Investigation on the Unexplored Photochemistry of 5,5-Dimethyl-1

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

An Investigation on the Unexplored Photochemistry of 5,5-Dimethyl-1-Pyrroline 1-Oxide (DMPO) Sindhuja Sen, Yasaswini Oruganti, and Anjan Chattopadhyay J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b08181 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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The Journal of Physical Chemistry

Figure. 1. Ground state optimized geometries of DMPO and 2-Me-DMPO at the CASSCF level 254x190mm (300 x 300 DPI)

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Figure. 2. The low-lying conical intersection geometries (a) CIA1 and (b) CIA2 of DMPO with their respective GD & DC vectors 254x190mm (300 x 300 DPI)


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Figure. 3. The two low-lying conical intersection geometries (a) CIB1and (b) CIB2 of 2-Me-DMPO with their respective GD & DC vectors 254x190mm (300 x 300 DPI)



Figure. 4. The optimized oxaziridine geometries of (a) DMPO and (b) 2-Me-DMPO 254x190mm (300 x 300 DPI)


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Figure. 5. The optimized transition state geometries (a) TSA of DMPO and (b) TSB of 2-Me-DMPO 254x190mm (300 x 300 DPI)



Figure. 6. (a) Optimized geometries of transition state TSA-OX and the corresponding product lactam PA of DMPO (b) Optimized transition state geometry TSB-OX of 2-Me-DMPO 254x190mm (300 x 300 DPI)


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Figure. 7. Overall reaction pathway of DMPO – Lactam conversion through oxaziridine 254x190mm (300 x 300 DPI)



An Investigation on the Unexplored Photochemistry of 5,5-Dimethyl-1-Pyrroline 1-Oxide (DMPO) Sindhuja Sen,† Yasaswini Oruganti,† Anjan Chattopadhyay*,† †

Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani –K.K. Birla

Goa Campus, Goa, 403 726, India

ABSTRACT A comparative study of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) and its 2-methyl-substituted analogue (2-Me-DMPO) has revealed their contrasting reaction pathways of oxaziridine and lactam (pyrrolidone) formation. The initial photo-excitation populates the second excited singlet states (S2) in both the systems with S0-S2 transition moment value of 3 Debye (oscillator strength 0.4); this subsequently undergoes (S0/S1) conical intersection through a structure having a CNOkink and situated around 35-40 kcal / mol below the vertically excited geometry of the first excited singlet state (S1). This conical intersection is found to be responsible for the formation of the oxaziridine photoproduct in these systems. In DMPO, this oxaziridine eventually forms the corresponding lactam compound through a [1,2] H shift after overcoming a barrier of 35 kcal / mol and following the imaginary frequency of 1517i cm-1. The reverse thermal process of parent nitrone formation proceeds through a transition state situated at 60 kcal / mol above the oxaziridine geometry and the corresponding imaginary frequency is 1514i cm-1. On the other

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hand, in 2-Me-DMPO, the oxaziridine formed is more stable and lactam formation does not happen from it in a similar manner.

1. Introduction The N-oxide systems are known to be important compounds to the organic chemists, over the last several decades. The N-oxide of imines, popularly known as nitrone is one such important category of compounds used for various purposes. Nitrones1 are known to be versatile intermediates in organic synthesis and have drawn special attention in recent years due to their pharmacological and various other applications.2 Nitrones are usually quite unstable and their photo-excitation normally results in transient oxaziridine species which eventually gives amide and other products. The photo-irradiation of nitrones has been the subject of interest over the last five decades. Analysis of the nitrone–oxaziridine conversion and cis–trans isomerization of nitrones was first attempted almost fifty years back. The study of the photo-irradiation products of several nitrones reported by Splitter et al.3,4 is still one of the major experimental work done on these systems. Experimental studies have revealed that the N-alkyl-α-arylnitrones give more stable oxaziridines on photo-irradiation, in comparison to the N,α-diarylnitrones.5 The stability of the oxaziridine ring and the nature of its cleavage were found to depend primarily on the types of substituents present on nitrogen and carbon. The stable and experimentally isolated oxaziridines 3-6

were found to contain alkyl groups on either or both nitrogen and carbon atoms; in contrast,

