Praveen Saini and
Anjan Chattopadhyay*
Department of Chemistry, Birla Institute of Technology and Science (BITS), Pilani – K.K. Birla Goa Campus, Goa, 403 726, India. E-mail: anjan@goa.bits-pilani.ac.in; anjan_chattopadhyay@yahoo.com; Fax: +91 832 2557033; Tel: +91 832 2580319
First published on 18th February 2015
This comprehensive spectroscopic analysis of α-(2-naphthyl)-N-methylnitrone has proposed its photochemical oxaziridine formation and thermal E–Z isomerization mechanisms. The activation energy for the conversion of the unstable non-planar E isomer to the stable planar Z-isomer is found to be 23.7 kcal mol−1 at the CASSCF/6-31G* level of calculation. A transition state with a negative frequency of 350 cm−1 is likely to be responsible for this process. Both CASSCF and ONIOM-based studies have revealed that the nitrone–oxaziridine photochemical conversion involves non-radiative decay channels which include biradicaloid conical intersection (CI) points through Hula-twist and terminal one-bond-flip motions, situated at 35–40 kcal mol−1 below the first excited singlet state (S1). Following the directions of their gradient-difference vectors, the optimized oxaziridine geometries are obtained. The nature of the low-lying singlet–singlet transitions of these α-naphthyl N-methylnitrones is found to be similar to that of the conjugated non-polar polyenes, and differs appreciably from our previously studied long-chain conjugated nitrone systems. The fluorescent S1 state with a radiative lifetime of nanosecond order is populated by the weak upward S0–S1 transition (transition moment: 0.3 Debye) and through the decay of the S2 state, which eventually gets involved in the S0/S1 conical intersections.
The photochemical conversion of nitrone–oxaziridine process has been experimentally attempted by several groups in the last few decades.19,20 The photoirradiation studies on nitrones can be primarily summarized into 2 important observations; the cis–trans isomerization reactions of nitrones occur thermally or through triplet excited states in presence of photosensitizers,21,22 while their normal photo-excitation involves singlet excited states which subsequently forms oxaziridines and other photo-products, such as amides. The other noticeable feature of the nitrone photo-excitation is the varying stability of the photoproduct oxaziridine, which largely depends on the substituents present on nitrogen or α-carbon.19,23 Experimental results have shown that N-alkyl substituents increase the stability of the oxaziridines and this has also been established by our CASSCF-based studies on model N-alkyl retinyl nitrone systems.18 This present work is aimed to reveal the unexplored mechanism of photochemical processes of one such nitrone system which is known to give a stable oxaziridine species on photo-irradiation; the experimentally studied α-(2-naphthyl)-N-methylnitrone was found23 to give stable and isolable oxaziridines, while its N-(p-tolyl) derivative was unable to produce such photoproduct. Photo-irradiation of neutral solution of this nitrone was found to increase the fluorescence intensity with fluctuations which was predicted to be arising due to the formation of efficiently fluorescing oxaziridines.
This study is an attempt to put forward a proper theoretical background of the nitrone–oxaziridine conversion and E–Z isomerization processes of this above-mentioned nitrone system through high-level quantum mechanical investigations. This analysis will include the study of optimized ground state, relaxed excited state, conical intersection points and important transition state geometries. In recent times, computational chemists have been able to develop tools to explore electronically excited state species. Intensive computational studies have revealed the behaviour of the photo-excited organic molecules, and highly systematic investigations are currently possible for the photochemical reaction pathways. In addition to the radiative emission, computational studies involving non-radiative processes have also drawn considerable attention in the last few decades. In a photochemical process, the electronically excited state has a high probability of passing through a region where the excited state crosses the ground state, thereby causing the breakdown of Born–Oppenheimer approximation. Such non-adiabatic crossing, commonly known as conical intersection,24–30 opens up a funnel-type radiationless decay channel, which results in subsequent photoproduct formation.
