum chemical characterization of the reactions of guanine with the phenylnitrenium ion

Density functional calculations at the B3LYP/6-311+G(2d,p)//pBP/DN level predict all cationic adducts combining guanine, at either its N2, O6, N7, or C8 positions, with phenylnitrenium ion, at either its N, 2, or 4 positions, to be lower in energy than the separated reactants. This relative stability of all adducts is preserved after addition of aqueous solvation free energies computed at the SM2 level, although some leveling of the adduct relative energies one to another is predicted. Cations having the lowest relative energies in solution correspond structurally to those adducts most commonly found when guanine reacts with larger, biologically relevant nitrenium ions in vitro and in vivo. One of these, the N-C8 adduct, is stabilized both by a rearomatized phenyl ring and by the operation of an anomeric effect not found in any of the others. On the basis of energetic analysis, direct conversion of an N-N7 cation to an N-C8 cation according to a previously proposed mechanism is unlikely; however, an alternative rearrangement converting a 2-N7 cation to an N-C8 cation via the intermediacy of a five-membered ring may be operative in nitrenium ions with aromatic frameworks better able than phenyl to stabilize endocyclic cationic charge.

Transient-resonance Raman and density functional theory investigation of 4-biphenylylnitrenium, 2-fluorenylnitrenium, and diphenylnitrenium ions

We present transient-resonance Raman spectra for the 4-biphenylylnitrenium, diphenylnitrenium, and 2-fluorenylnitrenium ions. These spectra display a number of fundamental vibrational bands whose frequencies exhibit good agreement with those computed using BPW91/cc-PVDZ density functional theory calculations for the singlet ground states of the 4-biphenylylnitrenium, diphenylnitrenium, and 2-fluorenylnitrenium ions. Comparison of these arylnitrenium ions with each other and with previous results for structurally similar biphenyl radical cations indicates that the degree of iminocyclohexadienyl character observed in these arylnitrenium ions depends on the relative orientation of the two phenyl rings, the nature of the nitrenium ion moiety, and the ability of the biphenyl-like group to accommodate positive charge through formation of a more planar-like structure with quinoidal-like character.

Transient resonance Raman and density functional theory investigation of the 2-fluorenylnitrenium ion

We report a transient resonance Raman spectrum for the 2-fluorenylnitrenium ion obtained after photolysis of 2-azidofluorene. The 10 experimental Raman band frequencies of the transient spectrum show very good agreement with the computed frequencies from BPW91/cc-PVDZ density functional theory calculations for the 2-fluorenylnitrenium ion. Our results confirm the assignment of the approximately 460 nm transient absorption band formed after photolysis of 2-azidofluorene in water/acetonitrile or water solution to the singlet ground electronic state 2-fluorenylnitrenium ion. Our study indicates the 2-fluorenylnitrenium has a large degree of iminocyclohexadienyl cation character with significant delocalization of the charge over both phenyl rings of the fluorene moiety. We compare our results for the 2-fluoreneylnitrenium ion to those previously reported for several other arylnitrenium ions.