Direct Observation of 4-Phenoxyphenylnitrenium Ion: A Transient

Article pubs.acs.org/JPCB

Direct Observation of 4‑Phenoxyphenylnitrenium Ion: A Transient Absorption and Transient Resonance Raman Study Jiadan Xue,*,†,‡ Yafang Li,‡ Lili Du,§ Yong Du,*,∥ Wenjian Tang,⊥ Xuming Zheng,‡ and David Lee Phillips§ †

Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China ‡ Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, 310018, China § Department of Chemistry, The University of Hong Kong, Hong Kong, S. A. R., China ∥ Centre for THz Research, China Jiliang University, Hangzhou, 310018, China ⊥ School of Pharmacy, Anhui Medical University, Hefei, 230032, China S Supporting Information *

ABSTRACT: Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with t = 346 ps or protonation in mixed aqueous solution with t = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.

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INTRODUCTION

Scheme 1.

Arylnitrenium ions are short-lived divalent reactive intermediates, which have been found to play an important role1−3 in the carcinogenesis of many aromatic amines and nitro-polyaromatic hydrocarbons (NPAHs).4,5 Recently, photochemical precursors to produce arylnitrenium ions developed by groups of McClelland,6−9 Falvey,10−14 and Novak15−18 make the laser flash photolysis (LFP)2,6,7,10−13 and time-resolved (TR) vibrational spectroscopies such as TR-IR19,20 and TRRaman21−25 techniques widely used, and provided much kinetic and structural information for some arylnitrenium ions in the solution phase. TR-IR and TR-Raman observed results with the help of theoretical calculations reveal that arylnitrenium ions have significant iminocyclohexadienyl character, and substantial positive charge delocalizes into the phenyl ring rather than the nitrenium nitrogen1 (Scheme 1). This property makes some substitutions greatly to stabilize arylnitrenium ions toward water and other similar nucleophiles. For example, some para-substituted (Cl, Me, Ph, MeO) Nmethyl-N-phenylnitrenium ions have lifetimes of 10 ns to 100 μs in water12 while the unsubstituted one (PhNMe+) was too short-lived to be detected by ns LFP.26 © 2015 American Chemical Society

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The types of nitrenium ions are determined by the photochemical precursors used in their generation.2,7,13,17,25 Certain primary arynitrenium ions (RNH+) can be produced by protonation of a singlet arylnitrene,8 a reactive intermediate after photolysis of an aryl azide in solution by extrusion of a nitrogen molecule. Some singlet arylnitrenes are strong bases, such as 4-biphenylnitrene, which has a pKb value of −2,9 so that they are able to be protonated with pretty high efficiency in protic solvents, even in aqueous solution. In aprotic solvents, most singlet arylnitrenes have the other two deactive pathways, intersystem crossing (ISC) to ground triplet nitrene and/or isomerization to a cyclic ketenimine by ring expansion.27 The Received: July 25, 2015 Revised: October 1, 2015 Published: October 26, 2015 14720

DOI: 10.1021/acs.jpcb.5b07218 J. Phys. Chem. B 2015, 119, 14720−14727

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The Journal of Physical Chemistry B

Figure 1. ns-TA spectra recorded in 30% MeCN/70% water (a) and in MeCN (b) after 266 nm excitation.