Our earlier studies on the model compounds of N-alkyl retinyl nitrones have predicted the presence of terminally twisted biradicaloid conical intersection (S0/S1) geometries with a C–N–O kink as the major player behind their photochemical oxaziridine conversion process. This was found to have some similarity with the prefulvenic conical intersection in benzene which appears during its channel-3 decay.30–32 It will be quite interesting to investigate whether our presently targeted nitrone system also involve similar conical intersection topography during the oxaziridine formation step as the retinyl nitrones or not. In addition to this non-radiative channel, analysis of their radiative transition properties will be also an important part of this work, and they are expected to give some equally important features. Their UV-vis peaks will be compared with the experimentally reported values. In addition to these comprehensive studies of the photochemical and photophysical properties of the α-(2-naphthyl)-N-methylnitrone, the probable mechanism of its E–Z isomerization process will be also analyzed in this present work.
In our earlier work18 it was concluded that the open-chain conjugated nitrone systems experience a twist at the terminal C–N–O moiety during the oxaziridine formation step without any significant change in the conjugated chain part. Based on the experimental and theoretical studies on the N-methyl retinyl nitrone system, the actual photochemical reaction path of such conjugated nitrone systems leading to oxaziridine is shown in Scheme 1. CASSCF studies had confirmed that the lowest-energy conical intersection geometry related to this terminal CNO twist is responsible for the formation of the oxaziridine ground state species. This study has indicated an involvement of the C–N π bond and a pz orbital on oxygen (holding the negative charge on oxygen) in the oxaziridine formation which results in a C–N σ bond and a possible carbon–oxygen bond (through a transient biradical species), after photo-excitation. Our currently studied α-naphthyl N-methyl nitrone is also likely to follow a similar path, as experimental studies have shown that no change in the naphthyl part was found to happen during the course of oxaziridine formation.
Different regions of a chemical system can be treated with a different level of theory in the hybrid ONIOM method. The basic idea behind this methodology is related to the fact that for a large molecule the major chemical process is usually localized only in a small segment, commonly known as the active site. This zone of our interest where the chemical process is actually happening can be treated with a higher level of quantum mechanical (QM) theory, while the less important part of the system can be treated with a lower-level of QM theory (QM′) or molecular mechanics (MM) theory. Considering the fact that a similar photochemical path as the model retinyl nitrone system is possibly operating in our currently studied nitrone system, it seems quite reasonable if we consider the terminal C–N–O moiety as the active site for the ONIOM calculation. It must be emphasized here that our major goal in this work is to find out the possible route of the experimentally studied oxaziridine formation process under photo-irradiation of the α-(2-naphthyl)-N-methylnitrone system. In the ONIOM extrapolation scheme,38–41 we can combine any two or three QM and MM methodologies into one calculation. In our present work, we have considered 2-layer QM:QM′ combination, which can be represented as CASSCF (4,4)/6-31G*:RHF/4-31G. The higher level quantum mechanical method is CASSCF and the lower level method is Hartree–Fock theory. This is expected to give a better interaction of the model part and the low-level system as both are treated at quantum mechanical level.46 The CASSCF-based higher level of theory has been treated with 4 active electrons in 4 active orbitals, considering the C–N–O moiety as the core region (model) of our photochemical interest which involves the C–N π bond and the pz orbital on oxygen in the nitrone–oxaziridine photo-conversion process. The total energy in the ONIOM methodology is expressed through an extrapolation scheme:
EONIOM (QM, QM′) = EQM, model + EQM′, real − EQM′, model |
In addition to this hybrid method, a separate calculation has been also employed for the whole nitrone system at the CASSCF (4, 4) level of calculation. It has already been discussed that our prior experience on the photochemical study of the model N-alkyl retinyl nitrone system18 has clearly indicated that the involvement of orbitals having electronic cloud distributed over the portions of the molecule away from the CNO moiety in the active space does not lead to our desired oxaziridine product. The same is also expected to be true for the α-naphthyl N-methyl nitrone system, as experimental results indicate. This implies that inspite of employing huge computational cost by using a large active space in this present photochemical study it is less likely to obtain our desired nitrone–oxaziridine photo-conversion process. Therefore, we have attempted to make an accurate minimal choice of the active space leaving out the probable less important naphthyl part from it, and this method is completely biased towards the possible reaction path39 using previous intuition from our earlier work and the experimental findings of Kochany et al.23 In this CASSCF (4, 4) calculation the chosen HOMO is of π symmetry on the CNO moiety while the LUMO is of corresponding π* symmetry (Fig. S1†). It must be added that we had actually started our CASSCF calculations using a (14, 12) active space, but we were unable to find any oxaziridine type species on the photochemical path unlike the (4, 4) active space calculation. It is a fact that there is no single correct active space in a molecule;39 the choice of active space depends on the particular process being carried out. Our present theoretical investigation is based on a comparison of these CASSCF calculated parameters and the ONIOM-based CASSCF (4,4)/6-31G*:RHF/4-31G derived values. CAS perturbation theory (CASMP2) has introduced dynamic correlation effect of Moller–Plesset perturbation (MP2) level through single point calculations on top of the CASSCF optimized geometries. Transition states have been located using the normal TS technique based on the Berny-algorithm47 and QST3 (ref. 48) methodology. Intrinsic reaction coordinate (IRC) method49–51 has been used to follow the minimum energy path from the transition state.
