Computational Investigation of the Photochemical Reaction Path of

Article pubs.acs.org/JPCA

Computational Investigation of the Photochemical Reaction Path of Some Synthesized and Experimentally Analyzed Small-Chain Conjugated Nitrones Praveen Saini,† Mainak Banerjee,† and Anjan Chattopadhyay*,† †

Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani−K.K. Birla Goa Campus, Zuarinagar, Goa 403 726, India S Supporting Information *

ABSTRACT: This combined theoretical and experimental study has revealed the photochemistry of two small open-chain conjugated N-methylnitrone systems with phenyl substitutions at the C-terminal positions. The UV spectra of these synthesized nitrones have shown intense peaks around 330 nm while the new bands formed near 260 nm after their photoirradiation are predicted to be arising from the photoproduct oxaziridine. Photoexcitation of α-styryl Nmethylnitrone populates the first excited singlet state which relaxes by 8 kcal/mol from the vertically excited state and subsequently goes toward the lowest-energy conical intersection (CI) geometry (situated 27−30 kcal/mol below) with a terminal CNO-kink. Following the gradient difference vectors of this CI, we have located the oxaziridine structure with its characteristic geometry at roughly 14 kcal/mol above the ground state. This whole process is triggered by a transfer of electronic cloud from oxygen to the conjugated chain side. On the other hand, the photoexcitation of the nonplanar 3,3-diphenylethylene N-methylnitrone has two strong singlet−singlet absorptions with almost 5 D transition moment values. Here the initial S2−S1 relaxation is followed by oxaziridine formation through the terminally twisted CI. However, the initially photoexcited S1 state in this nitrone is found to head toward some other direction with transfer of huge amount of nonbonding electron cloud of oxygen to the π* orbital, creating a stable excited state geometry with an elongated N−O bond which gets involved in a sloped CI with the ground state.

1. INTRODUCTION During the last couple of decades, computational chemists have developed various tools and methodologies to explore electronically excited state species. Computational photochemistry studies have revealed the behavior of the photoexcited organic molecules, and these investigations have correctly predicted their photochemical reaction pathways. In a photochemical process, the breakdown of Born−Oppenheimer approximation through the nonadiabatic crossing, commonly known as conical intersection, is usually responsible to open up funnel-type radiationless decay channels which eventually form the photoproducts. It has been well-accepted now that conical intersections play a key role in numerous photochemical and photobiological1−12 events. One such example is the photochemical oxaziridine conversion reaction of nitrones which we have thoroughly studied in recent years.13−16 These studies were mostly done on the nitrone systems with alkyl substituents on nitrogen. Choice of alkyl substituents in these nitrones was primarily based on the experimentally observed fact that N-alkyl group in nitrones stabilizes the oxaziridine17,18 while electron-withdrawing groups have an opposite effect.19,20 Our complete active space selfconsistent field (CASSCF)-based computational studies have © XXXX American Chemical Society

further confirmed this experimental observation. During these investigations, we have also tested the possibility of oxaziridine formation from nitrones with electron withdrawing substituents on nitrogen;16 however, we have found a different reaction pathway operating in those systems. Our previous studies include oxaziridine formation from α-(2-naphthyl)-N-methylnitrone,14 N-alkyl retinylnitrones13 and their model compounds.15 Analysis of their photochemical paths has predicted the presence of conical intersections as a key player behind these processes. A terminally twisted CNO-kinked geometry which appears as the lowest-energy conical intersection in all these cases is found to be responsible for the formation of the photoproduct oxaziridine which contains a terminal threemembered heterocyclic ring (Figure 1a). In our present study we have chosen two different categories of nitrone systems with α-styryl group and 1,1-diphenylethylene group substitutions on the C-terminal part of the C N bond. Photoirradiations on these terminally phenyl and biphenyl-substituted nitrones (Figure 1b) can be expected to Received: November 11, 2015 Revised: December 23, 2015

A

DOI: 10.1021/acs.jpca.5b11069 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

chemical processes are proposed from the computational studies. The following sections have elaborated this combined approach which has finally revealed the fact that these two systems follow completely different photochemical paths and have contrasting dissimilarities in their oxaziridine formation routes.