the presence of aryl groups on both these atoms decreases their stabilities significantly, and their existence can be seen only in solution. In recent times, the photochemistry of open-chain acyclic N-alkyl nitrones has been extensively studied by Chattopadhyay and co-workers. This includes the chemopreventive retinyl nitrone (and their model compounds), fluorescent naphthyl nitrone, α-styryl nitrone and some other open-chain nitrones,7-11 as well. Their comprehensive theoretical



investigations have put forward the explanations of several unanswered questions related to these acyclic systems. The formation of the photoproduct, oxaziridine, was found to involve the singlet excited state which passes through the lowest-energy conical intersection12-16 in the reaction path. This terminally twisted intersection geometry was found to produce the oxaziridine by following the gradient difference vectors. Various other conical intersection channels were also identified in the photochemical path of these acyclic nitrone systems, though all of them were higher in energy than the one leading towards oxaziridine. These studies on the acyclic N-oxide systems have triggered our interest in the photochemistry of various cyclic N-oxide systems which is still yet to be explored. First studies on the photo-isomerization of cyclic nitrones were reported almost six decades back by Todd and co-workers.17 The photo-conversion of 5,5-dimethyl-1-pyrroline-1-oxide, commonly known as DMPO (Figure 1a) , was reported to give the bicyclic oxaziridine. Under thermal condition, the latter was found to produce the 5,5-dimethyl-2-pyrrolidone (a lactam) instead of forming the parent nitrone. It must be mentioned here that DMPO is a well-known spin-trapping agent18-20 which is used to detect the free radicals. Subsequent experiments carried out by Kaminsky and Lamchen 21-23 have shown that methyl substitution on the 2-position of this nitrone can also produce the corresponding oxaziridine. The 2,5,5-trimethyl-1-pyrroline-1-oxide (Figure 1b) was found to form 2,5,5-trimethyl-6-oxa-1-azabicyclo hexane under photoirradiation.

22,23

This product had shown good stability under thermal condition and the

corresponding lactam or pyrrolidone was not found to form on heating. However, at very high temperature, the latter compound was obtained. They have studied substitutions on other positions of the 5-membered ring, as well. However, the oxaziridine produced from 2-MeDMPO was found to be highest in stability. There exists studies on several other kinds of cyclic


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nitrones, as well; one such example is the work of Sternbach and co-workers24 on the photoirradiation of 7-chloro-2-methylamino-5-phenyl-3H-1,4-C benzodiazepine 4-oxide which gives 7- Chloro-4, 5- epoxy-2- methylamino-5- phenyl-3H-l, 4- benzodiazepine. In our current work, we have targeted the photochemistry of the 5,5-dimethyl-1-pyrroline 1oxide (DMPO) and 2,5,5-trimethyl-1-pyrroline 1-oxide (2-Me-DMPO) systems. The reason for DMPO producing slightly less stable oxaziridine and leading to the cyclic amide type product (5membered lactam) thermally, in contrast to the nature of 2-Me-DMPO, which forms more stable oxaziridine, will be thoroughly analyzed in the work. Our computational investigation will mostly include Complete Active Space Self Consistent Field (CASSCF) based studies on the ground and excited states of these systems. Presence of non-radiative decay channels through surface crossings or avoided crossings between the low-lying states will be investigated. One of the major objectives of this work is to highlight the similarity or the difference in the photochemical behavior of our previously studied acyclic7-11 and currently studied cyclic nitrone systems. Overall, this work is going to be the first comprehensive approach towards revealing the photochemistry of the so far unexplored cyclic nitrone systems.

2. Computational Details The quantum mechanical studies presented in this work are mostly done using the Gaussian 09 suite of program.25 Calculations related to the optimizations of the equilibrium geometries, conical intersections and transition states have been carried out using the CASSCF26-31 method with the 6-31G* basis set. Experimental results indicate that the formation of oxaziridine involves the CNO part of the system while the other portions of the cyclic nitrone remain unchanged. This indicates that the C=N pi electrons and the electron pair on the oxygen atom are