In addition to these studies, GUGA-based configuration interaction singles and doubles (CISD) technique has also been used for some important calculations through the GAMESS52–56 suite of programs. Radiative transition57–59 calculations have been carried out between the two CI wavefunctions at the ground state equilibrium geometry, based on this GUGA CI code. Electrostatic potential-based atomic charges are calculated for the ground and excited state species using the Merz–Kollman60,61 scheme in Gaussian 09 program. For visualization of the output files, ChemCraft62 and Gaussview softwares have been employed throughout this computational work.
Molecular states | Level of calculationa | C–C | C–N | N–O | N–C | DC–C–C–N | DC–C–N–O |
---|---|---|---|---|---|---|---|
a Level of calculations are CASSCF (4,4)/6-31G* (level I), CASSCF (4,4)/6-31G*:RHF/4-31G (level II), CASSCF (14,12)/6-31G* (level III), RHF/6-311G** (level IV), B3LYP/6-311G**, (level V) and PM3/CI (level VI). | |||||||
Ground state of Z-isomer | I | 1.469 | 1.311 | 1.276 | 1.459 | 0.0 | 0.0 |
II | 1.456 | 1.291 | 1.319 | 1.455 | 0.0 | 0.0 | |
III | 1.465 | 1.303 | 1.265 | 1.462 | 0.0 | 0.0 | |
IV | 1.465 | 1.276 | 1.265 | 1.462 | 0.0 | 0.0 | |
V | 1.447 | 1.318 | 1.271 | 1.482 | 0.0 | 0.0 | |
VI | 1.455 | 1.334 | 1.243 | 1.507 | −24.2 | 0.1 | |
Ground state of E-isomer | I | 1.483 | 1.307 | 1.254 | 1.466 | −60.0 | 176.6 |
II | 1.473 | 1.296 | 1.274 | 1.495 | −50.5 | 176.8 | |
III | 1.481 | 1.307 | 1.254 | 1.466 | −57.5 | 176.6 | |
IV | 1.481 | 1.274 | 1.265 | 1.465 | −55.7 | 177.4 | |
V | 1.458 | 1.318 | 1.270 | 1.483 | −34.7 | 175.5 | |
VI | 1.460 | 1.330 | 1.247 | 1.500 | −49.1 | −179.7 |
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Fig. 1 Optimized excited state geometries of E and Z isomers at CASSCF (4,4)/6-31G* (I), CASSCF (14,12)/6-31G* (II) and ONIOM (QM:QM′) (III) level of calculations. |
Molecular state | Level of calculationa | Z-isomer | E-isomer |
---|---|---|---|
ΔE in kcal mol−1 | ΔE in kcal mol−1 | ||
a Level of calculations are CASSCF (4,4)/6-31G* (level I), CASMP2 (4,4)/6-31G* (level II), CASSCF (4,4)/6-31G*:RHF/4-31G (level III) and CASSCF (14,12)/6-31G* (level IV).b Experimental value is 327 nm (ref. 23). | |||
G. S. | I | 0 | 0 |
II | 0 | 0 | |
III | 0 | 0 | |
IV | 0 | 0 | |
E. S. | I | 110.68 | 88.62 |
II | 102.05 | 84.52 | |
III | 86.62 | — | |
IV | 84.38 | 82.70 | |
F. C. | I | 118.31 (241) | 91.30 (313)b |
II | 110.89 (257) | 84.83 (337)b | |
III | 98.02 (291) | 102.34 (279)b | |
IV | 95.83 (298) | 103.49 (276)b |
The calculated vertical excitation energies (Table 2) of the Z-isomer from the CASSCF method with (4, 4) and (14, 12) active spaces are 118 and 96 kcal mol−1, respectively. The CASMP2 calculations have predicted a value of 110 kcal mol−1 while the 2-layer ONIOM method has given a value (98 kcal mol−1) close to the calculated values at CASSCF (14, 12) level. These latter values have predicted an absorption peak position close to 295 nm. On the other hand, the VEE values predicted at the CASSCF (91 kcal mol−1, 315 nm) and CASMP2 (85 kcal mol−1, 335 nm) level using (4, 4) active space for the E-isomer are substantially lower than the CASSCF (14, 12) and the ONIOM predicted VEE values (102–103 kcal mol−1) for this isomer. The experimentally reported peak position23 of this isomer is close to 325 nm. After initial photo-excitation, the vertically excited state relaxes to a comparatively stable geometry which is situated at 8–12 kcal mol−1 below the excited state FC geometry for the Z-isomer. However, in case of the E-isomer this stabilization is smaller (1–2 kcal mol−1). The CASSCF calculated ESP-based atomic charges (Table S1†) are quite similar to the values observed in our earlier studied long-chain conjugated nitrone systems. A clear indication of initial electronic transfer from the oxygen atom to the nitrogen can be seen as the relaxation towards the optimized excited state is approached after the initial photo-excitation; this electronic transfer triggers the subsequent photochemical processes.
