Spectroscopic and theoretical studies of 4-nitropyridine N-oxide and of its related charge transfer compounds in their excited state T1

The geometries of both the singlet ground state and the lowest triplet excited state of NPO have been opti- mized through minimization of the total energy gradi- ent at the ab initio SCF level of calculation. Those optimizations have been carried out using basis sets of sizes (9s 5p) for C, N, 0 and (4s) for H [ 231, which were contracted into split-valence, that is [ 3s,2p] for C, N, 0 and [2s] for H. They will be referred to as basis sets I. Starting from the same set of primitive Gaussians for C, N, 0, but from a more extended (6s) set for H [ 241, a more flexible contraction scheme still minimal for the inner shell of C, N, 0, but triple-f for the valence shell (basis sets II), has been used for further optimization of the N-oxide distance. Finally, polarization functions have been added on the top of basis sets II in order to generate a quantitatively reliable description of the electron deformation density in both the ground state and the lowest triplet state. A p-type polarization function with exponent 0.8 has been used for hydrogen, d-type functions with exponents 0.63, 0.95 and 1.33 were used for C, N, and 0, respectively. Those polarized basis sets (basis sets III) have led to quantitative accuracy for peptide molecules when com- pared to static model distributions derived from X-ray diffraction experiments [ 251. All calculations have been carried out using the ASTERIX package [26], except for the geometry optimization of the open-shell state which has been performed using the HONDO program [ 271.

Chemical Physics

Spectroscopic and theoretical studies of 4-nitropyridine N-oxide

and of its related charge transfer compounds

in their excited state T1 Frangoise Briffaut-Le Guiner a, Pascal Plaza a, Nguyen Quy Dao a, Marc BCnard b

a Laboratoire de Chimie et Physico-Chimie Moltkxlaires, ERS 0070 CNRS, Ecole Centrale de Paris, Grande Voie des Vignes,

92295 Chritenay-Malabry, Cedex, France

b Laboratoire de Chimie Quantique, Institut Le Bel, 4 rue Blaise Pascal, 67ooO Strasbourg, France

Abstract

Lowest triplet state Tr Raman spectra of 4-nitropyridine N-oxide NPO, its deuterated derivative NPO-d., and its related

compound 3-methyl 4-nitropyridine N-oxide (POM) obtained by time-resolved resonance Raman spectroscopy (TR3) are

reported NPO and NPO-d, keep the CzV symmetry in their zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAT, state but the structure of the cycle is modified towards a pronounced

quinonoid conformation An important charge transfer from the N-oxide to the NO1 group is observed The TR3 activity of Tr-

POM vibrations is very similar to T,-NPO, the T, + T, transitions of these molecules are of the same nature Ab initio SCF

calculations have been performed and the results obtained for both geometry and electron deformation density are in good

agreement with spectroscopic data

1 Introduction

The molecule of 4-nitropyridine N-oxide (NPO) #l

plays an important role in chemical synthesis and its

related compound 3-methyl 4-nitropyridine N-oxide

(POM) is used as a very efficient non-linear optical

(NLO) medium NPO is the main chemical interme-

diate for the preparation of 4-pyridine substituted deriv-

atives and of pyridine N-oxide [2], and has an

excellent photochemical reactivity Unlike most aro-

matic oxide amines, its reactive site is not situated on

the N-oxide group but on the nitro group 13-51 It has

been shown [ 61 that photochemical reactions of NPO

are initiated by its T, state in desoxygenated water and

* Corresponding author

IT’ Preceding paper of the series; ref [ 11

by its S, state in alcohols NLO properties have been demonstrated for POM, which has proved through var- ious experiences to qualify as a highly efficient qua- dratic NLO material in the visible and near-IR range (from 0.5 to 1.8 pm) [ 7-91 This property can be explained by an internal charge transfer (ICT) phe- nomenon occurring in the singlet excited state (S,) between the N-oxide donor group and the nitro accep- tor group, through the aromatic ring

In order to quantify the ability of these compounds

to undergo solvent-induced ICT in their ground state, vibrational spectroscopic studies of those two mole- cules have been done in this laboratory using both experimental and semi-empirical methods [ l,lO-151

As far as ICT in the Si state is concerned, it seems that the extremely short lifetime of this state [ 161 ( < 100 fs) prevents easy spectroscopic investigation Never-