likely to be primarily involved in the oxaziridine formation process. Our chosen (4,4) active space for these calculations is mostly centred on this CNO moiety which has accurately captured our desired reaction path. Some important orbitals of this active space are shown in Fig. S1. It must be mentioned here that some prior knowledge of the system under photochemical investigation always help to choose an accurate minimal active space by eliminating the less important orbitals from the calculations for that particular reaction. Dynamic correlation effect (CASPT2) has been included32-34 through single point calculations on top of the CASSCF/631G* optimized geometries in the Molpro program.35 The transition states have been located using the normal TS technique based on the Berny-algorithm.36 Additionally, GUGA (Graphical Unitary Group Approach)-based configuration interaction singles and doubles (CISD) technique has been used for some important calculations through the GAMESS37 suite of programs. In this CISD calculations, the RHF/6-31G* method has been used in the first step for the self-consistent molecular orbital (SCFMO) calculations of the ground states, and these MOs are subsequently used for the configuration interaction steps. Radiative transition38-39 calculations based on this GUGA CISD code40-43 have been carried out between the two configuration interaction wavefunctions at the ground state equilibrium geometries. Electrostatic potential-based atomic charges are calculated at several geometries using the Merz-Kollman44,45 scheme in Gaussian 09 program. For visualization of the output files, ChemCraft software has been employed throughout this work. 3. Results & Discussion: a. Ground & excited states The ground state geometries (GSA and GSB) of the two systems (Figure 1) are optimized at the CASSCF level with 6-31G* basis. The vertical excitation energy (VEE) to the first excited


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(a)

(b)

Figure. 1. Ground state optimized geometries of (a)DMPO and (b)2-Me-DMPO at the CASSCF level singlet state of DMPO is found to be 115 kcal/mol (Table 1) while it is slightly lower (101 kcal/mol) in case of Me-DMPO. On the other hand, the inclusion of dynamic correlation treatment gives a slightly higher value of VEE in the latter system (113 kcal/mol) in comparison to the other one (109 kcal/mol). The transition property analysis shows that at the Franck Condon geometry, the first excited singlet state is dominated by the HOMO-1

LUMO excitation and

some substantial contribution from the doubly excited configuration (HOMO2

LUMO2) is

there, as well. The second excited singlet state at this geometry is almost exclusively dominated by the HOMO

LUMO excitation with small contribution from the HOMO2

LUMO2

configuration. Transition moment study reveals that the S0-S1 transition is almost forbidden (Table 2) with negligible oscillator strength value while the S0-S2 is a strongly allowed one. This is quite similar to our previously studied N-methyl naphthyl nitrone systems10 and opposite to the long-chain conjugated nitrones,7 where in the latter, the S0-S1 is the stronger transition. Interestingly, we were unable to locate any relaxed excited state geometries of S1 and S2 states at the CASSCF level. However, optimized excited state geometry of the second excited singlet state (S2) for 2-Me-DMPO was obtained at the TDDFT (Time Dependent Density Functional 6 ACS Paragon Plus Environment


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Theory) level with B3LYP functional using 6-31G* basis. This geometry has a highly stretched N-O bond which is slightly twisted downward with a very short adjacent N-C bond. The S2 and S1 state energies are very close at this geometry and with further stretching of the N-O bond they are found to become degenerate. Similar results have been obtained for the DMPO system, as well. We consider this as the possible channel through which the S1 state becomes populated. Further discussions on these geometries with the figures can be seen in the supplementary information. Table 1a Absolute and relative energies of different geometries of DMPO at CASSCF and CASPT2 level of theories Geometry

CASSCF Absolute energy Relative energy (in Hartree) (in kcal/mol)

CASPT2 Absolute energy (in Hartree)

Relative energy (in kcal/mol)

GSA VEE CIA1 CIA2 TSA

-362.90265 -362.72030 -362.78078 -362.78076 -362.80246

0 114.8 76.7 76.4 63.1

-363.97391 -363.80111 -363.86430 -363.86442 -363.87593

0 109 69 68.7 62

OXA1 OXA2 PA

-362.89343 -362.90317 -363.01698

5.8 -0.3 -72

-363.97029 -363.97061 -

2.3 2.1 -

TSA-OX

-362.83870

40.3

-363.91000

40.3

Table 1b Absolute and relative energies of different geometries of 2-Me DMPO at CASSCF and CASPT2 level of theories Geometry

CASSCF

CASPT2

Absolute energy (in Hartree)


Absolute energy (in Hartree)