2. EXPERIMENTAL SETUP General Information. The reagents were purchased from commercial sources (Sigma-Aldrich and Loba Chemi) and were used without further purification. All solvents were of research grade and obtained from local suppliers. 1H NMR and 13C NMR spectra were recorded on Bruker Avance (300 or 400 MHz, respectively) with tetramethylsilane (TMS) as internal standard. The chemical shifts are reported in parts per million (δ) units relative to the solvent peak or TMS. Mass spectra were recorded on Bruker maXis TOF LC/MS using ESI as ion source. The IR spectra of the samples were recorded on IR Affinity 1 FTIR spectrophotometer, Shimadzu with ATR probe. Absorption spectra were recorded on a JASCO V570 UV/vis/ NIR spectrophotometer at room temperature. Photoirradiations of samples were done in a UV chamber at 254 nm. Column chromatography was performed on silica gel (60−120 mesh, Merck). The reactions were monitored by thin layer chromatography (TLC) carried out on 0.25 mm silica gel on aluminum plates (60F-254) using UV light (254 or 365 nm) for visualization.

Figure 1. (a) Photoexcitation of open-chain conjugated Nmethylnitrone15 leading to oxaziridine and (b) structures of currently studied nitrone systems (nitrone A and nitrone B).

lead to some very interesting observations. During our investigations on the open-chain conjugated nitrones (Figure 1a) and comparisons of their photoexcitation properties with conjugated polyenes and iminium ions (proptonated Schiff bases), we had experienced an interesting dissimilarity related to their transition properties.15 The small conjugated openchain nitrones (with two conjugated CC bonds) without any C-terminal substitutions were found to have strongly allowed S0−S1 and almost forbidden S0−S2 transitions which resemble the transition properties of the protonated Schiff bases. On the other hand, when naphthyl substitution14 was present at the Cterminal side of the CN bond, the transition properties inclined toward the conjugated nonpolar polyenes with an almost forbidden lowest energy singlet−singlet transition and a strongly allowed S0−S2 transition. These dissimilar transition properties led to their slightly different oxaziridine formation routes. This concluded that substituting an α-aryl moiety at the C-terminal position of a nitrone can give contrasting transition properties from the conjugated chain-substituted nitrone systems. However, it is quite possible that if aryl substitution is done on the C-terminal end of the conjugated chain then the transition properties may vary from a normal open-chain and terminally unsubstituted conjugated nitrone, and subsequently, depending on these transition properties the nitrone-oxaziridine photochemical route may vary significantly from our previously studied nitrone system. Furthermore, the presence of one phenyl group and two phenyl groups at the terminal position of the small conjugated chain nitrones may not have comparable effects on this photochemical path, and this may have huge significance from the nitrone−oxaziridine photochemistry point of view, as well. The presence of one phenyl group is expected to keep the system planar while two phenyl rings are supposed to create steric repulsion and consequently the parent nitrone will be nonplanar. This difference in planarity may cause difference in their photochemical reaction path and nature of subsequent photoproducts can vary. In this work, we have synthesized these two phenylsubstituted nitrones (Figure 1b) and attempted a comprehensive photochemical study on them, which include their characterization through spectroscopic analysis and complete investigation of their photochemical reaction path by means of high-level computational techniques. The experimentally observed findings are compared with our theoretically predicted results, and finally, the overall mechanisms of these photo-

3. COMPUTATIONAL METHODS Computational studies on the low-lying electronic states of the above-mentioned nitrones are primarily based on the CASSCF level of calculations with 6-31G* basis sets using Gaussian 0921 suite of program. The CASSCF22−27 method has been employed for locating the minimum energy geometries, transition states and conical intersection points on the potential energy surfaces (PES). Ground state geometries are also optimized at the RHF and DFT (B3LYP) level with 6-311G** basis sets. For locating the transition states, the normal TS technique, based on the Berny-algorithm28 are employed. Dynamic correlation treatments on top of the CASSCFoptimized geometries have been introduced through single point studies at the CASMP2 level29−31 using Gaussian 09 and at the CASPT2 level32−34 using MOLPRO programs.35 Our CASSCF calculations include use of three different active space sizes, (4, 4), (6, 6), and (12, 12). However, similar to our earlier observations for other nitrones, here also we have found the success of the properly chosen (4, 4) active space calculations to track the nitrone-oxaziridine photoconversion path for both the studied systems. These are discussed in the subsequent sections and in the supplementary part. It should be emphasized here that use of a large active space need not be always successful to track a particular reaction path and may often lead to a completely different direction instead of leading toward the actual photochemical reaction of our interest. In both the systems, our CASSCF results have confirmed that an oxaziridine species is formed through the lowest-energy conical intersection geometry with a terminal CNO kink, and this process has been found to be perfectly captured by a properly chosen (4, 4) active space. It must be further added in this context that there is no single correct active space in a molecule;36 for different processes of the same system, the active space choice can be different depending on the process B


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Figure 2. Photoirradiation study of (a) nitrone A and (b) nitrone B.