0301-0104/94/$07.00 0 1994 Elsevier Science B.V All rights reserved

SSDIO301-0104 (94)00050-K

theless the long-lived T, excited state of NPO has been

shown to exhibit also an ICT character [ 171 The aim

of the present work is to study this ICT character of the

T, state, for NPO and POM, by time-resolved reso-

nance Raman (TR3) spectroscopy and ab initio cal-

culations

2 Experimental

2 I Chemicals

NPO was purified by recrystallisation of NPO

(Merck 98%) NPO-d, was synthetised from pyridine-

d5 (99.8%) using Ochiai procedure [ 181 Pure POM

single crystals were prepared by Zyss and co-workers

[ 191 These compounds were dissolved in bidistilled

water at the concentration of lo-’ M for TR3 experi-

ments

2.2 The TR3 setup

The S, state (IT-~* of ‘Ai symmetry) of NPO has

a strong absorption band in the UV range and the T,

state (n-n* of 3A, symmetry) has a strong and broad

T, -+T, absorption band in the visible range (maxi-

mum absorption at 550 nm) [ 71 The T, -+ T, absorp-

tion of POM is still unknown, but owing to the

similarity of structures, it is expected in the same region

as for NPO It is possible under these conditions to

perform time-resolved “pump-probe” resonance

Raman experiments using a single pulsed laser to study

the T, state of these compounds

The T, states of NPO, NPO-d, and POM were pop-

ulated by the third harmonic (355 nm, IO ns duration)

of a Q-switched Nd : YAG Laser (Quanta Ray, model

GCR 4,10 Hz), in the S, -+ S, absorption band (whose

maxima in water lie respectively at 3 14 and 307 nm for

NPO and POM) followed by an ultra-fast intersystem

conversion S, -+ T, Energy of 1 mJ/pulse is used for

the experiments in order to avoid multiple absorption

[20] For the generation of the Raman exciting line,

the second harmonic (532 nm, 10 ns duration) of the

same laser was used to pump a home-made Littman-

type dye laser [ 21,221 The dye solution were rhoda-

mine 575 (Exciton), rhodamine 610 (Exciton) and

DCM (Exciton) in ethanol for tunable excitation

between 550 and 620 nm The probe beam intensity

was fixed at 3.5 mJ/pulse at the level of the sample The probe beam was delayed by 10-15 ns with regard

to the pump beam in an optical delay line and then focalized colinearly in a capillary tube where the sam- ple solution was circulating The transit time of the solution in the pumped volume was about lo-” s so that solution was refreshed before each laser pulse The Raman spectrum was collected at 90” with the help of a triple monochromator Raman spectrometer (Jobin-Yvon S3000) equipped with a 700 intensified photodiodes multichannel detector The spectra were recorded between 500 and 2000 cm-’ accumulating

100 spectra of 10 s integration time each, which cor- responds to 10000 pulses per spectrum

2.3 Ab initio calculations: computational details

The geometries of both the singlet ground state and the lowest triplet excited state of NPO have been opti- mized through minimization of the total energy gradi- ent at the ab initio SCF level of calculation Those optimizations have been carried out using basis sets of sizes (9s 5p) for C, N, 0 and (4s) for H [ 231, which were contracted into split-valence, that is [ 3s,2p] for

C, N, 0 and [2s] for H They will be referred to as basis sets I Starting from the same set of primitive Gaussians for C, N, 0, but from a more extended (6s) set for H [ 241, a more flexible contraction scheme still minimal for the inner shell of C, N, 0, but triple-f for the valence shell (basis sets II), has been used for further optimization of the N-oxide distance Finally, polarization functions have been added on the top of basis sets II in order to generate a quantitatively reliable description of the electron deformation density in both the ground state and the lowest triplet state A p-type polarization function with exponent 0.8 has been used for hydrogen, d-type functions with exponents 0.63, 0.95 and 1.33 were used for C, N, and 0, respectively Those polarized basis sets (basis sets III) have led to quantitative accuracy for peptide molecules when com- pared to static model distributions derived from X-ray diffraction experiments [ 251 All calculations have been carried out using the ASTERIX package [26], except for the geometry optimization of the open-shell state which has been performed using the HONDO program [ 271