The initial guess geometry corresponding to the Z-isomer has lead to two different types of conical intersections at the higher level of ab initio calculations. Rotation around the C–C bond and C–N bond causes these two types of intersection points. The one with simultaneous change of the C–C–C–N and C–C–N–O dihedral angles (Table 3) gives rise to the conical intersection point CI1, which seems to originate from a hula twist (HT)-type motion, normally responsible for the kinked conical intersection in conjugated polyenes. On the other hand, the terminally-twisted CI2 intersection geometry (Fig. 2) resembles the lowest-energy conical intersections earlier reported by us for the long-chain conjugated nitrone systems. This latter geometry involves an out-of-plane C–N–O kink or oxygen-bridge structure due to a rotation about the C–N bond resulting in a change in the C–C–N–O dihedral angle. Both of them are having almost similar atomic charges and these are close to the values reported for the lowest energy CI geometry for the model retinyl nitrone systems. In other words, similar to the retinyl nitrone systems, the ESP-derived charges for this α-naphthyl N-methylnitrone indicate biradicaloid conical intersection geometries with an odd electron on the α-C and a reduced electronic cloud on oxygen with a lone pair on nitrogen. Though the geometries obtained from the CI optimization run from the two different methods are apparently similar, the predicted C–C and C–N bond lengths differ by 0.05 Å. The hybrid method has predicted these structures at 20–26 kcal mol−1 below (Table 4) the optimized excited state while the CASSCF and CASMP2 methods have predicted these values between 33 and 38 kcal mol−1. In both the cases (CI1 and CI2), the gradient difference vectors indicate (Fig. 2) the possibility of a C–N–O triangle formation. Following these vectors by shortening the C–O distance we have obtained optimized ground state geometries (Fig. 3) in both the cases (Ox1 and Ox2) with a CNO kink having bond lengths and bond angles closely resembling the reported parameters of oxaziridine structure.63 The CASSCF predicted values of geometrical parameters of Ox1 and Ox2 (RC–O = 1.38 Å, RN–O = 1.44 Å, RC–N = 1.40 Å, ∠OCN = 62°, ∠ONC = 58°) are close to the hybrid ONIOM-predicted values of these two geometries, with some exceptions in the C–O and N–O bond distances. The experimentally reported oxaziridine geometry has a C–N–O triangle with C–O and N–O bond lengths of 1.40 Å and 1.50 Å, respectively, and the C–N bond length is approximately 1.44 Å; the bond angles ∠OCN = 63.7° and ∠ONC = 56.8° are also close to our predicted results. Energy-wise, the CASSCF and CASMP2 values suggest that these oxaziridine geometries are situated between the optimized ground and excited states; on the other hand, the ONIOM study has located their positions below the starting ground state geometry. Molecular orbital analysis of both these structures clearly gives the evidence of C–O bond formations. In fact, from the figure of HOMO, a three-centered C–O–N-type of bond formation (Fig. 3) is clearly visible in Ox1 and Ox2.
