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1H-14N and 1H-17O nuclear quadrupole resonance in thermochromic N-5-chlorosalicylideneaniline
Chemical Physics 142 (1990) 237-244 North-Holland
IH-14N AND IH-l~O NUCLEAR QUADRUPOLE RESONANCE IN T H E R M O C H R O M I C N - 5 - C H L O R O S A L I C Y L I D E N E A N I L I N E J. SELIGER, V. ZAGAR, R. BLINC J. Stefan Institute, E. Kardelj University of Ljubljana, Ljubljana, Yugoslavia
E. H A D J O U D I S and F. MILIA N CR Demokritos Institute of Material Sciences, 153 10 Ag Paraskevi, Attikis, Athens, Greece Received 11 August 1989; in final form 5 December 1989
The temperature dependenceof the m4Nand ~0 NQR frequencieshas been measured in N-5-chlorosalicylideneanilinewith the help of nuclear quadrupole double resonance. The proton--oxygendistance has been determined from the dipolar structure of the 170 NQR lines. The results show that there is a fast exchange of protons between two inequivalent equilibrium sites in the O-H...N hydrogen bond. The energydifferenceof the two proton sites is 74 meV.
1. Introduction
Salicylideneanilines exhibit photochromic or thermochromic phenomena in the crystalline state [ 1,2 ]. In thermochromic compounds an absorption band is observed near 480 nm the intensity of which diminishes with decreasing temperature. The colour of these compounds changes from white at low temperature to yellow or orange at high temperatures. In photochromic compounds an absorption band in the same spectral region develops upon irradiation with UV light. Both processes are reversible and mutually exclusive [ 3 ]. The presence of an ortho-OH group is essential for the occurrence of thermochromism and photochromism [ 1 ]. In the thermochromic crystals the molecules are planar and closely packed while in the photochromic crystals the salicylaldimine part of the molecule is planar but the aniline ring is rotated 400-50 ° out of this plane and there is no close contact between the molecules [4,5 ]. Some of the compounds may crystallize in two polymorphic forms, one of which is thermochromic and another photochromic [6]. All the compounds which are either photochromic or thermochromic in the crystalline state become photochromic in rigid glassy solutions. In all the compounds photochromism has also been observed in dilute solutions by the flash photolysis 0301-010419015 03.50 © Elsevier Science Publishers B.V. (North-Holland)
technique. This leads to the conclusion [ 7 ] that the photochromism of salicylideneanilines is an intrinsic property of the molecules while the thermochromism is related to the crystal structure. It is generally agreed that in the thermochromic compounds there is a temperature sensitive equilibrium between two tautomers of the molecules: (i) the enol form in which the chelating hydrogen is covalently bonded to the oxygen and (ii) the cis-keto form in which the hydrogen is bonded to the nitrogen, //
/
\\
\ --..- ~
c\\
N
/I---~,, \ : /
\OH
)
/ H
/ ~ \ \\
i --
\\ 0
C\ N-I H
//~\ \ - - /
(I)
In the photochromic compounds the photoproduct is described as a trans-keto configuration of the cis-keto form:
238
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
so that
lH
/=\_c \\ / -
\
N
-\\
I
0
_
V+~V_>/V,.
//-\\
\-/
-
H H
1
/=\ \\
-\\
,
/ = c,
I N-y-,-\\
H
(11)
In case of a fast exchange between the molecular configurations 1 (ground state) and 2 (excited state), with energies E, and Ez respectively and with the corresponding EFG tensors VI and V2, the NQR frequencies depend on the time averaged EFG tensor*
0
(V) = (1 -P)V, The microscopic nature of thermochromism and photochromism in the Schiff bases as well as the relation of those properties to the molecular and crystal structure are not yet completely understood. In order to test the proton transfer model of thermochromism in N-5-chlorosalicylideneaniline, we decided to perform a precise measurement of the temperature dependence of the 14Nand “0 nuclear quadrupole resonance (NQR) frequencies and of the dipolar structure of the “0 lines. Preliminary results of the 14NNQR study of the same compound have already been published [ 8 1. A comparative study of various substituted photochromic and thermochromic salicylideneanilines and salicylideneaminopyridines will be published later.