315

T Intensity (arbitrary unit)

+

Wavenumbers (cm-‘)

Fig 1 Raman spectra of POM recorded with a 550 nm excitation

(A) With a pump beam at 355 nm (B) Without the pump beam

3 Results

Spectra of NPO, NPO-d, and POM in aqueous solu-

tion obtained without the pump beam show as expected

only Raman lines of the So state The ones recorded

with the pump beam show extra lines, due to the tran-

sient species The spectra of the transient species were

obtained by difference and then scaled, as shown in

Fig 1 for POM Fig 2 gives the Raman spectra of

NPO, NPO-d, and POM in their excited state T,

Before going to the assignment of the Raman lines,

some preliminary remarks must be drawn:

(i) Raman spectra of low3 M solutions of NPO and

POM were first recorded with 550 nm excitation With-

out the pump beam, no Raman band was observed, the

concentration of the molecules in the So state is too low

to be detected With the pump beam, Raman peaks

appear for both compounds The number and positions

coincide with the transient spectra obtained with lo-’

M solutions These results show that the resonance

Raman phenomenon occurs for both NPO and POM

(ii) The Raman intensity of the transient species

varies linearly with the pump beam intensity around 1

ml/pulse energy, which means that this specifies is

effectively created by a one-photon process as expected

(iii) The scaling factor gives an estimation of the depopulation ratio of the So state For a pump beam of

1 mJlpulse in power, it is found to be equal to 10% for both NPO and POM

(iv) All the spectra were recorded with 0.01 M solu- tions of NPO, NPO-d, and POM Several wavelengths, around 550 nm, corresponding to the maximum posi- tion of the T, + T, absorption band, were used in order

to check the intensity variation of the Raman lines at resonance, pre-resonance and off-resonance wave- lengths Figs 3a and 3b show the Raman enhancement profiles of the strongest peak of the transient species, which lie at 982 and 940 cm-’ respectively for NPO and POM

This peak is about 50 times stronger for the 550 nm resonance excitation than for the 620 off-resonance excitation in the case of NPO The Raman enhancement profile is the same as for the Tr -+ T, absorption band, suggesting that the transient species observed on the resonance Raman spectra is the T, state of NPO Fur- ther proof of the observation of the T, state by oxygen

400 600 800 1000 ,200 1400

Wavenumbers (cm-‘)

Fig 2 Ramaa spectra in their T, state of (A) NPO, (B) NPO-d, and (C) POM

316 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAF Bri@ tr- Le Guiner et al / Chemicul Phy sics 182 (1994) 313- 323 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

, i(l’,) (arbitrary linear unit)

560 580 600 620

Wavelength (nm) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

540 860 580 600 620

Wavelength (nmf

Fig 3 Kaman enhancement profile of the T, strongest Raman peak

respectively at 982 cm-’ for (A) NPO and at 940 cm-’ for (B)

POM

quenching could not be achieved, due to the low solu-

bility of oxygen in water and to the fixed pump-probe

delay time, = 15 ns, which does not allow the obser-

vation of the decay kinetics

For POM, the T, ) 1;, absorption spectrum was still

unknown but according to the resonance Raman

enhancement profile, its maximum can be located

approximately at 560 nm zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

4 Assignment

In order to make the assignment of the bands, fol-

lowing the Franck-Condon principle [28] for reso-

nance Raman spectra, the enhanced modes must belong

to the A, symmetry of a common point subgroup shared

by the molecular geometries of the T, and T,, states If

both states have CzV symmetry, which means that the

molecule retains its planar configuration in T, and T,,

one should expect I 1 observable A, modes If the sym-

metry is lowered, for example if the NO, group is

twisted, the symmetry of the molecule would be C, and

there woufd be 18 observable A, modes Experimen-

tally, there are only 11 observed frequencies in the TR3

spectrum of NPO, 10 frequencies and a shoulder for

NPO-d, Therefore, the symmetry of Ti-NPO is CzV as

it is for S,-NPO

The T,-POM spectrum exhibits 13 Raman peaks

including two unresolved doublets Its general feature

is the same as in T,-NPO The two extra peaks at 1026 and 1420 cm-’ can be assigned without any doubt respectively to p(CH,) and s(CH3) They are only slightly shifted when passing from S, to T, states ( 1038 and 1417 cm- ’ in S,,-POM crystal)