pump) were focused into an optics fiber coupled to a multichannel spectrometer with a CMOS sensor. The time delay between the pump and probe pulse was controlled by an optical delay line, and the instrument response time was ∼300 fs. Transient Resonance Raman Technique. A single beam transient Raman spectrum32 was acquired with a homemade resonance Raman spectroscopic apparatus described previously.23,33−35 The 266 nm laser pulses were obtained from the fourth harmonics of a Nd:YAG Q-switched laser, and the laser beam was focused onto a flowing liquid stream of sample using a near-collinear geometry. The Raman-scattered light was collected in a backscattering geometry and detected by a liquidnitrogen-cooled charge-coupled device detector (CCD). The Raman signal was read by an interfaced PC, and the Raman spectra were obtained from subtraction of an appropriately scaled low-power spectrum from the corresponding high-power spectrum. The transient Raman spectra were calibrated by utilizing the known wavenumbers of the acetonitrile (MeCN) Raman bands. Nanosecond Transient Absorption (ns-TA) Technique. The ns-TA measurements were performed on a LP-920 Laser flash photolysis setup (Edinburgh Instruments, UK). The 266 nm pump laser pulse was obtained from the fourth harmonic output of an Nd:YAG Q-switched laser, and the probe light was provided by a 450 W Xe arc lamp. These two light beams were focused onto a 1 cm quartz cell. The signals analyzed by a symmetrical Czerny−Turner monochromator were detected by a Hamamatsu R928 photomultiplier, and the signal was processed via an interfaced computer and analytical software. DFT Calculations. All of the density functional theory (DFT) calculations reported here made use of the Gaussian 09W program in PC.36 The complete geometry optimization and vibrational frequency computations were done analytically using the BPW9137,38 method with the cc-PVDZ39 basis set for nitrenium ions and (U)B3LYP40,41/TZVP42,43 for singlet and triplet nitrenes, respectively. Vertical excitations were obtained using time-dependent density functional theory (TDDFT).44−46 The calculated Raman vibrational frequencies and relative intensities were obtained with BPW91/cc-PVDZ. Preparation of 4-Phenoxyphenyl Azide Precursor Compounds. The sample of 4-phenoxyphenyl azide was synthesized according to the literature,47 and prepared in MeCN, 70% MeCN/30% water solvents for fs-TA experiment, 40% MeCN/60% water for transient Raman, and 30% MeCN/ 70% water for ns-TA experiments, respectively. Spectroscopic

parent singlet phenylnitrene under ambient conditions in pentane has a lifetime of ∼1 ns and ISC rate (kISC) of 3 × 106 s −1 ,28 while, in 100% formic acid, the decay of the pheneylnitrene produces the phenylnitrenium ion with a time constant of 12 ps.29 The phenylnitrenium ion has a lifetime of 110 ps in formic acid recorded by ultrafast UV−vis spectroscopy, while its lifetime can be prolonged to the order of milliseconds30 when with some para-alkoxy substitutions. As strong π-donors, the alkoxy groups such as methoxy and ethoxy are able to accelerate the ISC rate of singlet nitrene to the triplet one significantly.31 Meanwhile, it is interesting to be noted that the para-alkoxyphenylnitrenium ions are relatively unreactive toward the 2′-deoxyguanosine (kdG < 2 × 107 M−1 s−1)30 compared to other para-substituted phenylnitrenium ions, such as 4-biphenylnitrenium ion, being able to be quenched by guanosine derivatives with the diffusion limit (∼109 M−1 s−1).1,2 All of these prior results motivated this investigation on the photochemical property of 4-phenoxyphenylnitrenium ion, aiming to provide some kinetics and structural information on this kind of nitrene and nitrenium ion. In this paper, with the tool of ultrafast transient absorption (TA) spectroscopy, we first observed the formation of 4phenoxyphenylnitrenium ion from singlet nitrene. We also reported the transient resonance Raman spectrum for the phenoxyphenylnitrenium ion. After comparison with the DFT computation results for 4-phenoxyphenylnitrenium ion and the experimental results for other relevant arylnitrenium ions, some explanations for the character of 4-phenoxyphenylnitrenium ion could be provided.

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EXPERIMENTAL AND COMPUTATIONAL METHODS Femtosecond Transient Absorption (fs-TA) Technique. The fs-TA measurements were performed on the basis of a commercial femtosecond Ti/sapphire regenerative amplifier laser system and Helios Transient Absorption Spectrometer. A flow cell was used in order to prevent the accumulation of photodecomposition products. For the present experiments, the sample solution was excited by a 267 nm pump beam (the third harmonic of 800 nm, the regenerative amplifier fundamental) and probed by a white light continuum produced from a two-dimensional movable CaF2 plate pumped by 800 nm fundamental laser pulses. The pump and probe laser beam spot sizes at the sample were about 500 and 200 μm, respectively. The detection signals (with and without 267 nm 14721


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Figure 2. fs-TA spectra obtained after 267 nm excitation of 4-phenoxyphenyl azide in MeCN (a, b) and 70% MeCN/30% water (c, d). The star symbol indicates the laser subtraction artifacts.