Figure 3. FT-IR spectra of (a) nitrone A and (b) nitrone B at room temperature and their change caused by photoirradiation (zoomed).

being carried out. For the oxaziridine conversion process from these nitrones, this smaller active space is found to be successful and correctly captures this photochemical route; however, for some other processes of these phenyl-substituted conjugated nitrones, this same active space may not be correct.

A separate level of study has been carried out at the graphical unitary group approach (GUGA) CI37−40 level for some other studies, such as the radiative transition properties41,42 for the low-lying vertical transitions at the ground state equilibrium geometries of the nitrones through the GAMESS43 program C


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Table 1. Structural Parameters of Optimized Ground and Excited States of Nitrone A and Nitrone B at Various Levels of Calculation system

geometry

methoda

RC3−C2

RC2−C1

RC1−N

RN−O

RC−O

RN−C

DC−C−C−N

DC−C−N−C

DC−C−N−O

nitrone A

GSA

I II III IV V III IV V I II III IV III IV

1.329 1.353 1.329 1.328 1.329 1.463 1.429 1.430 1.335 1.362 1.334 1.335 1.453 1.452

1.451 1.427 1.454 1.454 1.454 1.441 1.405 1.402 1.453 1.427 1.457 1.456 1.332 1.332

1.277 1.323 1.302 1.301 1.302 1.395 1.401 1.388 1.278 1.324 1.304 1.303 1.406 1.405

1.265 1.269 1.267 1.266 1.266 1.272 1.266 1.309 1.265 1.269 1.265 1.265 1.393 1.393

2.253 2.300 2.274 2.272 2.287 2.282 2.295 2.308 2.256 2.303 2.276 2.276 2.295 2.295

1.459 1.477 1.459 1.459 1.459 1.449 1.448 1.447 1.459 1.477 1.460 1.460 1.458 1.457

180.0 180.0 179.9 180.0 180.0 170.8 177.1 176.7 176.8 177.8 176.8 176.9 175.6 175.6

180.0 180.0 179.9 180.0 180.0 165.2 165.9 163.2 178.1 177.1 177.9 178.0 129.1 129.1

0.0 0.0 0.0 0.0 0.0 10.9 5.9 9.0 −1.8 −2.5 −2.0 −2.0 10.3 10.2

ESA

nitrone B

GSB

ESB a

Where I = RHF/6-311G*, II = B3LYP/6-311G*, III = CASSCF (4,4)/6-31G*, IV = CASSCF(6,6)/6-31G*, and V = CASSCF(12,12)/6-31G*

and 1541 cm−1 for nitrone B; they have almost disappeared after the photoirradiation. Another important characteristic peak for N−O bonds observed at 1138 and 1122 cm−1 for nitrone A and nitrone B, respectively, were also found missing on photoirradiation. In contrast, peaks at 1228 and 1226 cm−1 have appeared for the photoirradiated products of these two nitrones in the IR spectrum, and these are characteristic of the ring vibration (cyclic C−N−O moiety) of oxaziridine. These changes in the IR spectra have given clear indication that the main product in both the cases is an oxaziridine species. Similar line of study for confirming oxaziridine formation from IR analysis can be also found from experiments done by other groups on various nitrone systems.51 (b). Computational Part. (i). Ground and Excited Singlet States. The ground state geometries (S0) of the nitrone systems are optimized at different level of calculations (Table 1) which include the restricted Hartree−Fock (HF) method, density functional theory (DFT), and the CASSCF level of theory. The HF and DFT calculations are done using 6-311G* basis sets while the CASSCF studies are attempted with three different active space sizes using 6-31G* basis sets. No significant differences are noticed in the geometries at these different levels of calculations, except slight deviations in the case of DFT calculated values. The two phenyl rings in nitrone B are expectedly nonplanar with respect to the remaining portions of the molecule. This is to avoid strong interactions between the ortho-hydrogens on the two phenyl moieties. Their planar arrangement would have brought the two hydrogens at only 0.62 Å distance creating a huge strain while in the actual optimized ground state these are 3.48 Å apart with the rings becoming nonplanar. This geometry also releases the possible repulsion at the other two orthohydrogens of the two rings, one with the hydrogen on αcarbon and the other on the carbon atom of the CN bond. Excited states (S1) of these nitrones are optimized at the CASSCF level. An interesting feature has been noticed for the optimized geometry of the first excited singlet state (ESB) of nitrone B; the C1−C2 bond is almost 0.12 Å shorter (Table 1) than that in the ground state while the N−O bond length is larger than its ground state value by almost similar margin. The alternation of the single and double bonds in the C−C−C-N region and the stretching of the N−O bond length are remarkably different from nitrone A (discussed later on) and all