Molecular geometry | Level of calculation | RC–C | RC–N | RN–O | RC–O | RN–C | DC–C–C–N | DC–C–N–O |
---|---|---|---|---|---|---|---|---|
CI1 | I | 1.453 | 1.392 | 1.360 | 2.125 | 1.445 | 176.0 | −80.5 |
II | 1.392 | 1.474 | 1.366 | 2.355 | 1.461 | −174.5 | −116.5 | |
CI2 | I | 1.464 | 1.389 | 1.387 | 2.128 | 1.444 | 17.2 | −71.0 |
II | 1.413 | 1.446 | 1.385 | 2.227 | 1.446 | −1.0 | −79.2 | |
CI3 | I | 1.376 | 1.466 | 1.257 | 2.340 | 1.450 | −173.1 | −168.8 |
II | — | — | — | — | — | — | — | |
CI4 | I | 1.453 | 1.409 | 1.368 | 2.173 | 1.446 | 8.0 | 82.0 |
II | 1.428 | 1.428 | 1.406 | 2.206 | 1.448 | 3.0 | 78.1 | |
Ox1 | I | 1.493 | 1.406 | 1.443 | 1.378 | 1.449 | −125.1 | −108.9 |
II | 1.486 | 1.405 | 1.425 | 1.529 | 1.450 | −135.1 | −109.6 | |
Ox2 | I | 1.490 | 1.404 | 1.442 | 1.381 | 1.450 | 36.8 | −108.8 |
II | 1.484 | 1.403 | 1.528 | 1.427 | 1.450 | 36.7 | −109.5 | |
Ox3 | I | 1.490 | 1.404 | 1.404 | 1.384 | 1.450 | −36.7 | 108.8 |
II | 1.483 | 1.403 | 1.427 | 1.528 | 1.450 | 36.7 | 109.5 |
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Fig. 2 Optimized conical intersection geometries (CI1, CI2, CI3 and CI4) at the CASSCF/6-31G* level. The corresponding gradient difference and derivative coupling vectors are also shown. |
Molecular geometry | CASSCF | CASMP2 | ONIOM (CAS:RHF) |
---|---|---|---|
ΔE in kcal mol−1 | ΔE in kcal mol−1 | ΔE in kcal mol−1 | |
E.S. (Z isomer) | 0 | 0 | 0 |
E.S. (E isomer) | 0.20 | 0.80 | — |
CI1 | −35.49 | −36.29 | −20.34 |
CI2 | −33.50 | −37.68 | −26.15 |
CI3 | 12.31 | 10.27 | — |
CI4 | −41.52 | −36.87 | −27.23 |
Ox1 | −81.49 | −93.57 | −91.27 |
Ox2 | −82.68 | −94.22 | −92.20 |
Ox3 | −96.96 | −93.61 | −92.20 |
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Fig. 3 Optimized geometries of the oxaziridines (Ox1, Ox2 and Ox3) at the CASSCF/6-31G* level. The HOMOs of the corresponding geometries are also shown. |
Following the guess structure provided by the semi-empirical configuration interaction result for the E-isomer, two conical intersection geometries, namely CI3 and CI4, are obtained at the CASSCF and ONIOM level of calculations. The first one (CI3) has a twist in the opposite direction of CI1, however, situated at very high-energy level; on the other hand, the energy of the low-lying CI4 intersection point with a terminal C–N–O kink on the backside is found to be close to the CI2 geometry at the CASMP2 and ONIOM level, though CASSCF energies have predicted this geometry (CI4) as the lowest-energy CI point. This conical intersection point is situated at 60–69 kcal mol−1 (Table 4) above the ground state optimized geometry of the Z-isomer. The turn of the C–C–N–O dihedral angle in this species is exactly in the reverse direction in comparison to that of the CI2 geometry. The CASSCF and ONIOM-based geometrical parameters are in close agreement with each other, while the latter method has predicted this geometry at a higher energy value (51 kcal mol−1) than the CASSCF and CASMP2 reported values (47 kcal mol−1) with respect to the ground state geometry of E-isomer. The gradient difference vectors of this conical intersection point is clearly showing a chance of C–N–O triangle formation through a C–O bond. Following these vectors we have obtained an oxaziridine geometry (Ox3) which almost matches the reported geometrical parameters of this 3-membered heterocyclic species. Unlike the other two oxaziridine structures, the Ox3 species is situated below (8.5–9.9 kcal mol−1) the ground state geometry of the E-isomer at the CASSCF and CASMP2 level of studies, while the hybrid method has predicted its location at 13.7 kcal mol−1 lower than this ground state geometry. Similar to the other two oxaziridine structures, a clear overlap of orbitals on carbon and oxygen can be noticed from the picture of HOMO of Ox3 geometry along with the formation of a 3-centred C–O–N bond. Though the oxaziridine geometry Ox3 and the conical intersection CI4 are mentioned to be arising from the E-isomer based on the rough estimation of semi-empirical guess structures, it must be added that these two geometries may originate from the Z-isomer, as well. In fact, the back-side turn of the C–C–N–O dihedral angle (57.3°) in the optimized excited state of the Z-isomer is also quite consistent with the formation of the CI4 geometry where the angle increases to 82°.