employed to investigate the ultrafast photochemistry of 4phenoxyphenyl azide. Figure 2 displays fs-TA spectra upon 267 nm excitation recorded in MeCN and 70% MeCN/30% water mixed solvent, respectively. Examination of Figure 2 shows that, within the instrument time resolution (300 fs), in both MeCN and aqueous solutions, a sharp band at 360 nm and a wide absorption at 500 nm with very slight intensity are detected. The 500 nm absorption decays with a time constant of ∼300 fs (see the Supporting Information for details), while the 360 nm band decays to the residual absorption recorded at the longest delay time used in our experiment. The assignments of the 500 and 360 nm species are based on the literature precedent for the phenyl analogue and the kinetics of these species. The 500 nm broad transient absorption band is assigned to an excited state of 4-phenoxyphenyl azide, since it resembles the absorption of the excited phenyl azide, which is centered at 520 nm.29 The 360 nm transient absorption band is identified as the singlet 4-phenoxyphenylnitrene, as it agrees well with the observations for singlet phenylnitrene carrying the absorption maximum at 350 nm.28,48 This singlet nitrene absorption band (360 nm) has a measurable intensity at the earliest delay time in the fs-TA experiment, and no increasing was observed for the 360 nm band during the decay of the excited azide state at 500 nm. This result indicates that an upper excited azide state fragments directly to singlet 4-phenxoyphenylnitrene. Our TDDFT calculation predicts that the 267 nm laser pulse used in our spectroscopic experiments excites the azide to its Sn (n ≥ 2) state directly. Previous studies on aryl azides show that their Sn (n ≥ 2) states can fragment as fast, or faster, as they undergo internal conversion (IC).49,50 In MeCN, the decay of the singlet nitrene can be fitted well by a biexponential function with time

grade MeCN and deionized water were used in preparing the samples.

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RESULTS AND DISCUSSION

ns- and fs-TA Spectra Acquired after Photolysis of 4Phenoxyphenyl Azide in MeCN and Mixed Aqueous Solutions. Figure 1 presents the nanosecond transient absorption (ns-TA) spectra in the UV−Vis region with 266 nm photolysis of 4-phenoxyphenyl azide recorded in MeCN and 30% MeCN/70% water. The observable earliest delay time (indicated as 0 ns) spectrum acquired in aqueous solution (a) has a maximum absorption below 300 nm, two close absorption bands at 400 and 430 nm, resembles the spectrum reported for 4-phenoxyphenylnitrenium ion.30 This nitrenium ion only has a lifetime of ∼50 ns,30 and as displayed in Figure 1a, it converts completely to a new species within 200 ns. HPLC analysis showed that the decay of the nitrenium ion generates 4benzoquinone with a yield of 40%.30 In nonaqueous solution, photolysis of 4-phenoxyphenyl azide produces a species at early delay time (10 ns) with the absorption at 310, 400, and 500 nm (shown in Figure 1b) and a big band at 360 nm at post observation window. The 10 ns species was assigned to the triplet nitrene, since its spectrum resembles that for the triplet 4-methoxyphenyl nitrene.51 The 360 nm species in MeCN has a very long lifetime and is tentatively assigned to an azo compound formed by dimerization of triplet nitrene according to literature precedents.51 The ns-TA result in Figure 1 demonstrates that water acts as an effective proton donor for the generation of 4-phenoxyphenylnitrenium ion. In order to examine how fast the nitrenium ion forms, femtosecond transient absorption (fs-TA) measurement was 14722


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Figure 3. Decay of singlet nitrene recorded after photolysis of 4-phenoxyphenyl azide (λexc. = 267 nm) in MeCN (a) and 70% MeCN/30% water (b).