14N (I= 1) has in zero magnetic field three quadrupole energy levels. The corresponding NQR frequencies depend on the nuclear quadrupole moment eQ and on the eigenvalues V,,, I’,, and V,, of the electric field gradient (EFG) tensor (V)ij=@ V/ 3XiaXjat the nuclear site as:
v,,)/2h=eQ1/,,(3+r1)/4h,
v-=eQ(V,,-1/,,)/2h=eQV,,(3_tt)/4h, vg=v+-
v_ =eQ( Vxx-
(la) (lb)
Vy,)/2h=eQVzzq/2h. (lc)
Here V is the electrostatic potential at the nuclear site, eQV_&h is the quadrupole coupling constant and tl= (V,W- V,,) / Vzz is the asymmetry parameter of the EFG tensor. The eigenvalues of the EFG tensor are ordered in the following way:
V,) )
(2)
where P is the probability of finding a molecule in the configuration 2: P={l+exp[(&-E,)/kT]}-‘.
(3)
From the temperature dependence of the NQR frequencies one can thus determine the energy difference El-E,. When the exchange becomes slow so that the inverse value of the lifetime r2 of the excited state becomes comparable to the NQR frequencies, the spin-lattice relaxation time of nitrogen shortens and a dynamical broadening of the NQR lines occurs [ 91. “0 (I= 5/2) has in zero magnetic field three doubly degenerate quadrupole energy levels with the energies Ei= eQVzz&/20. Here Xi are the solutions of the secular equation X3-7(3+~2)X-20(1-~2)=0.
2. Experimental
v+ =eQ(V==-
+A!2 =v, +P(vz-
(4)
The corresponding NQR frequencies are labeled in the following way: v5/2,1/2
’
v5/2,3/2
3
v3/2,1/2
.
None of the transitions is forbidden when the asymmetry parameter q is nonzero. In a hydroxyl group the strong magnetic dipolar coupling between the protons and “0 nuclei removes the degeneracy of the quadrupole energy levels. Each energy level splits into a quartet. The resonance lines become broad and structured [ 10 1. From the structure and width of the NQR lines it is possible to determine the proton-oxygen distance R(OH) and the orientation of R (O-H) in the eigenfiame of the EFG tensor [ 10,111. This orientation may be described by the polar angle 19between the direction of the O-H bond and the Z axis of the EFG tensor and by the azimuthal angle Q,between the projection of R (O-H ) on the XY plane and the X axis.
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
If in a hydrogen bond the proton jumps with a high rate between two equilibrium sites with the protonoxygen distances R (O-H ) and R ( 0. ..H ) , the dipolar broadening of the NQR lines depends on the time averaged O-H distance R defined as [ 12 ] R-3=(1-P)R-3(0-H)+PR-3(0...H).
(5)
Here P is the probability of finding the proton in the site where the O-H distance is R (O...H ). Both 14N and “0 NQR frequencies have been measured with the help of a nuclear quadrupole double resonance technique based on magnetic field cycling and two-frequency irradiation. A measuring cycle consists of the following steps: First the sample stays in a high magnetic field B0 until the proton magnetization reaches its equilibrium value M,,= CB,/ T,_. Here C is the Curie constant and T, is the lattice temperature. Then the sample is adiabatically transferred into a low magnetic field B, which results in a significant decrease of the proton spin temperature. During the stay in low magnetic field the proton spin temperature increases due to the spinlattice relaxation. If in addition two rf magnetic fields are applied with frequencies close to a “0 NQR frequency or close to the upper two 14N NQR frequencies, the proton relaxation rate increases. In the former case the proton relaxation increases when B=O while in the latter case it increases when the proton Larmor frequency vH = y,B/2n is equal to the lowest nitrogen NQR frequency vo. After a time T which is approximately equal to the proton spin-lattice relaxation time T, in the low magnetic field B, the sample is transferred back into the initial magnetic field B. and the proton free induction decay signal is measured. It is proportional to the remaining proton magnetization and it decreases when the proton relaxation rate in the low magnetic field increases. The measurement of the “0 NQR spectra consists of two steps: (i) rough determination of the NQR frequencies and (ii) measurement of the dipolar structure of the NQR lines. First the low magnetic field B is set to zero. Then the frequency difference of the two rf magnetic fields v, - v2 is set to a fixed value Au smaller than the widths of the dipolarly broadened “0 NQR lines and the measuring cycles are repeated at different center frequencies (vi + v2) / 2. A decrease of the proton free induction signal is observed when the two frequencies vi and v2 are lying
239