A certain number of the Raman lines on the three spectra can be assigned unambiguousIy either because

of their comparable intensity, or due to the isotopic effects for NPO and NPO-d,, or by comparison with the spectra of the S,, states

The two very strong peaks at [ 1631, 1589, 1621 cm-‘] and [ 1434, 1368, 1401 cm-‘] for respectively [ NPO, NPO-I-I,, POM] can be assigned to the C-C or C-N stretching ring modes 8a and 19a (Wilson’s nota- tion for aromatic ring) They are observed at [ 1606,

1573, 1611 cm-‘] and [ 1476, 1388, 1478 cm-‘] for the S,) states in aqueous solutions

The two medium peaks at [ 1335,1325,1322 cm-‘]

and at [ 835,822,730 cm- ‘1 are assigned respectively

to v,(NO,) and mode I (breathing of the cycle) They appear at [ 1359, 1341, 1353 cm-‘] and at [ 875, 849, (no.) cm- ’ ] in the ground state It is to be noted that mode 1 is not observed f n.o.) for POM in the S, state

The weak and very weak peaks at [ 1289,12 lo,1302 cm ‘], [1098, 1072, 1085 cni-‘I, [631, 612, 615

cm - ’ ] and [ 368,362,368 cm ~ ’ ] are assigned respec- tively to modes 7a, 13, 12 and 6a These modes result from a coupling of the in plane (ip) cycle deformation and the stretching vibrations of the substituents They appear respectively at [ 1248, 1208, 1296 cm-‘], [1125, 1084,1098cm”“], [647,631,647cm-i] and [ 362, 366, 366 cm _ ’ ] in the ground state So

The in-plane deformation vibrations of NFO and POM are expected between 1000 and 1300 cm-’ and shifted to 700-1000 cm-’ ’ for NPO-& The very strong peaks at 982 cm- ’ for T,-NPO and 940 cm-’ for Ti- POM can be assigned to the f8a mode: they appear at

103 1 and 1032 cm ’ respectively for S,-NPO and So- POM A comparison of the frequency shifts of 18a for the St, and T, states of those two molecules suggests that the corresponding mode of T,-NPO-d, should appear around 730 cm- I However, it is not observed

It is to be noted that this mode is not observed either in the Raman spectrum of So-NPO-d, but only in its IR spectrum [ 13,141 The 9a mode is present in the three spectra at 1158 cm-’ for Ti-NPO, 888 cm-’ for T,- NPO-d, and 1144 cm- ’ for T,-POM The assignment

is based only on the comparison of the frequency shifts, since the intensities greatly vary from one compound

Table 1

Tl state Raman frequencies (cm- ‘) of NPO, NPO-d, and POM and their assignment The corresponding frequencies at the So state of these

compounds in aqueous solution are given for comparison (values in brackets are only observed in the crystalline state)

“Ref [24]

b Frequencies observed and assigned in this work (IR and Raman spectra of NPO-d, in zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAaqueous solution were not recorded before)

‘Seeref [35]

d o: in-plane cycle deformation; ox in-plane cycle deformation and substituents stretching; S,, (S,,) : in-plane deformation of C-H (C-D)

bonds; v,( NO*) : symmetrical stretching of the nitro group; p( CH,) : rocking of the methyl group; 8 (CH,) : deformation of the methyl group

to the other: weak for NPO, medium for NPO-d, and

strong for POM

These ten above-mentioned frequencies observed for

the Tr state were assigned without difficulty to the

totally symmetrical mode of the molecules possessing

the CZV symmetry The last peak of the T, state still

remains unassigned: the peak situated at 767 cm- ’ ( w)