GSB VEE CIB1 CIB2 TSB

-401.91805 -401.75721 -401.81692 -401.81425 -401.86398

0 101.3 63.7 65.4 34.06

-403.15114 -402.97132 -403.02423 -403.04935 -403.07395

0 113 79 64 48

OXB1 OXB2 TSB-OX

-401.95567 -401.95567 -401.86579

-23.69 -23.69 32.92

-403.14829 -403.14831 -403.07434

1.8 1.8 48.4


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Table 2 Transition moment studies at the ground state optimized geometries of DMPO and 2-Me DMPO System

Transition

DMPO

S0-S1 S0-S2 S0-S1 S0-S2

2-Me DMPO

Transition Moment (Debye) 0.109 3.094 0.184 3.190

Oscillator Strength 0.0004 0.3910 0.0010 0.4370

b. Conical intersections & oxaziridine formation paths Two low-lying conical intersection (S0/S1) geometries (Figures 2 & 3) of these systems (CIA1 and CIB1) have been detected around 35-40 kcal / mol below the vertical excitation energy of S1 states (Table 1a) which seem to be responsible for their respective oxaziridine geometries. The N-O bond becomes elongated in this geometry and it turns towards upside giving a C-N-O kink structure. The gradient difference (GD) vectors at this geometry in DMPO clearly indicate the chance of formation of a 3-membered cyclic system. Following these vectors, we have obtained the oxaziridine geometry (OXA1) around 5.8 kcal / mol at the CASSCF level (2.3 kcal / mol at CASPT2 level) above the ground state geometry (Figure 4) in DMPO. On the other hand, in 2Me-DMPO, the derivative coupling (DC) vectors at the conical intersection (CIB1) geometry (Figure 3a) have shown clear possibility of oxaziridine formation. Following these vectors, this product (OXB1) is found to form with the characteristic C-N-O kink around 24 kcal / mol below the optimized ground state of 2-Me-DMPO (GSB). Almost similar conical intersection geometries (CIA2, CIB2) have been identified for these two compounds at similar energy values having C-N-O kink downwards (Figures 2b & 3b). The CASPT2 energies of OXB1 and OXB2 geometries lie marginally (1.8 kcal/mol) above the nitrone ground state energy at the same level.



Figure. 2. The low-lying conical intersection geometries (a) CIA1 and (b) CIA2 of DMPO with their respective GD & DC vectors The bond lengths and bond angles of the cyclic C-N-O moiety in all the cases (OXA1, OXA2, OXB1, OXB2) closely resemble the geometrical parameters of the oxaziridine structure reported by Murphe etal.46 The N-O bond length in DMPO increases during the reaction path starting from the Franck Condon geometry (1.27 Å) to the S0/S1 conical intersection (1.34 Å) to oxaziridine (1.56 Å), while the C-O bond distance decreases in each of these steps. Similar trend can be noticed in the other compound, however, the N-O bond length in the S0/S1 conical geometry (CIB1) in Me-DMPO is almost 0.25 Å larger than the ground state equilibrium bond length. The adjacent N-C bond is significantly smaller (1.33 Å) than the similar conical intersection geometry of the other compound by roughly 0.11 Å. The atomic charge analysis (Table S1) shows that the negative charge accumulation on the nitrogen atom in CIB1 structure in 2-Me-DMPO is much higher than that of the analogous CIA1 structure of DMPO. A significant 9 ACS Paragon Plus Environment

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increase in the positive charge on the adjacent carbon atom holding the methyl group can be also noticed in the former system. This seems to be connected to the weakening of the N-O bond and the shortening of the adjacent N-C bond in CIB1 of 2-Me-DMPO. The latter bond is significantly ionic in nature in comparison to its more covalent character in the other compound. The positive charge on the carbon atom at 2-position is stabilized by the methyl group and favors a heterolytic cleavage of this C-N π bond; consequently a high negative charge is created on the nitrogen atom. This increased charge in the antibonding π* orbital of nitrogen in 2-Me-DMPO is related to the elongated N-O bond at this geometry.