constants of 7 and 350 ps, respectively, as shown in Figure 3a, and the fast component corresponds to the vibrational cooling (VC) process. During the 360 nm band decays, an absorption band at 440 nm is clearly observable, which has the same decay time constant as the 360 nm band (see the Supporting Information for the 3D contour), and should also be carried by the singlet nitrene. The residual spectrum at the post delay time (Figure 2b) was assigned to the mixture of triplet nitrene and ketenimine, based on the following reasons: (1) According to the literature, the ketenimine has a very broad absorption centered at 350 nm, and the triplet has the maximum absorption in the region 300−350 nm, and the medium around 400 nm, as well as a small absorption above 500 nm.51 (2) Para substitutions have a very slight influence on the energy barrier of ring expansion, while in fs-TA experiments the decay of singlet nitrene (τ = 350 ps) is clearly much faster than the ring expansion of singlet phenylnitrene or other parasubstituted phenylnitrenes (τ ≈ 1 ns).28 (3) Para π-donating substitution will accelerate the ISC rate of corresponding arylnitrene hugely,31 such as 4-methoxyphenyl azide, in aprotic solution phase largely yielding azobenzene on photolysis.52 This observation is not inconsistent with that in the ns-TA experiment, since there could be an equilibrium between the ketenimine and the singlet nitrene.51 In aqueous solution, the decay of singlet 4-phenoxyphenylnitrene is much faster than that in MeCN, with a time constant of only 37 ps (Figure 3b), and produces the absorptions at 400 and 430 nm, in agreement with the result in ns-TA (Figure 1a). Transient Resonance Raman Spectrum for 4-Phenoxyphenylnitrenium Ion. Single-pulse transient resonance Raman has been proved to be an effective method to investigate short-lived photogenerated species that appear within the laser pulse width.32 In this experiment, the same laser pulse was used to excite the sample and induce the Raman scattering. A Raman spectrum recorded with low-power laser pulse contains little information for intermediate or photoproduct, while a Raman spectrum obtained with high-power laser pulse includes appreciable information for intermediate or photoproduct. Then, the “difference” Raman spectrum contains mainly the information for intermediate or photoproduct that generated within the laser pulse width. Figure 4 presents typical single-pulse Raman spectra for (a) MeCN, (b) low laser power, (c) high laser power with 266 nm, 10 ns pulse excitation and the transient Raman spectrum (d) obtained by subtraction of the lower laser power and solvent spectra from the high laser power spectrum. According to our fs- and ns-TA results, the 4-

Figure 4. Single-pulse Raman spectra of MeCN (a), low laser power of 4-pheoxyphenyl azide in 40% MeCN/60% water (b), high laser power of 4-pheoxyphenyl azide in 40% MeCN/60% water (c), and (d) = (c) − X(b) − Y(a), where X and Y are scaled parameters. All spectra were obtained with 266 nm 10 ns laser pulse excitation, and the star symbols indicate subtraction artifacts.

phenoxyphenylnitrenium ion can be generated in mix aqueous solution with a time constant of several tens of picoseconds, and has strong absorption at and below 300 nm, so here the transient Raman spectrum (d) is tentatively assigned to 4phenoxyphenylnitrenium ion. The experimental transient Raman spectrum (Figure 4d) has Raman shifts at 1635, 1583, 1559, 1454, 1164, 1126, 856, 620, and 463 cm−1, and exhibits a difference of ∼3.5 cm−1 on average for nine vibrational frequencies between the experimental ones and the DFT calculation predicted vibrational frequencies for singlet 4phenoxyphenylnitrenium ion, as shown in Table 1. Table 1 also displays the DFT calculated Raman activities for the singlet 4phenoxypheylnitrenium ion, as well as the vibration mode descriptions for selected vibrational frequencies (see the Supporting Information for more vibrational frequencies). Inspection of Table 1 shows that Raman shifts observed at ∼1600 cm−1 are due to the aromatic C−C stretching, which is consistent with our previous observations for other arylnitrenium ions.23,24,33,34 The C−O stretching (1401 cm−1) has a very strong Raman activity predicted by DFT calculation but not observed in the experimental spectrum, which is due to the 14723


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Table 1. Experimental Transient Raman Frequencies Observed in Single-Pulse Resonance Raman Experiment and the DFT Calculation Predicted Vibrational Frequencies for 4-Pheoxyphenylnitrenium Ion singlet 4-phenoxyphenylnitrenium ion

ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18 ν19 ν25 ν26 ν27 ν28 ν29 ν42 ν43 ν51 ν56 a

vibrational mode possible description

calc. Raman activity (a.u.)

BPW91/cc-pvdz calc. frequencies (cm−1)

expt. Raman shift (cm−1)

C−C str.a (ring 1b) C−C str. (ring 2b) C−C str. (ring 2) C−N str. C−C str. (ring 1) C−H bend (ring 2, in the ring plane) C−H bend (in the ring plane) + C−C str. (ring 1) C−H bend (in the ring plane) + N−H bend C−O str. C−H bend (ring 2, in the ring plane) C−H bend (ring 2, in the ring plane) C−H bend (ring 1, in the ring plane) + N−H bend C−H bend (ring 1, in the ring plane) + N−H bend C−H bend (ring 2, in the ring plane) + C (ring 2)−O str. C−H bend (ring 1, out of ring plane) ring def.c ring def. CCC bend (ring 2)

1099 79 1923 31 43 57 282 96 3196 149 12 330 68 3004 1 6 5 106

1630 1588 1580 1559 1541 1456 1451 1441 1401 1161 1152 1139 1138 1110 858 851 622 465

1635 1583 1559 1454

1164 1126

856 620 463

str. = stretching. bring 1 and ring 2 refer to the phenyl rings close to and far from the nitrenium nitrogen, respectively. cdef. = deformation.