for T,-NPO, at 809 cm-’ (m) for T,-NPO-d, and 789

cm-’ (m) for Tr-POM There are only two possible

assignments: the first one would be to assign it to

y (NO,) an out-of-plane (op) vibration which appears

at [752,700,753 cm-‘] [ 13-171 in the S,, state But

since no other op deformation modes were observed,

neither the op CH deformations nor the cycle op bend-

ing modes, this assignment would be very unlikely

The other possible assignment is the symmetricaldefor-

mation 6,(NO,) mode of the NO, group which is the

last totally symmetrical mode of the CZV symmetry, also

expected in this region Such an assignment would

make this mode the most perturbed one when passing

from Se to Tr : the frequency is lowered by 9 1 cm- ’ for

NPO, by 48 cm- ’ for NPO-d,, and by 59 cm-’ for

POM This important frequency shift will be discussed

in the next section

The assignment of the Raman peaks is summarized

in Table 1

5 Discussion 5.1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAThe cases of NPO and NPO- d,

Only the vibration modes are totally symmetrical under the constraints of the CZV point group are enhanced for T,-NPO and T,-NPO-d4 As already stated, these molecules retain their CZV symmetry in the

T, states Moreover, the enhanced modes are not only active on a specific molecular fragment but are delo- calized on the whole molecule However, the strongest intensities are the cycle vibrations (8a, 19a, 18a), which basically locate the T, + T, chromophore on the cycle

Although modifications in the description of the nor- mal modes are unavoidable when passing from the So

to the T, state, we will base our discussion about the frequency shifts on the results of the normal coordinate analysis (NCA) previously performed for NPO and NPO-d, in their ground states [ 161

The ip deformation modes 1,8a and 19a are modified

by the promotion of one electron from the bonding v orbital to the antibonding n* orbital:

(i) The important lowering of the cycle breathing frequency 1 (A V= v(T,) - v(T,) = - 40 and - 27 cm-‘) shows that, as for the benzene [ 291 and toluene

318

Fig 4 Some vibrational normal modes of NPO (ref [ 151) The

atomic motions of C N 0 and of H atoms are represented respec-

tively at the scale 10 and 2 (the atomic normalized coordinates are

taken as unity)

Fig 5 Two resonant forms of NPO at the ground state

[30] molecules, this transition induces a decrease of

the global charge on the cycle

(ii) In S,-NPO and S,-NPO-d4, 8a is due mainly to

the stretching vibration of the C-C bonds parallel to

the axis of the molecule This mode is very sensitive to

the quinonoid distorsion of the cycle The increase of

the frequency (A Y= 25 and 16 cm-‘) for NPO-d,

when passing from So to T, means that there is a rein-

forced quinonoid ring structure Malar and Jug [31]

reached the same conclusion for the pNA molecule

(iii) This is also confirmed by the frequency

decrease of the 19a mode (A Y = - 42 and - 20 cm ’ ) :

this mode is due to the C-C ring bonds which are not

parallel to the axis of the molecule

The lowering of the global charge of the cycle also

implies a weakening of the ip deformation of the C-H

bonds and consequently a frequency decrease of the 9a

and 18a modes While the NCA results [ 151 showed

an important contribution of the C-C stretching vibra- tions of the bonds parallel to the molecular axis for the 9a mode, on the contrary it showed for the 18a mode contribution of the C-C stretching vibrations of the bonds non-parallel to this axis (Fig 4) This can explain why the frequency shift is less important for 9a (Av= -22 cm-‘) than for 18a (Av= -49 cm-‘) between the S,-NPO and T,-NPO states Concerning NPO-d,, it is essentially the C-C stretching vibrations

of the bonds non-parallel to the molecular axis, in addi- tion to the deformation vibrations 6CD, who contribute

to the 9a mode; this is the reason why the frequency shift is more important for NPO-d, than for NPO

The u,(N02) vibration frequency mode is practi- cally the same for NPO and NPO-$ and is mainly due

to the symmetrical stretching of the NO bonds (Fig

4) The modes leading to the deformation of NO, and

to the stretching C-NO2 bonds also give a small con- tribution The frequency shift for u,(NO,) mode is nearly the same for NPO and NPO-d, ( A Y = - 24 and

- 16 cm- ’ ) and corresponds to a loss of charge density

on the NO, bonds

Normal modes 7a, 13, 12 and 6a are more complex and difficult to analyze Several coordinates implying both cycle and substituent motions contribute to those modes and the effect on the frequency shifts can be opposite (Fig 4) Only mode 7a is simple to describe

as it mainly originates in the N-O bond of the N-oxide group, at least in the case of NPO The increase of the frequency of this mode (A Y = + 4 1 cm- ’ ) then cor- responds to a significant increase of the N-O bond order

in the T, state It is worth noting that the frequency shift of this mode in NPO-d, is very small (A V= 2 cm- ’ ) This can be explained by the fact that the N-O stretching vibration contributes much less to 7a in this molecule than in NPO Moreover the deformation motions of the cycle angles also contribute to this mode and tend to lower its frequency as the global charge decreases