within a dipolarly broadened “0 NQR line [ lo]. Thus the position and the width of the NQR lines are determined. Then one of the frequencies, vI, is fixed at the lower edge of a NQR line and the shape of the resonance line is scanned with another frequency v2. A decrease of the proton free induction signal is observed always when the frequency v2 hits a resonance frequency of the OH group corresponding to the same NQR transition. The dipolar structure of the lower part of the NQR line is determined with a similar measurement in which vi is fixed at the upper edge of the NQR line. The 14N NQR frequencies are also measured in two steps. First the lowest NQR frequency v. is determined. The measuring cycles without the rf irradiation are repeated at different values of the low magnetic field B. When the proton Larmor frequency becomes equal to vo/2 or to v. a decrease of the proton magnetization is observed. It is due to a single resonant coupling of nitrogens and protons with unequally populated energy levels. A larger decrease of the proton magnetization is observed when the nitrogens relax fast so that the spin-lattice relaxation rate of the resonantly coupled proton-nitrogen system is higher than the proton spin-lattice relaxation rate. In samples in which the nitrogens relax slowly, a large decrease of the proton magnetization at vu = v. is observed when during the stay in the low magnetic field, a rf magnetic field is applied the frequency of which is sweeping many times between vi < v_ and v2 > v+. When the irradiation frequency passes a NQR frequency the populations of the corresponding two nitrogen energy levels change. This is in fact similar to a fast spin-lattice relaxation of nitrogens. When the lowest NQR frequency v. is determined, the frequencies of the rf magnetic fields are set to v and v+ v. and the cycles are repeated at different frequencies v. In resonance when v= v_ and v+ vo= v+ a large decrease of the proton signal is observed. In this way the nitrogen NQR frequencies are measured with high sensitivity and with a resolution which is tyljically a few kHz. The nitrogen spin-lattice relaxation rates can be determined from the Larmor frequency dependence of the proton spin-lattice relaxation rate. When vu> v. the proton spin-lattice relaxation rate is equal to WH while in resonance when vu = vo it is equal to
240
+~[w,+w+w_/(w++w_)].
J. Seliger et cil. / NQR in N-5-chlorosalicylideneaniline
(6)
Here E is twice the number of the chemically equivalent nitrogens per molecule divided by three times the number of protons per molecule and W+, W_ and W,, are the nitrogen spin-lattice relaxation rates corresponding to the NQR transitions with the frequencies v,, v_ and vo. When during the stay in the low magnetic field a rf magnetic field is applied with frequency v= v,, the nitrogen contribution to the proton spin-lattice relaxation rate in resonance changes to c( Wo+ W_). When v= v_ and vu= v. the nitrogen contribution to the proton spin-lattice relaxation rate becomes equal to e ( Wo+ W+ ). From the three nitrogen contributions to the proton spin-lattice relaxation rate on resonance one can in principle calculate the three nitrogen spin-lattice relaxation rates W,, W_ and W,.
IlitY*l N-5-
“N NQR IN CHLOROSALICYLIOENEANILINE
e 0 0
Z.!l-
0
2.8,_
0
2.1,_
> 0 0 0
3. Results and discussion The temperature dependences of the 14NNQR frequencies v, and v_ in N-5-chlorosalicylideneaniline are shown in fig. 1. The temperature dependences of the eigenvalues Vzz and VXXof the EFG tensor at the nitrogen site multiplied by eQ/h are shown in fig. 2. In contrast to the previously published data [ 13 ] only a single nitrogen site (site a) has been observed. Since the sensitivity and resolution of the present experimental technique are much higher than the sensitivity and resolution of the solid-effect technique applied in the earlier work, we believe that the additional lines (site b) were an experimental artefact. Another possibility is that the additional NQR lines are the consequence of the y irradiation of the sample which had been used in the earlier work in order to shorten the proton spin-lattice relaxation time T,. As seen from fig. 2, Vzz strongly decreases with increasing temperature while Vxx is nearly temperature independent. From the electric charge distribution around the nitrogen nuclei we may conclude that the Z axis of the EFG tensor lies within the molecular plane and is close to the intermediate molecular axis. Another principal axis X or Y is perpendicular to the molecular plane while the third principal axis Y or X
,2.6 _^^ -1LU
I _^
-IN
I ._
-4U
I
U
I ._
”
P
T[“C
1
Fig. 1. Temperaturedependences of the “N NQR frequencies v+ and v_ in N-5-chlorosalicylideneaniline.