package. Electrostatic potential-based atomic charges are calculated for the ground and excited state geometries using the Merz−Kollman (MK) scheme44,45 in Gaussian. The time dependent density functional theory46,47 has been employed for the study of the absorption peak positions of the photoproduct formed on photoirradiation of nitrones. Visualization softwares like Chemcraft,48 Gaussview, and Chemissian are used to analyze the output files throughout the work. The various levels of quantum mechanical studies are expected to give significant amount of information as the outcome of this work, which is likely to establish a certain mechanism for the photoexcitation process of the studied nitrone systems.

4. RESULTS AND DISCUSSIONS (a). Experimental Part. (i). Photoirradiation of Synthesized Nitrones. The ultraviolet absorption spectra of the synthesized nitrones were recorded in methanol by irradiation of 254 nm light at room temperature. The intense absorption peaks for both the nitrones are found to arise near 330 nm while the weaker peaks are found around 240 nm (Figure 2). The nature of the change in their spectra on continued photoirradiation indicates the formation of the photoproduct which has been identified as oxaziridine and discussed later on. However, the decay of the longer wavelength peak and the formation of the photoproduct absorption band are not quite similar in the two systems. In nitrone A, the 326 nm peak decays much faster than the 334 nm peak of nitrone B. On prolonged irradiation (25 min for nitrone A, whereas it is 90 min for nitrone B), the longer wavelength bands have totally disappeared, indicating complete conversion to the photoproducts. In both the cases, the newly formed bands on the blue side have characteristic of oxaziridine. A comparative analysis of some sample oxaziridine absorption peaks studied on photoirradiation of some different types of nitrones by other groups49,50 with ours has indicated that the produced photoproducts in our present study are probably oxaziridines. However, this has been further confirmed by the FT-IR and computational studies. (ii). FT-IR Spectroscopic Studies. Infrared spectroscopic studies were conducted to confirm the formation of oxaziridine from the photoirradiation of the nitrones. As shown in Figure 3, the characteristic stretching bands of CN in the starting materials (nitrone) are observed at 1561 cm−1 for nitrone A D


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Table 2. Absolute and Relative Energy Values at CASSCF Level and with Dynamic Correlation Corrections on the Optimized Structures of Ground and Excited States with dynamic correlation correction (CASMP2 and CASPT2)

CASSCF/6-31G* molecular states

E (Hartree)

GSA

−514.3267

FCA

−514.1893

88.03

ESA

−514.2004

79.25

GSB

−743.8729

FCB

−743.7360

85.90

ESB

−743.7764

60.55

nitrone B

a

ΔE (kcal/mol)

system nitrone A

0

0

E (Hartree)

ΔE (kcal/mol)

−515.9523a −515.8886b −515.7806a −515.7396b −515.7888a −515.7501b −746.2934a −746.1911b −746.1316a −746.0406b −746.1781a −746.0642b

0a 0b 107.74a 93.49b 102.59a 86.91b 0a 0b 101.53a 94.41b 72.35a 79.60b

CASMP2. bCASPT2.

though in the S2 state, the dominance of the HOMO → LUMO excitation is comparatively lower. Interestingly, at the optimized excited state geometry (ESB) of S1, this state is still found to be dominated by these three above-mentioned configurations in almost similar manner in this nitrone. In contrast, in nitrone A, the dominance of HOMO2 → LUMO2 increases significantly from the vertically excited geometry (FCA) to the optimized S1 geometry (ESA) and becomes almost equally contributing as the HOMO → LUMO configuration. In order to investigate further differences in these two excited states, we have analyzed the MK charges of the ground state and excited state geometries (Table 4) of these two nitrones.