The frequency lowering and especially of the 6(NO,) mode, and to some extent, of v,(NO,) mode clearly imply that the N-O bond order of the NO, group diminishes in the T, state, and that charges are more localized on the oxygen atoms

The analysis of the frequency shifts between the S, and T, states unambiguously indicates that the elec- tronic structure of the T, state is derived from the Mul-

Ii

1

0

Fig 6 A major resonant form of NPO at the T, state The arrows

represent unpaired electrons

0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA0

Fig 8 Major resonant forms of POM at its T, state

liken resonant form II of Fig 5 This is at variance

from the So state for which it had been shown in polar

solvents that form I is largely predominant [ 141 (fig

5) Fig 6 displays the resonant forms that are most appropriate to describe the triplet state of NPO and NPO-d,

5.2 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAThe case of PO M

The great similarity of T,-POM and T,-NPO spectra suggests that the T, + T, transitions are quite compa- rable in the two molecules The resonant activity of the two methyl vibrations v(CH,) and S,(CH3) appears quite surprising This means that the motion of the methyl substituent should be coupled with that of the atoms implied in the T, + T,, transition process How- ever, these two modes in the T, state are practically not shifted with respect to the So state, thus suggesting that they are insensitive to the m + 7~* electronic transition

The frequency decrease of the C-H deformation modes 9a and 18a is quite similar to what was observed for NPO Mode 1 was not observed in the So state but was calculated by NCA to show up at 793 cm- ’ [ 321

This frequency is also expected to decrease when the molecule is promoted to the T, state in conformity with the behaviour characteristic of a diminution of the global charge on the cycle

The important decrease of the 19a mode frequency, which is composed of the stretching vibrations of the cycle bonds zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA1,2,5 suggests a diminution of the charge along those bonds On the opposite way, the slight

increase in frequency of the 8a mode, mainly implying the cycle bonds 2, 4 and 5, suggests a charge increase along bond 4 (Fig 7) Concerning the two last cycle bonds (3 and 6)) no straightforward conclusion can be deduced from the rest of the Raman spectrum of T,- POM

As for NPO, the frequency lowering of the v,( NO&

and 6( NOz) modes can be interpreted in terms of a loss

of the electronic density along the N-O bonds of the nitro group The potential energy distribution of mode 7a in the So [ 321 state shows equal contributions from the N-oxide stretching vibration and from the N-O stretching vibrations of the nitro group The increase in frequency of this mode indicates a strengthening of the N-oxide bond in the T,-POM The various contribu- tions to the modes w,, 13, 12 and 6a, namely the ip deformation of the cycle and of V( N-O) and u( NOa) have opposite effects on the frequency shifts which makes unreliable any attempt to interpret them

320 F Bnffaut-Le Guiner et al /Chemical Physics I82 (1994) 313-323 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

I 326

SO

Fig 9 The optimized geometry of the ground state and of the lowest

triplet state of NPO as obtained from ab initio SCF calculations

As for NPO, trends for the structure of the T,-POM

can be proposed from this analysis Figs 8(A) and

8(B) represent the two most probable structures, but

unlike T,-NPO for which the cycle has the quinonoid

conformation, in Tr-POM only bond 4 is strengthened

while bonds 1, 2 and 5 are weakened Those results

suggest that resonance form B probably competes with

the quinonoid form A

5.3 Comparison with ab initio calculations

The geometries optimized for the ground state and

for the lowest singlet state (IT f n*) of the NPO mol-

ecule are displayed in Fig 9 It should be noted that the

molecule remains planar in the T, state A comparison

between the structure computed for the ground state

and the geometry observed by Coppens and Lehmann

from neutron diffraction data [ 331 is displayed in Fig

10 The geometry optimizations have been carried out

with basis sets I For the ground state, an excellent

agreement with the experimental results has been

obtained, except for the N-O bond length, which was

computed to be 1.333 A, longer by 0.036 A, longer by

0.036 A than the experimental distance Calculations

carried out with the more flexible basis sets II decreased

the N-O distance to 1.326 A, and further decrease

should be expected from the influence of polarization

functions Apart from the N-O distance, the maximum

difference between experimental and calculated geom-

etries is 0.017 A,for the distances and 0.8’ for the angles

(Fig 9)