Table 2. Comparison of Bond Lengths for Some Selected Arylnitrenium Ions 4-MeOPh-NH+ N1−H2 N1−C3 C3−C4 C3−C8 C4−C5 C7−C8 C5−C6 C6−C7 C6−O9 (C)

+0.0004 −0.0021 +0.0016 +0.0019 −0.0055 +0.0026 +0.0106 −0.0023 −0.0129

24

(Å)

4-EtOPh-NH+

24

(Å)

+0.0002 −0.0023 +0.0010 +0.0028 −0.0048 +0.0005 +0.0073 +0.0055 −0.0167

4-PhOPh-NH+ (Å)

4-biphenyl-NH+ (Å)

1.0404 1.3063 1.4689 1.4691 1.3696 1.3662 1.4380 1.4436 1.3228

+0.0004 +0.0020 −0.0045 −0.0039 +0.0011 +0.0031 +0.0061 +0.0056 +0.1266

frequency. This property is more evident by comparing the bond lengths of the phenoxyphenylnitrenium ion and the biphenylnitrenium ion shown in Table 2. The phenoxyphenylnitrenium ion has longer C3C4 and C3C8 bond lengths but shorter C4C5, C7C8, and N1C3 bond lengths compared to the biphenylnitrenium ion, and was observed in Raman experiment to have aromatic CC stretching at 1635 cm−1 while at 1625 cm−1 for the biphenyl one.

fact that the DFT calculation is for normal Raman while our experiment is resonance Raman spectra. Influence of the 4-Phenoxy Substituent on Structures and Reactivities of Arylnitrenium Ion and Singlet Nitrene. The para-phenoxy substitution has a slight influence on the structure of nitrenium ion compared to methoxy and ethoxy groups. As shown in Table 2 and Scheme 2, the phenoxy substitution makes the C3C4, C3C8, C7C8, and C5C6 bond lengths a little shorter and the N1C3 and C4C5 bonds longer, which suggests the phenoxyphenylnitrenium ion has comparable quinoidal character with the methoxy- and ethoxy-phenylnitrenium ions. This is also in agreement with the experimental observation in the Raman spectra if considering the instrument spectral resolution (5 cm−1) used in the Raman experiments, since the aromatic C C aromatic stretching was observed to have the vibrational frequencies at 1632, 1636, and 1635 cm−1 for the methxoy,24 ethoxy,24 and phenoxy substituted phenylnitrenium ions, respectively. All of these results are consistent with previous observations recorded by some vibrational spectroscopic experiments such as TR-IR and TR3 that the specific frequencies in the region of ∼1600 cm−1 reflect the degree of quinoidal character present in arylnitrenium ions, and the more quinoidal character imparts the higher CC stretching

Scheme 2

The methoxy, ethoxy, and phenoxy substituted phenylnitrenium ions have similar atomic charge distributions revealed by a full natural bond orbital (NBO) analysis (see the Supporting Information). The calculation results show that in all three nitrenium ions, the positive charge localizes on the phenyl ring close to the nitrenium nitrogen, and the carbon connected to the aroxyl oxygen bears the major positive charge. 14724


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constant of 37 ps, also slower than the methxoy one (5.4 ps in 60% MeCN/40% water53). The observations in fs-TA are consistent with previous results that the electron donation groups will accelerate the ISC of singlet nitrene. The single pulse transient Raman spectrum was used to characterize the phenoxyphenylnitrenium ion and with the help of DFT calculation suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character with methoxy- and ethoxyphenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.