We have tried to analyze the reason behind the difference in stabilities of the three important conical intersection geometries (CI1, CI2, and CI4). The Hula-twist motion involved in the CI1 geometry is known to be a volume demanding process and therefore its energy is expected to be higher. On the other hand, the CI2 and CI4 geometries have terminal-twist (OBF-type) with C–C–N–O turn in the front and back sides, respectively, which requires less volume change. However, the energy gaps between the CI1 geometry and the other two low-lying CI geometries are not very high; this is due to the fact that unlike our previously studied conjugated-chain N-alkyl nitrone systems, there exists an additional strain in this presently investigated nitrone system due to the naphthyl ring and the methyl group/oxygen atom, when they come to close vicinity. As a consequence, the out-plane Hula twist motion though expected to be at significantly high energy level, seems to be sterically more favored, and therefore energy-wise not too different from the terminally twisted structures.
A similar investigation of transition states on the ground state surface of the Z-isomer has led to two TS geometries (Fig. 4) at CASSCF level (TSgs1 and TSgs2) while two different transition states have been obtained at the ONIOM level (TSgs3 and TSgs4). The only negative frequency of TSgs2 which indicates stretching and contracting along the C5–O bond is 1305 cm−1. This is consistent with the IR stretching frequency of the C–O bond and there is enough reason to conclude that this TS connects the oxaziridine (Ox2) and the Z-isomer of the nitrone, and indicates that the C5–O bond will open up to give back the parent nitrone passing through this transition state. The ONIOM-predicted TSgs3 and TSgs4 structures are characterized by negative frequencies of 332 cm−1 and 323 cm−1, respectively. In both the cases the vectors are clearly showing chances of C–O bond formation. It must be added here that the gradient difference vectors of CI1 and CI2 are parallel to the vectors corresponding to the negative frequencies of TSgs3 and TSgs4, respectively, and both seems to be leading towards their respective oxaziridine structures. The TSgs3 geometry is situated at 40 kcal mol−1 below the CI1 intersection point and 30 kcal mol−1 above the Ox1 geometry, while the TSgs4 geometry lies half-way between the CI2 and the Ox2 geometries. On the whole, both these transition states are situated on the nitrone–oxaziridine photochemical conversion pathway.
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Fig. 4 Optimized transition states geometries (TSgs1-4) on ground state surface with displacement vectors of their respective negative frequencies. |
The fifth transition state detected on the ground state surface (TSgs5) has been identified as the link between the E–Z isomers (Fig. 5). The QST3 methodology in Gaussian has been employed for finding out this transition state where the E-isomer has been specified as the reactant molecule while the Z-isomer is taken as the product. This transition state, which is responsible for the E–Z thermal isomerization, is characterized by a negative frequency of 350 cm−1. The unstable E isomer which is situated at 22 kcal mol−1 above the stable Z-isomer, reaches this transition state with activation energy of 23.7 kcal mol−1 (Table S2†). Thereafter following the negative frequency, the Z-isomer is formed. This optimized ground state of the latter isomer is situated at 46 kcal mol−1 below the TSgs5 geometry. An IRC run has been given on this transition state both in the forward and reverse directions; the corresponding IRC plot is shown in Fig. S4.† It should be mentioned here that in recent times E–Z isomerization studies64,65 on different types of nitrones have been extensively reported at the DFT level of calculations.