is close to the long molecular axis. The analysis of the X-ray data in N-5chlorosalicylideneaniline [ 41 had shown the presence of two dominant molecular motions: the isotropic motion which does not change the EFG tensor at the nitrogen site and the librational motion around the long molecular axis which influences the principal values of the EFG tensor Vzz and Vxx or Vrr. A nearly linear temperature dependence of the corresponding two principal values of the EFG tensor is expected for this motion. Since Vxx is nearly temperature independent we may assume that the X axis is close to the long molecular axis and the Y axis is perpendicular to the molecular plane. The observed temperature dependence of V,, and Vzz is rather strong and nonlinear. The influence of the librational modes can be as well observed on the temperature dependence of the WI NQR frequency at the position 5 of the salicylaldehyde ring. Since this position is far from the 0-H...N bond, the influence
241
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
lLN N-5-
NQR
IN
CHLOROSALICYLIOENEANILINE
eWZ
h [MHz1
’
o
0
3.60 t -120
,
,
-80
- 40
,
,
o’
b
Ez - E, = 74 meV derived from P is in good agrcement with the thermal [ 1 ] and optical [2] data. The temperature dependence of the “0 NQR freqUenCieS v5/2,3/2 and V3/2,~/2iS shown in fig. 3. The corresponding quadrupole coupling constant and asymmetry parameter are plotted as a function of temperature in fig. 4. Since the temperature variation of the principal values of the EFG tensor is weak we may in eq. (2 ) consider only the part of the EFG tensor V2.which is diagonal in the eigenframe of the EFG tensor V, . The experimentally determined principal values of the EFG tensor at the “0 site can be again well described by eq. ( 2 ) ; eQr/zz/h=8035
kHz-Px2000
kHz ,
(8a)
- 5925 kHz+ Px 3900 kHz ,
eQr/uJh=
(8b)
eQvXX/h = - 2110kHz-Px1900kHz, q
ID
D,D
0:
-40
0
with the same probability Pas in the case of nitrogen.
04,0:;
170 - 120
-80
40
l[‘Cl
Fig. 2. Temperaturedependences of the “N eQV,/h and eQV,l h in N-5-chlorosalicylideneaniline.
of the proton transfer to the 35C1NQR frequency is expected to be small. The experimental results of ref. [ 81 show that in the temperature interval between - 100°C and 50°C the relative change Au/v of the “Cl NQR frequency is approximately 0.01 while in the same temperature interval the relative change of the i4N and “0 NQR frequencies is approximately 0.06 which is much more than in the case of chlorine. Therefore we can safely assume that the temperature dependences of the 14Nand 0” NQR frequencies are mainly due to the proton transfer. The temperature dependence of V,, V,, and VZZ at the nitrogen site can be well described by eq. (2). In this case we have: eQVz-/h=3767
Wz-Px24OOkHz,
eQVrr/h=
-2152 kHz+P~23OOkHz,
eQI’&h=
- 1615 kHz+Px
1OOkHz ,
(8~)
(7a) (7b) (7c)
where P is given by eq. ( 3 ). The energy difference
NOR
IN
N-5-CHLOROSALICYLIOENEANILINE 2.35
0
0
0 0
2.30
0 0 0
1.50
1.45
0
1.40
I
-100
I
-60
I
-20
I
2o
I
TVCI
Fig. 3. Temperature dependences of the “0 NQR frequencies us,2,3,2and v,,~.,,~ in N-S-chlorosalicylideneaniline.
242
J. Seliger et al. / NQR in N-khlorosalicylideneaniline
170
NQR IN
‘H -“O NQOR LINESHAPES IN N- 5- CHLOROSALlCYLlDENEANltlNE
N-5-CHLOROSALICYLIOENEANltlNE
T =23”C
7.95 7.9C
0
I
-100
I
-60
I
-20
I
2o
T[“Cl
T 0.48
.
G
0
z
0
0
0
I
-100
2.25
E
0.46
I
-60
I
-20
2.30
2.35 v#lHzl
I
2o
11°C
I
Fig. 4. Temperature dependences of the “0 quadrupole coupling constant eQV&h and the asymmetry parameter q in N-5chlorosalicylideneaniline.