This result is in agreement with the observation that the nitrenium ions have lifetimes of 370, 550, and 50 ns for the methoxy, ethoxy, and phenoxy ones in water respectively recorded with LFP. The shorter lifetime of phenoxyphenylnitrenium ion in water may be due to another intramolecular reaction pathway present, since only 40% quinone product30 was observed for the phenoxyphenylnitrenium ion reaction with water. The electron-donating group can accelerate the ISC rate, as a previous report53 shows that the singlet methoxyphenylnitrene undergoes ISC to the triplet nitrene with a time constant of 108 ps in MeCN. In our experiments, the singlet phenoxyphenylnitrene was observed to decay with a time constant of 346 ps. This value is close to the ISC of carbine which has a closedshell configuration, much faster than the parent phenylnitrene whose lowest singlet has an open-shell configuration and the spin orbit coupling (SOC) mechanism is forbidden. In our fsTA experiments, a transient absorption spectrum for the singlet nitrene was identified and has the maximum absorption at 360 nm bearing a shoulder at ∼440 nm, which was tentatively assigned to the open-shell rather than the closed-shell singlet on the basis of the DFT calculation predictions for the two singlets, as shown in Figure 5 (see the Supporting Information

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07218. Full description of experimental transient Raman frequencies observed in the single-pulse resonance Raman experiment and the DFT calculation predicted vibrational frequencies for 4-pheoxyphenylnitrenium ion; the early time decay curve monitored at 523 nm; fs-TA 3D contour obtained after 267 nm excitation of 4phenoxyphenyl azide in MeCN; NBO analysis for the phenoxy-, methoxy-, and ethoxy-phenylnitrenium ion; vertical transition for the phenoxyphenylnitrene triplet nitrene and open-shell and closed-shell singlet nitrenes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Grant No. 21202032 to J.X., 21205110 to Y.D., 21473163 to X.Z.), Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 1206841-Y, and Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (YR2013009), was partially supported by grants from the Research Grants Council of Hong Kong (HKU 7035/13P) and the Special Equipment Grant (SEG HKU/07), and Zhejiang Provincial Natural Science Foundation of China (Y16B030023 to J.X.).

Figure 5. Transient absorption spectrum of singlet 4-phenoxyphenylnitrene recorded at 4 ps after 267 nm excitation of 4phenoxyphenyl azide. The computed oscillator strengths ( f, right axis) of the absorption bands are depicted as vertical lines, blue for open-shell and red for closed-shell singlet. For very small f, it is presented by multiplying by 10.

for detailed calculation results). The calculation predicts the open-shell singlet has transitions at 409 nm (f = 0.06), 346 nm (f = 0.02), and 332 nm (f = 0.03). This result shows better agreement with the experimental transient spectrum than that for the closed-shell singlet does, as the calculated transitions for the latter are at 319 nm ( f = 0.02), 317 nm (f = 0.03), and 272 nm (f = 0.44, maximum).

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REFERENCES

(1) McClelland, R. A.; Gadosy, T. A.; Ren, D. Reactivities of Arylnitrenium Ions with Guanine Derivatives and Other Nucleophiles. Can. J. Chem. 1998, 76, 1327−1337. (2) McClelland, R. A.; Ahmad, A.; Dicks, A. P.; Licence, V. E. Spectroscopic Characterization of the Initial C8 Intermediate in the Reaction of the 2-Fluorenylnitrenium Ion with 2′-Deoxyguanosine. J. Am. Chem. Soc. 1999, 121, 3303−3310. (3) Kim, E. J.; Matuszek, A. M.; Yu, B.; Reynisson, J. Theoretical Investigations into the Role of Aryl Nitrenium Ions’ Stability on Their Mutagenic Potential. Aust. J. Chem. 2011, 64, 910−915. (4) Talaska, G.; Al-Juburi, A. Z. S. S.; Kadlubar, F. F. Smoking Related Carcinogen-DNA Adducts in Biopsy Samples of Human Urinary Bladder: Identification of N-(Deoxyguanosin-8-yl)-4-amino-

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CONCLUSION The singlet phenoxyphenylnitrene was the first intermediate observed in fs-TA spectra after photolysis of the azide in both MeCN and mixed aqueous solution. In MeCN, the singlet phenoxyphenylnitrene undergoes ISC and ring expansion with a time constant of 346 ps, slower than that of the methoxyphenylnitrene, 108 ps observed with ps-TR3 experiment.53 In 70% MeCN/30% water mixed aqueous solution, the singlet phenoxyphenylnitrene was protonated with a time 14725


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on November 5, 2015. Text and graphics were updated. The revised paper was reposted on November 9, 2015.

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Direct Observation of 4-Phenoxyphenylnitrenium Ion: A Transient

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