other nitrones studied by us previously.13−15 The energy of this optimized excited state is found to be roughly 60 kcal/mol above the ground state minimum energy geometry (GSB) and 25 kcal/mol below the vertically excited state point (FCB) of this state at the CASSCF level (Table 2). On the other hand, for nitrone A, the optimized excited singlet state (ESA) is situated around 80 kcal/mol above the ground state optimized geometry (GSA) and the energy difference with the vertical excitation point (FCA) is 8 kcal/mol; this is quite similar to our earlier studied nitrone systems. It must be noted that the energy values discussed in this section are corresponding to the (4, 4) active space calculations while the (6, 6) calculation values are shown in the Supporting Information (Table S1). This is primarily due to the fact that only the smaller (4, 4) active space calculations have correctly tracked the nitrone− oxaziridine path which is our primary objective in this work. All energy values are corrected using the dynamic correlation methods of CASMP2 (in Gaussian 09) and CASPT2 level (in Molpro) of theories. Analysis of the radiative transition properties of S0-S1 and S0-S2 transitions of these two nitrones at their optimized ground state geometries has revealed some interesting facts. It has been noticed that in nitrone B both S0− S1 and S0−S2 vertical transitions are almost equally strong (∼5 D) with high oscillator strength values (Table 3). On the other hand, in nitrone A, the S0−S1 transition is significantly stronger than the S0−S2 transition at the ground state geometry, where the latter has negligible transition moment value. Both vertically excited S1 and S2 states of nitrone B are found to have HOMO → LUMO, HOMO2 → LUMO2 and HOMO − 4 → LUMO as their leading configurations with dominance of the first one;

Table 4. Atomic Charges Determined Using Merz−Kollmen (MK) Scheme at Various Important Geometries on PES system nitrone A

nitrone B

Table 3. Radiative transition properties of nitrone A and nitrone B at their optimized ground state geometries

system

transitions

transition moment (Debye)

nitrone A

S0−S1 S0−S2 S1−S2 S0−S1 S0−S2 S1−S2

6.92 0.68 0.20 5.22 4.47 1.77

nitrone B

oscillator strength

Einstein coefficient A (1/s)

1.9320 0.0202 0.0001 0.7980 0.6207 0.0055

9.47(+09) 1.10(+05) 1.64(+03) 2.05(+09) 1.80(+09) 5.14(+04)

point on PES

C1

N

O

C (of methyl)

GSA ESA TSA CIA OXA GSB ESB SP TSB1 TSB2 CIB1 CIB2 OXB

−0.1609 −0.0997 −0.2470 0.0555 0.4106 −0.1349 −0.1355 −0.3013 0.1258 −0.2496 0.2160 0.0381 0.4329

0.4675 0.1634 0.3155 −0.0741 −0.3094 0.4705 −0.2229 0.3552 −0.2710 0.3234 −0.2712 −0.1823 −0.2829

−0.5961 −0.3064 −0.3577 −0.1578 −0.2995 −0.5978 −0.0899 −0.3624 −0.1297 −0.3569 −0.1416 −0.0748 −0.3127

−0.3958 −0.3070 −0.2549 −0.3195 −0.0295 −0.4193 −0.1835 −0.2736 −0.1510 −0.2520 −0.2191 −0.3693 −0.1623

Our earlier proposed electron transfer theory which was predicted to happen from oxygen to nitrogen in the beginning of the photoexcitation of conjugated open-chain nitrones is found to hold good in nitrone A. In contrast, in the biphenyl system, a huge shift of oxygen lone pair cloud can be observed from the atomic charge analysis which consequently creates a lone pair cloud or negative charge on nitrogen in the optimized excited state geometry of this nitrone. This feature clearly explains the structural differences observed in the optimized excited state geometry of the biphenyl system. In other nitrones, a smaller amount of electron cloud transfer from E


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Table 5. Absolute and Relative Energy Values at the CASSCF Level and with Dynamic Correlation Corrections on the Optimized Transition States, Conical Intersection Points, and Oxaziridine Structures with dynamic correlation correction (CASMP2 and CASPT2)