Figure. 3. The two low-lying conical intersection geometries (a) CIB1and (b) CIB2 of 2-MeDMPO with their respective GD & DC vectors



Figure. 4. The optimized oxaziridine geometries of (a) DMPO and (b) 2-Me-DMPO Two transition states have been identified (TSA and TSB) for these two systems on their respective ground state surfaces (Figure 5). The transition vectors of TSA corresponding to the high imaginary frequency (1514i cm-1) in DMPO give indications of heading towards the nitrone structure. Probably this transition state acts as a barrier between the oxaziridine and the parent nitrone. However, the barrier being substantially high (~ 60 kcal/mol), indicates less chance of formation of parent nitrone from the oxaziridine species in this compound, a fact which was observed experimentally. The analogous transition state geometry of 2-Me-DMPO (TSB) can be reached from the optimized ground state at roughly 34 kcal/mol of energy at the CASSCF level (48 kcal/mol at CASPT2 level). The imaginary frequency value at this geometry is comparatively less (911i cm-1). The corresponding transition vector shows possibility of C-O bond formation (Figure 5) and indicates this as a possible thermal route towards oxaziridine-type product. It is quite possible that this is connected to the previously mentioned photo-chemical reaction path. 11 ACS Paragon Plus Environment

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(a) (b) Figure. 5. The optimized transition state geometries (a) TSA of DMPO and (b) TSB of 2-MeDMPO

c. Oxaziridine to Pyrrolidone In the next stage of work, we have tried to explore the ground state reaction path of oxaziridinelactam conversion. The lactam structure of DMPO (PA) has been optimized at the same level of calculations and this resulting geometry is shown in Figure 6a. It is quite obvious that a kind of [1,2]-H shift in DMPO is required to form the lactam compound PA from the parent oxaziridine. We were able to track the possible transition state (TSA-OX) linking the oxaziridine (OXA1) and pyrrolidone (PA) structures (Figure 6a). The transition state situated around 34-38 kcal/mol above the oxaziridine structure in 5,5-dimethylpyrroline-1-oxide has shown the clear possibility of lactam formation through [1,2]-H shift. The single imaginary frequency of 1517i cm-1 leads (TSA-OX) to the breaking of the N-O bond and the formation of the N-H and C-O bonds. On the other hand, in the 2-methyl compound, a similar transition state (TSB-OX) with the single imaginary frequency of 1213i cm-1 has been identified on the ground state surface (Figure 6b). This is situated almost 58 kcal / mol above the corresponding oxaziridine and it does not lead to a lactam structure. Analysis of the vibration mode corresponding to the imaginary frequency of TSB-OX shows the breaking of the C-CH3 bond, which probably indicates the formation of methyl radical. The barrier to form TSB-OX from OXB1 is almost 20 kcal/mol higher than the barrier to form TSA-OX from its corresponding oxaziridine. This indicates that the formation of methyl 12 ACS Paragon Plus Environment


radical may happen on strong heating of oxaziridine; this subsequently may attach on the

nitrogen atom to form lactam at elevated temperatures.

Figure. 6. (a) Optimized geometries of transition state TSA-OX and the corresponding product lactam PA of DMPO (b) Optimized transition state geometry TSB-OX of 2-Me-DMPO d. Summary of the overall reaction paths The above-mentioned discussion indicates that the reaction paths of DMPO and 2-Me-DMPO follow slightly different trajectories after the oxaziridine formation. In both the cases, an initial electron transfer from the oxygen atom initiates the photo-excitation process. The S2 state with single electron excitation to LUMO becomes populated which subsequently populate the S1

state; a conical intersection follows which brings back the population to S0 state and the reaction path goes towards the photoproduct oxaziridine. The less stable oxaziridine formed in DMPO is expected to form the lactam compound through [1,2]-H shift (Figure 7) after overcoming a


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barrier of 34 kcal/mol. However, in case of 2-Me-DMPO, the highly stable photoproduct is not likely to give a lactam compound directly like the other one.

Figure. 7. Overall reaction pathway of DMPO – Lactam conversion through oxaziridine e. Reported experimental findings & current results Experimental reports have suggested that pyrrolidone is formed as the end product16 in case of DMPO. However, the intermediate oxaziridine was reported to be stable which eventually leads to the cyclic amide (lactam) compound through a thermal route. On the other hand, in 2-MeDMPO, no lactam or pyrrolidone was reported to form21-24 even after heating the oxaziridine at 1250C and the bicyclic oxaziridine was detected as the product. The latter was found to form the lactam only at very high temperature. Our above-mentioned discussion gives a similar indication, as well. The reactions paths, DMPO