In order to find the O-H distance, the sign of V,,and the orientation of the principal axes of the EFG tensor in the molecular frame we performed the measurement of the dipolar structure of the “0 NQR lines at 296 and 145 K. The experimental results at 296 K are shown in fig. 5. The quadrupole coupling constants, asymmetry parameters, O-H distances and the angles 19and @are presented in table 1. As we see from table 1 at both low and high temperature the Z axis lies in the molecular plane and forms an angle rY= 57’ with the direction of the O-H bond. The Y axis is perpendicular to the molecular plane. On going from the room temperature to 145 K the dipolar structure of the “0 NQR lines does not change while the width increases by approximately 5%. The experimentally determined O-H distance is thus at room temperature approximately 2% longer than at low temperature. So if we set the probability of finding the molecule in the excited state at room temperature equal to P=O.O5 then the above results
3.65
3.70
3.75
v2[MHzl
Fig. 5. Dipolar structures of the “0 NQR lines in N-S-chlorosalicylideneaniline at 23°C as obtained with the help of the twofrequency irradiation technique. The arrows indicate the fixed irradiation frequencies v,. The positions of the resonance lines calculated with the parameters given in table 1 are indicated on the frequency scales.
are consistent
with
eq.
(5)
if we assume
that that the proton is jumping in the 0-H...N hydrogen bond between two inequivalent equilibrium sites so that in the ground state it is close to the oxygen while in the excited state it is close to the nitrogen. The jump rate may be in principle determined from the proton or nitrogen spin-lattice relaxation rate or from the temperature dependence of the width of the 14N NQR lines. The width of the 14N NQR lines is within the experimental resolution equal to 5 kHz and temperature independent. It is mainly determined by the resonant proton-nitrogen dipolar coupling. The proton spin-lattice relaxation time T, at 32 MHz is equal to 40 s and is nearly temperature independent. In the low frequency region between 1 and 2 MHz, T, is equal to 15 s and is nearly temperature indepen-
R 3( O...H ) 3 R 3(O-H ) . We may thus conclude
243
J. Seliger et al. / NQR in N-5-chlorosakylideneanilidene
Table 1 Quadrupole coupling constants, asymmetry parameters, O-H distances and the angles 6 and 4 from the dipolar structureof the “0 NQR lines Temperature (K)
eQVz.zlh &Hz)
1
R(O-H) (nm)
6 (deg)
Q (de@
145 296
8030 7940
0.413 0.445
0.100 0.102
57f5 57f5
Ok20 Of20
SPIN- LATTICE
RELAXATION
dent below room temperature. No nitrogen contribution to T, at vu = v. has been observed at any temperature. Since the fluctuations of the 14NEFG tensor are not small we may conclude that the absence of the nitrogen contribution to the proton T, is due to the fact that the lifetime of the excited state is rather short, presumably shorter than 10-‘” s. Above room temperature, a frequency dependence of the proton T, has been observed at low Larmor frequencies. The Larmor frequency dependence of the proton T, at 50 and 90°C is shown in fig. 6. At 50°C the correlation time of the fluctuations which relax the proton system is equal to 0.12 us and it decreases to 0.06 us at 90°C. The extrapolation of the spin-lattice relaxation rate which is due to the slow motion to zero frequency is nearly temperature independent. Such a behavior of the proton spin-lattice relaxation rate is expected when an exchange between the ground state and one or more excited states takes place [ 14 1. This slow motion does not affect the t4N NQR frequencies and the spin-lattice relaxation. One possible assumption is that the aniline ring rotates for a small angle about the C-N bond with respect to the rest of the molecule. Further experiments are needed to prove this assumption.