CASSCF/6-31G* system

molecular states

E (Hartree)

ΔE (kcal/mol)

E (Hartree)

ΔE (kcal/mol)

nitrone A

TSA

−514.1991

80.15

CIA

−514.2431

52.45

OXA

−514.3030

14.87

SP

−743.7241

93.37

TSB1

−743.7702

64.44

TSB2

−743.7121

100.90

CIB1

−743.7865

54.21

CIB2

−743.7402

83.27

OXB

−743.8586

8.97

−515.7792a −515.7455b −515.8257a −515.7974b −515.9367a −515.8673b −746.1111a −746.0467b −746.1487a −746.0579b −746.0932a −746.0205b −746.1707a −746.0724b −746.1375a −746.0508b −746.2665a −746.1656b

108.62a 89.79b 79.44a 57.22b 9.78a 13.36b 114.39a 90.61b 90.80a 83.58b 125.62a 107.05b 76.99a 74.46b 97.78a 88.04b 16.88a 16.00b

nitrone B

a

CASMP2. bCASPT2.

Figure 4. Optimized lowest-energy conical intersection (S0/S1) geometries of (a) nitrone A and (b) nitrone B with their gradient difference (GD) and derivative coupling (DC) vectors.

oxygen orbital to an antibonding π* orbital decreases the N−O bond order slightly which results in the marginal increase of this bond length in the excited state, whereas in case of nitrone B, a huge transfer of electron cloud from oxygen results in a

significant increase of this bond length. The lone pair cloud on nitrogen converts it into a pyramidal or sp3 hybridized center and the planarity of the C−C−N−O part of the excited state geometry is lost. This can be also understood from the huge F


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Figure 5. Optimized oxaziridine geometries of (a) nitrone A and (b) nitrone B with structural parameters.

the other hand, for nitrone A, the lowest-energy conical intersection point with the terminal twist (CIA) is obtained around 23−29 kcal/mol below the optimized excited state geometry. Following its gradient difference vectors (Figure 4a), the optimized structure of oxaziridine (OXA) is obtained below (∼40 kcal/mol) the intersection point. Structurally the oxaziridines obtained from both the nitrones (Figure 5a and 5b) are quite similar to a slight difference of 10° in their C−C− C−N dihedral angle and their geometrical parameters exactly resemble the experimentally known52 oxaziridine structure (RC−O= 1.40 Å, RN−O= 1.50 Å, ∠OCN = 63.7°, ∠ONC = 56.8°). Our attempt to obtain conical intersections with similar possibility of oxaziridine formation was not successful using higher active space (Figure S2) calculations. For nitrone A, the reaction path leading toward the OXA geometry seems to be quite similar to our previously studied nitrone systems; the single electron excitation from oxygen kick start the process leading to the optimized excited state geometry (and possibly to TSA thereafter) which passes through the lowest energy conical intersection point (CIA) with a CNO-kink and this eventually forms oxaziridine as the photoproduct. However, the photochemical path followed by nitrone B is not as simple as this. At the saddle point (SP) on its first excited singlet state surface, which is obtained only at the (4, 4) level of calculation, the gap between the S1 and S2 states has considerably gone down in comparison to the S1−S2 energy difference at the vertically excited geometry and this indicate that this saddle point might be a result of an avoided crossing between the second and the third singlet roots. The vertically excited S2 state probably brings this saddle point into the reaction path. This geometry is found to have a good contribution from the HOMO2 → LUMO2 excitation. Atomic charge analysis (Table 4) reveals that this geometry maintains our earlier discussed single electron transfer property, as seen in other nitrones. The ESP-derived charges on the carbon, nitrogen and oxygen suggest a continuity of the SP−TSB1− CIB1 route during the passage toward oxaziridine (OXB) where the negative charge on nitrogen increases along this path. Therefore, there remains enough reason to believe that in nitrone B this saddle point lies in the path of the nitroneoxaziridine route following the S2−S1 relaxation. A separate conical intersection (CIB2) between the S0 and S1 states has been identified (Figure S3) above the ESB geometry of nitrone B. Unlike the peaked topography of the CIB1, the