The deformation density distribution computed

using the polarized basis sets III (Figs 11 and 12) can

also be compared with multipole model maps derived

from the X-ray measurements Except for the oxygen

lone pair peaks which are as usual much sharper in the computed maps, the height of the density peaks dis- pIayed in the experimental model map is reproduced within an accuracy of = 10% Results of similarquality have been obtained for Leu-enkephalin [25] and for other polypeptides [25] using the same atomic basis sets It should be noted that the electron accumulation along the N-O bond is strongly displaced toward the

N atom in both the experimental and the theoretical maps

It is therefore of interest to correlate the spectro- scopic results obtained for NPO in the S, and T, states with the changes in the molecular geometry and in the density distribution evidenced from ab initio calcula- tions carried out on those two states The deformation

of the cycle toward a quinonoid structure in the triplet state is illustrated by the shortening of the C-C bonds parallel to the symmetry axis and the concomitant lengthening of the four other bonds of the cycle (Fig

9)

The only variation of interatomic distance which does not seem to be in straightforward agreement with the previous interpretation of the spectroscopic results

is the N-oxide distance Even though the Raman spec- tra indicate that the bond order is higher in Tr than in

So, the calculated N-O distance is longer in T, by 0.01

A However, the change in the equilibrium interatomic distance should account not only for the evolution of the bond order, but also for the variation of the electro- static interaction According to theMulliken population analysis, this interaction is globally attractive in the So state (computed point charges: + 0.10 e for N, - 0.59

I> n zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA0 0

Fig 10 The geometry of the ground state of NPO after theoretical calculations (A) and as observed by neutron diffraction (B) (ref

F Bri@ ~ut- Le Guiner et (11 /Chemiml Phy sics 182 (1994) 313- 323 321 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

,, I I -s.aoI,,,,,,,,,,,~,,,;~~~‘~-~~~~~~~~~~~~~~~,~~~~~~~~I,~~~~~~~~~,~~~~~:~~~,~~I~~~~~~i

-6.00 -4.00 -2.00 0.00 2.00 4.00 6.00 6.00

Fig 11 Deformation density maps for NPO obtained as the difference between the molecular density distribution and the density of a superposition

of spherical atoms, both obtained from ab initio SCF calculations (basis sets III) Zero contour bold, negative contours dashed Contour interval:

322 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAF Brijfuut- Le Guiner et al /Chemical Phy sics 182 (1994) 313- 323

Fig 12 Deformation density maps for NPO, defined as in Fig 10 (A) plane perpendicular to the molecular plane and containing the symmetry axis; singlet ground state So (B) same plane as in A; lowest triplet state T,

e for 0)) but becomes repulsive in the T, state ( - 0.09

e for N, - 0.18 e for 0) This modification of the

electrostatic balance should be attributed to the impor-

tant charge transfer occurring from the IT orbital of

oxygen to the 7~ orbital of nitrogen and eventually lead-

ing to an interchange between the regions of density

accumulation and depopulation along the IT system of

the N-oxide bond (Fig 12) Simultaneously, the N-O

u bond acquires in the triplet state more covalent char-

acter, as illustrated by the displacement of the bond

accumulation from the nitrogen side to the center of the

bond (Fig 11) Quite logically, this strengthening of

the covalent character of the N-O o bond has an influ-

ence on the curvature of the potential well near the

equilibrium Calculations carried out for several N-O

distances in the vicinity of the equilibrium position in the So and T, states show that the potential well is significantly sharper in the triplet state Assuming the potential energy curve to remain strictly parabolic, a 0.1 bohr deviation of the N-O distance with respect to the equilibrium position would correspond to an energy destabilization of 2.59 X lo-’ hartree for T, compared

to 1.84 X 10m3 hartree for S, This computed change in the curvature of the potential well associated with the N-O stretching is in agreement with the observed var- iation of the associated Raman frequency, in spite of the N-O distance being slightly longer in the triplet state