4. Conclusion The temperature dependences of the 14N and “0 NQR frequencies show a fast proton exchange between the ground and excited states in the asymmetric 0-H...N double minimum H-bond potential of NS-chlorosalicylideneaniline. The energy difference of the two states is 74 meV, consistent with the value determined previously from other experiments. The exchange is fast on the NQR time scale and does not influence the width of the NQR lines. The nitrogen spin-lattice relaxation rates are lower than 1 s-r,
PROTON N-5-
IN
CHLOROSALlCYLlOENEANltlNE
A-
0
o
00
0
0 0
12 -
0
10 -
0 a6-
0
0
0
0
0
T=50”C
o” I
0
I
I
I
vL [ MHz1
2
1
Is1TI -
0 0
12 -
0 0
10 -
II-
66 0
I
I
0
T =9o”C
0 0 0
1
0
0
0
0 0
0
2
vJMHz1
Fig. 6. Larmor frequency dependence of T, in the N-5chlorosalicylideneaniline.
which is consistent with the fast exchange limit. The time averaged O-H distance as determined from the dipolar structure of the “0 NQR lines decreases with decreasing temperature, directly demonstrating that the proton is moving rapidly in the 0-H...N hydrogen bond. At room temperature the proton spends about 5% of its time in the O...H-N configuration and 95% in the 0-H...N configuration.
244
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
References [ 1 ] M.D. Cohen and G.M.J. Schmidt, J. Phys. Chem. 66 ( 1962) 2442. [ 21 E. Hadjoudis, M. Vittorakis and I. Moustakali-Mavridis, Tetrahedron 43 ( 1987) 1345. [ 31 M.D. Cohen, G.M.J. Schmidt and S. Flavian, J. Chem. Sot. (1964) 2041. [4] J. Bregman, L. Leiserowitz and G.M.J. Schmidt, J. Chem. Sot. ( 1964) 2068. [ 5] J. Bregman, L. Leiserowitx and K. Osaki, J. Chem. Sot. ( 1964) 2086. [ 61 A. Senier, F.G. Shepheard and R. Clarke, J. Chem. Sot. 101 (1952) 1912.
[ 71 G.M.J. Schmidt, in: Reactivity of the Photoexcited Organic Molecules (Interscience, New York, 1967) p. 227. [ 81 F. Milia, E. Hadjoudis and J. Seliger, J. Mol. Struct. 177 (1988) 191. [ 91 A. Abragam, The Principles of Nuclear Magnetism (Clarendon Press, Oxford, 196 1). [ 1OlS.G.P. Brosnan and D.T. Edmonds, J. Magn. Reson. 38 (1980) 47. [ 111 J. Seliger, V. Zagar, R. Blinc and A. Novak, J. Chem. Phys. 84 (1986) 5857. [ 121 S.G.P. Brosnan and D.T. Edmonds, Phys. Letters A 81 (1981) 243. [ 131 E. Hadjoudis, F. Milia, J. Seliger, R. Blinc and V. Zagar, Chem. Phys. 47 (1980) 105. [ 141 D.C. Look and I.J. Lowe, J. Chem. Phys. 44 (1966) 3437.
IH-14N AND IH-l~O NUCLEAR QUADRUPOLE RESONANCE IN T H E R M O C H R O M I C N - 5 - C H L O R O S A L I C Y L I D E N E A N I L I N E J. SELIGER, V. ZAGAR, R. BLINC J. Stefan Institute, E. Kardelj University of Ljubljana, Ljubljana, Yugoslavia
E. H A D J O U D I S and F. MILIA N CR Demokritos Institute of Material Sciences, 153 10 Ag Paraskevi, Attikis, Athens, Greece Received 11 August 1989; in final form 5 December 1989
The temperature dependenceof the m4Nand ~0 NQR frequencieshas been measured in N-5-chlorosalicylideneanilinewith the help of nuclear quadrupole double resonance. The proton--oxygendistance has been determined from the dipolar structure of the 170 NQR lines. The results show that there is a fast exchange of protons between two inequivalent equilibrium sites in the O-H...N hydrogen bond. The energydifferenceof the two proton sites is 74 meV.