deviation of the C−C−N−C dihedral angle from 180° which supports the formation of the pyramidal nitrogen. This lack of planarity of this part may well be the reason for the shorter central C1−C2 bond; as soon as the C−N π bond breaks, the adjacent C−C bond becomes a double bond; however, as the negatively charged nitrogen now favors the tetrahedral structure (sp3 hybridized nitrogen) it twists instantly and any further delocalization in the C−C−C-N moiety gets disrupted. It must be added in this section that we have identified some important saddle points on the S1 surfaces of both the nitrones (Figure S1). In nitrone A, a transition state (TSA) is found to be situated at only 1 kcal/mol above the relaxed excited state geometry (and 7 kcal/mol below the vertically excited geometry) with an imaginary frequency of 156i cm−1. On the other hand, we have identified a saddle point (SP) on the first excited state surface of nitrone B which lies around 6−7 kcal/ mol (Table 5) above the vertically excited geometry of this state at the CASSCF level. However, this geometry is found to be situated slightly below the Franck−Condon excited state at the CASPT2 level. It seems that this saddle point leads toward a transition state (TSB1) with an imaginary frequency of 51i cm−1 showing twist in the CNO region (quite similar to the twist shown by 31i cm−1 frequency of SP). Our results have indicated that these geometries play a key role during the passage toward the oxaziridine formation. These are discussed in the following section in detail. Interestingly, this important saddle point found on the first excited state surface of nitrone B has been identified only at the (4, 4) level of calculations. Another transition state (TSB2) in nitrone B has been detected on the first excited state surface which is found to create a very high barrier and is situated around 25 kcal/mol above the vertically excited point at the CASSCF level. (ii). Conical Intersections and Oxaziridines. The lowest energy conical intersection (S0/S1) geometries (Figure 4, parts a and b) for both the systems are found to have a terminally twisted CNO moiety, quite similar to our earlier studied nitrone systems. This intersection point in nitrone B (CIB1) has been detected at roughly 5 kcal/mol below the ESB geometry. The gradient difference vectors of this species reveal the fact that oxaziridine (Figure 5, parts a and b) formation is quite possible from this intersection point and we have obtained the oxaziridine geometry (OXB) by following the vector directions below (47 kcal/mol at CASSCF and 52−58 kcal/mol using dynamic correlation) the intersection geometry (Table 5). On G


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Figure 6. Comparison of theoretically obtained UV−visible spectrum with the experimental peaks of (a) OXA and (b) OXB.

Figure 7. Schematic representation of the entire photoconversion process of (a) nitrone A and (b) nitrone B. Energy values are shown in parentheses (in kcal/mol) with respect to the optimized ground state geometries of the respective nitrones.

sloped nature of this intersection indicates that the decay through this point to the ground state will be very slow. In fact, the substantial value of ΔE (CIB2−ESB) may well indicate that the molecule along this path will equilibrate in the excited state. It has already been discussed in section 4(a)(i) about the

difference in the decay pattern of the 330 nm absorption peaks of nitrone A and nitrone B with time. The latter was found to decay comparatively slowly than that of the styryl nitrone which may have a direct correlation with the sloped topography of CIB2, which is reached after relaxation to ESB following the H


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decay of the near 330 nm peak have supported these mechanistic steps where no alternate relaxation route like nitrone B exists which can have a slower decay rate through a sloped CI.