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Fig. 5 Schematic representation of E–Z isomerization of α-(2-naphthyl)-N-methylnitrone through TSgs5. |
Systems | Transition moment in Debye | Oscillator strength | Einstein's coefficient in s−1 | ||||||
---|---|---|---|---|---|---|---|---|---|
S0–S1 | S0–S2 | S2–S1 | S0–S1 | S0–S2 | S2–S1 | S0–S1 | S0–S2 | S2–S1 | |
a The values in parentheses are the powers to the base 10. | |||||||||
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0.304 | 5.616 | 2.099 | 0.003 | 1.322 | 0.010 | 1.7280 (+7) | 7.0138 (+9) | 1.8008 (+5) |
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0.371 | 3.862 | 1.890 | 0.005 | 0.644 | 0.010 | 2.7374 (+7) | 3.6303 (+9) | 2.5472 (+5) |
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0.961 | 7.764 | 4.303 | 0.028 | 1.861 | 0.010 | 7.7773 (+7) | 5.3540 (+9) | 9.1551 (+3) |
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4.861 | 0.112 | 0.069 | 0.706 | 0.000 | 0.000 | 2.1472 (+9) | 1.0559 (+6) | 1.2264 (+0) |
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7.543 | 1.891 | 1.935 | 1.695 | 0.109 | 0.019 | 4.5380 (+9) | 3.1182 (+8) | 1.5590 (+6) |
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Fig. 6 Schematic representation of the low-lying singlet–singlet transitions and involvement of some important low-lying conical intersections (S0/S1) in α-(2-naphthyl)-N-methylnitrone system. |
A comparative analysis of the four above-mentioned conjugated systems has revealed that the vertically excited S1 state of 2,4-pentadien-1-iminium ion is completely dominated by the configuration arising due to the HOMO → LUMO excitation (Table S3†) while it partly dominates this state in the conjugated N-methyl nitrone system. In contrast, this configuration dominates the S2 state of the 1,3,5-hexatriene system and our presently studied α-naphthyl N-methyl nitrone system at the FC geometry. The S1 states of these latter systems are mostly contributed by configurations arising due to the HOMO → LUMO + 1 and HOMO − 1 → LUMO excitations at the above-mentioned geometries. The conjugated non-polar polyene has also a major contribution from the HOMO2 → LUMO2 configuration for this first vertically excited singlet state (at FC geometry). Interestingly, unlike in the iminium ion, this doubly excited configuration becomes a key player in the S1 state of the nitrones and the polyenes as the optimized geometry of this state is approached. This optimized excited state in the α-naphthyl N-methyl nitrone is almost equally contributed by configurations due to HOMO2 → LUMO2, HOMO → LUMO + 1 and HOMO − 1 → LUMO excitations (Table S3†).
Experimental studies have revealed23 that the photoproduct oxaziridines are also fluorescent in nature which results in increase and fluctuation of fluorescence intensity of the parent nitrone system during their irradiation. GUGA CISD-based radiative transition studies have been carried out on the ground state oxaziridine systems and the results (Table S4†) are found to be quite similar to those of the parent nitrone systems. The S0–S1 transitions are weaker than the S0–S2 and S2–S1 transitions; however, the transition moment and oscillator strength values of the latter-mentioned transitions are significantly lower in comparison to those values of nitrones. On the other hand, these parameters of the lowest transition (S0–S1) are found to decrease slightly in this heterocyclic species.
Footnote |
† Electronic supplementary information (ESI) available: Orbitals involved in the (4, 4) active space in CASSCF/6-31G* calculations; Fig. S1; semi-empirical CI (PM3/CI) level of studies; Fig. S2; ESP-derived atomic charges using Merz–Kollman scheme; Table S1; transition states; Fig. S3, S4 and Table S2; dominant configurations of S1 and S2 states Tables S2 and S3; Cartesian coordinates. See DOI: 10.1039/c4ra16375c |
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