1. Introduction
Salicylideneanilines exhibit photochromic or thermochromic phenomena in the crystalline state [ 1,2 ]. In thermochromic compounds an absorption band is observed near 480 nm the intensity of which diminishes with decreasing temperature. The colour of these compounds changes from white at low temperature to yellow or orange at high temperatures. In photochromic compounds an absorption band in the same spectral region develops upon irradiation with UV light. Both processes are reversible and mutually exclusive [ 3 ]. The presence of an ortho-OH group is essential for the occurrence of thermochromism and photochromism [ 1 ]. In the thermochromic crystals the molecules are planar and closely packed while in the photochromic crystals the salicylaldimine part of the molecule is planar but the aniline ring is rotated 400-50 ° out of this plane and there is no close contact between the molecules [4,5 ]. Some of the compounds may crystallize in two polymorphic forms, one of which is thermochromic and another photochromic [6]. All the compounds which are either photochromic or thermochromic in the crystalline state become photochromic in rigid glassy solutions. In all the compounds photochromism has also been observed in dilute solutions by the flash photolysis 0301-010419015 03.50 © Elsevier Science Publishers B.V. (North-Holland)
technique. This leads to the conclusion [ 7 ] that the photochromism of salicylideneanilines is an intrinsic property of the molecules while the thermochromism is related to the crystal structure. It is generally agreed that in the thermochromic compounds there is a temperature sensitive equilibrium between two tautomers of the molecules: (i) the enol form in which the chelating hydrogen is covalently bonded to the oxygen and (ii) the cis-keto form in which the hydrogen is bonded to the nitrogen, //
/
\\
\ --..- ~
c\\
N
/I---~,, \ : /
\OH
)
/ H
/ ~ \ \\
i --
\\ 0
C\ N-I H
//~\ \ - - /
(I)
In the photochromic compounds the photoproduct is described as a trans-keto configuration of the cis-keto form:
238
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
so that
lH
/=\_c \\ / -
\
N
-\\
I
0
_
V+~V_>/V,.
//-\\
\-/
-
H H
1
/=\ \\
-\\
,
/ = c,
I N-y-,-\\
H
(11)
In case of a fast exchange between the molecular configurations 1 (ground state) and 2 (excited state), with energies E, and Ez respectively and with the corresponding EFG tensors VI and V2, the NQR frequencies depend on the time averaged EFG tensor
0
(V) = (1 -P)V, The microscopic nature of thermochromism and photochromism in the Schiff bases as well as the relation of those properties to the molecular and crystal structure are not yet completely understood. In order to test the proton transfer model of thermochromism in N-5-chlorosalicylideneaniline, we decided to perform a precise measurement of the temperature dependence of the 14Nand “0 nuclear quadrupole resonance (NQR) frequencies and of the dipolar structure of the “0 lines. Preliminary results of the 14NNQR study of the same compound have already been published [ 8 1. A comparative study of various substituted photochromic and thermochromic salicylideneanilines and salicylideneaminopyridines will be published later.
14N (I= 1) has in zero magnetic field three quadrupole energy levels. The corresponding NQR frequencies depend on the nuclear quadrupole moment eQ and on the eigenvalues V,,, I’,, and V,, of the electric field gradient (EFG) tensor (V)ij=@ V/ 3XiaXjat the nuclear site as:
v,,)/2h=eQ1/,,(3+r1)/4h,
v-=eQ(V,,-1/,,)/2h=eQV,,(3_tt)/4h, vg=v+-
v_ =eQ( Vxx-
(la) (lb)
Vy,)/2h=eQVzzq/2h. (lc)
Here V is the electrostatic potential at the nuclear site, eQV_&h is the quadrupole coupling constant and tl= (V,W- V,,) / Vzz is the asymmetry parameter of the EFG tensor. The eigenvalues of the EFG tensor are ordered in the following way:
V,) )
(2)
where P is the probability of finding a molecule in the configuration 2: P={l+exp[(&-E,)/kT]}-‘.
(3)
From the temperature dependence of the NQR frequencies one can thus determine the energy difference El-E,. When the exchange becomes slow so that the inverse value of the lifetime r2 of the excited state becomes comparable to the NQR frequencies, the spin-lattice relaxation time of nitrogen shortens and a dynamical broadening of the NQR lines occurs [ 91. “0 (I= 5/2) has in zero magnetic field three doubly degenerate quadrupole energy levels with the energies Ei= eQVzz&/20. Here Xi are the solutions of the secular equation X3-7(3+~2)X-20(1-~2)=0.
2. Experimental
v+ =eQ(V==-
+A!2 =v, +P(vz-
(4)
The corresponding NQR frequencies are labeled in the following way: v5/2,1/2
’
v5/2,3/2
3
v3/2,1/2
.