oxaziridine

pyrrolidone and 2-Me-DMPO

oxaziridine are properly captured by our CASSCF and CASPT2 based calculations. The stability of oxaziridine is significantly higher in the latter case which indicates better chance of this as the final photoproduct. In case, we assume that the lactam product is forming like in DMPO then it must be through a [1,2]-methyl shift process and this will certainly create some steric repulsion with the two methyl groups at the 5-position. Moreover, in contrast to TSA-OX, the transition vectors corresponding to the imaginary frequency of the transition state TSB-OX shows no indication of forming the pyrrolidone product. This latter transition state (TSB-OX) can be reached 14 ACS Paragon Plus Environment


after overcoming a high barrier (58 kcal/mol) and its imaginary frequency (1213i cm-1) has given clear indication of the breaking of its C-CH3 bond; the detached CH3 group may attach with the nitrogen atom subsequently to form lactam as discussed in section 3c. This was exactly the prediction done by Lamchen and co-workers23 when they obtained lactam from the oxaziridine of 2-Me-DMPO at 3000C. The reason for the high stability of the oxaziridine OXB1 (and OXB2) is due to the presence of the one methyl group and two methyl groups at the 2 and 5 positions, respectively. The positive charge on the oxaziridine ring at the 2-position gets stabilized by the methyl group in Me-DMPO, the effect which is missing in the DMPO system.

4. Conclusion This work is the first comprehensive computational study on the photochemical reaction path of cyclic nitrone systems. The experimentally reported difference in the nature of the final products of 5,5-dimethyl-1-pyrroline 1-oxide and its 2-methyl-substituted analogue under photoirradiation (and subsequent heating) has been accurately reproduced by our ab initio-based studies. The investigation reveals that the process of oxaziridine formation happens through a low-lying conical intersection channel in the first compound which finally leads to a [1,2]-H shift to form a cyclic amide (lactam) known as pyrrolidone, after overcoming a barrier. The 2-methyl substituted compound follows a similar route through a low-lying conical intersection towards the oxaziridine, however, the photoproduct here is highly stable and found to be not involved in lactam formation in a similar manner.


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AUTHOR INFORMATION Corresponding Author* Dr. Anjan Chattopadhyay, Department of Chemistry, BITS-Pilani, K.K. Birla Goa Campus, Zuarinagar, Goa, 403726, India *E-mail: [email protected], [email protected] Fax: +91 832 2557033

Acknowledgement We gratefully acknowledge the financial support provided to our department by the Department of Science and Technology (DST) under the DST-FIST program.

SUPPORTING INFORMATION Atomic charge analysis; Important orbitals of the (4,4) active space of DMPO and 2-Me-DMPO; Highest occupied molecular orbitals of oxaziridines; Optimized excited state geometries at TDDFT & the 3-state degeneracy at CASSCF level. References (1) Feuer, H. Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2008. (2) Floyd R. A.; Kopke, R. D.; Choi, C-H.; Foster, S. B.; Doblas, S.; Towner, R. A. Nitrones as Therapeutics. Free Radic. Biol. Med. 2008, 45, 1361-1374. (3) Splitter, J. S.; Calvin, M. Oxaziridines. I. The Irradiation Products of Several Nitrones. J. Org. Chem. 1965, 30, 3427−3426. (4) Splitter, J. S.; Su, T.-M.; Ono, H.; Calvin, M. Orbital Symmetry Control in the NitroneOxaziridine System. Nitrone Photostationary States. J. Am. Chem. Soc. 1971, 93, 4075−4076. (5) Kochany, E.L.; Kochany, J. Photochemistry of Some α-(2-naphthyl)-nitrones. J. Photochem. Photobiol., A 1988, 45, 65-79. (6) Balogh-Nair, V; Nakanishi, K. Retinylnitrones: A New Class of Retinoids with Chemopreventive Action. Pharm. Res. 1984, 1, 93-95. (7) Saini, P.; Chattopadhyay, A. Spectroscopic Features of the Low-lying Singlet States of Some N-alkyl Retinylnitrone Model Systems and Their Involvement in Oxaziridine Formation. RSC Adv. 2014, 4, 20466-20478. (8) Saini, P.; Chattopadhyay, A. Revealing the Active Role of the Terminal CNO Moiety in the Photochemical Oxaziridine Conversion Process of Some Chemopreventive Retinylnitrones Through Hybrid QM:QM and QM:MM ONIOM Calculations. Chem. Phys. Lett. 2015, 633, 612.



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TOC Graphic


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