initial photoexcitation to FCB. The oxaziridine formation route is through the peaked CIB1 which this takes place through the slightly less populated S2 state and requires an initial relaxation to S1. (iii). Oxaziridine Absorption Peaks. The absorption peak positions (Figure 6a and 6b) of the oxaziridines are analyzed at the TDDFT/B3LYP level with 6-311G** basis sets. For nitrone A, the major experimentally observed peak which rises along with the decay of the 330 nm peak is observed near 263 nm while the peak position from the theoretical calculations of oxaziridine (OXA) is found at 252 nm. In the biphenyl system, the predicted position of the OXB peak is 263 nm which is very close to the value of the experimentally detected 261 nm peak, which is found to develop during the course of photoirradiation and during the decay of the nitrone peak. These TDDFT results have shown that for both the systems these peaks (OXA and OXB) are arising due to strong S0−S2 transitions with high oscillator strength (∼0.20) values. Primarily the HOMO → LUMO excitations are responsible for these strong UV transitions. A second peak observed near 220 nm has been also produced at the TDDFT level with slight deviations from the experimental positions. This close matching of the theoretically derived peak positions of oxaziridines with the experimentally observed ones clearly establishes further that the photoproduct formed on photoirradiation of both the nitrones are oxaziridines. (iv). Summary of the Overall Photochemical Path. The whole photochemical conversion process operating in the two systems has been summarized in Figure 7, parts a and b. In nitrone B, the nonplanar ground state on photoexcitation populates both S1 and S2 states simultaneously, which is evidenced from the high transition moment values of S0-S1 and S0-S2 transitions. The S1 state then relaxes to a stable excited state geometry which involves a considerable amount of charge transfer from the oxygen to the nitrogen (or conjugated part) with an elongation of the N−O bond. This is uncharacteristic of the nitrone excited states investigated by us previously where oxaziridines were found to form from the relaxed excited states which involve less amount of electronic charge flow from oxygen. Both (4,4) and (6,6) calculations have produced the stable excited state geometry of nitrone B. Above this geometry lies the sloped S0/S1 conical intersection, and this can be related to the slow decay of the 330 nm peak of this nitrone. However, the oxaziridine formation does not lie along this path in this system and it arises through a different trajectory; the wellpopulated S2 state decays to the S1 state and proceed toward the S0/S1 lowest-energy conical intersection point which eventually forms oxaziridine. This path seems to involve a saddle point (probably due to an avoided crossing with S2) on the S1 surface which has been tracked only at the (4,4) level of calculation. Atomic charge analysis has shown that this path is quite consistent with the single electron transfer mechanism which triggers the oxaziridine formation process, as described earlier. On the other hand, the nitrone A photochemistry is quite simple and similar to our earlier studied conjugated chain nitrone systems. The initial photoexcitation takes the system to a relaxed excited state geometry which involves single electron flow from oxygen. Here the vertical excitation happens to the S1 state only and S2 state has almost no population. The relaxed excited state then goes toward the lowest-energy CNO-kinked CI after crossing a small barrier of 1 kcal/mol which finally leads toward the oxaziridine geometry. The comparatively faster

5. CONCLUSION The computational results based on the high-level quantum mechanical treatments have justified our experimentally observed photochemical features of the two studied nitrone systems. Their proposed mechanisms have explored their contrasting behavior on photoexcitation. The final photoproduct in both the cases has been identified as oxaziridine from the matching of the gradually developed UV peaks near 260 nm with the theoretically predicted TDDFT peak positions of this terminal heterocyclic species; analysis of the IR peaks have also justified this species as the possible photoproduct. The photoirradiation of 3,3-diphenylethylene N-methyl nitrone (nitrone B) is found to involve two well-populated excited states, which lead to two different reaction paths. The lowestexcited singlet state goes toward a stable optimized excited state with a sloped conical intersection at a substantial height from its minimum energy geometry. This indicates the possibility that the molecule might equilibrate at this excited state and consequently may not decay very quickly from here. The other route involves a relaxation of the second excited singlet to the first one which eventually forms the photoproduct oxaziridine through the lowest-energy conical intersection geometry. On the other hand, the nitrone-oxaziridine formation route of the styryl nitrone (nitrone A) is quite similar to our previously investigated open-chain conjugated nitrones with no substitution at the terminal position. Here a single nonradiative decay route is followed by the first excited singlet state which passes through a low-barrier transition state followed by the lowest-energy conical intersection with a CNO-kink and goes toward the photoproduct. This finding can be correlated with the faster decay of the absorption band of this nitrone. In both the cases, following the vectors corresponding to the gradient difference, we have obtained oxaziridine as the photoproduct. Finally, it can be concluded that the two-way decay channel of the conjugated nonplanar biphenyl nitrone with two different conical intersection topographies has made the photochemical behavior of this system significantly different from that of the planar conjugated phenyl system which is almost parallel to the features of the other planar nitrone systems studied by us previously.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11069. General procedure for the synthesis of nitrones, saddle points on the excited state surfaces, calculations with the (6,6) active space, sloped conical intersection of nitrone B, and Cartesian coordinates of important geometries (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(A.C.) E-mail: [email protected]; anjan_ [email protected]. Fax: +91 832 2557033. Notes

The authors declare no competing financial interest. I


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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the Council of Scientific and Industrial Research (CSIR), Government of India, under the Scheme No. 01(2681)/12/ EMR-II, for the present work.

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Computational Investigation of the Photochemical Reaction Path of

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