None of the transitions is forbidden when the asymmetry parameter q is nonzero. In a hydroxyl group the strong magnetic dipolar coupling between the protons and “0 nuclei removes the degeneracy of the quadrupole energy levels. Each energy level splits into a quartet. The resonance lines become broad and structured [ 10 1. From the structure and width of the NQR lines it is possible to determine the proton-oxygen distance R(OH) and the orientation of R (O-H) in the eigenfiame of the EFG tensor [ 10,111. This orientation may be described by the polar angle 19between the direction of the O-H bond and the Z axis of the EFG tensor and by the azimuthal angle Q,between the projection of R (O-H ) on the XY plane and the X axis.
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
If in a hydrogen bond the proton jumps with a high rate between two equilibrium sites with the protonoxygen distances R (O-H ) and R ( 0. ..H ) , the dipolar broadening of the NQR lines depends on the time averaged O-H distance R defined as [ 12 ] R-3=(1-P)R-3(0-H)+PR-3(0...H).
(5)
Here P is the probability of finding the proton in the site where the O-H distance is R (O...H ). Both 14N and “0 NQR frequencies have been measured with the help of a nuclear quadrupole double resonance technique based on magnetic field cycling and two-frequency irradiation. A measuring cycle consists of the following steps: First the sample stays in a high magnetic field B0 until the proton magnetization reaches its equilibrium value M,,= CB,/ T,_. Here C is the Curie constant and T, is the lattice temperature. Then the sample is adiabatically transferred into a low magnetic field B, which results in a significant decrease of the proton spin temperature. During the stay in low magnetic field the proton spin temperature increases due to the spinlattice relaxation. If in addition two rf magnetic fields are applied with frequencies close to a “0 NQR frequency or close to the upper two 14N NQR frequencies, the proton relaxation rate increases. In the former case the proton relaxation increases when B=O while in the latter case it increases when the proton Larmor frequency vH = y,B/2n is equal to the lowest nitrogen NQR frequency vo. After a time T which is approximately equal to the proton spin-lattice relaxation time T, in the low magnetic field B, the sample is transferred back into the initial magnetic field B. and the proton free induction decay signal is measured. It is proportional to the remaining proton magnetization and it decreases when the proton relaxation rate in the low magnetic field increases. The measurement of the “0 NQR spectra consists of two steps: (i) rough determination of the NQR frequencies and (ii) measurement of the dipolar structure of the NQR lines. First the low magnetic field B is set to zero. Then the frequency difference of the two rf magnetic fields v, - v2 is set to a fixed value Au smaller than the widths of the dipolarly broadened “0 NQR lines and the measuring cycles are repeated at different center frequencies (vi + v2) / 2. A decrease of the proton free induction signal is observed when the two frequencies vi and v2 are lying
239
within a dipolarly broadened “0 NQR line [ lo]. Thus the position and the width of the NQR lines are determined. Then one of the frequencies, vI, is fixed at the lower edge of a NQR line and the shape of the resonance line is scanned with another frequency v2. A decrease of the proton free induction signal is observed always when the frequency v2 hits a resonance frequency of the OH group corresponding to the same NQR transition. The dipolar structure of the lower part of the NQR line is determined with a similar measurement in which vi is fixed at the upper edge of the NQR line. The 14N NQR frequencies are also measured in two steps. First the lowest NQR frequency v. is determined. The measuring cycles without the rf irradiation are repeated at different values of the low magnetic field B. When the proton Larmor frequency becomes equal to vo/2 or to v. a decrease of the proton magnetization is observed. It is due to a single resonant coupling of nitrogens and protons with unequally populated energy levels. A larger decrease of the proton magnetization is observed when the nitrogens relax fast so that the spin-lattice relaxation rate of the resonantly coupled proton-nitrogen system is higher than the proton spin-lattice relaxation rate. In samples in which the nitrogens relax slowly, a large decrease of the proton magnetization at vu = v. is observed when during the stay in the low magnetic field, a rf magnetic field is applied the frequency of which is sweeping many times between vi < v_ and v2 > v+. When the irradiation frequency passes a NQR frequency the populations of the corresponding two nitrogen energy levels change. This is in fact similar to a fast spin-lattice relaxation of nitrogens. When the lowest NQR frequency v. is determined, the frequencies of the rf magnetic fields are set to v and v+ v. and the cycles are repeated at different frequencies v. In resonance when v= v_ and v+ vo= v+ a large decrease of the proton signal is observed. In this way the nitrogen NQR frequencies are measured with high sensitivity and with a resolution which is tyljically a few kHz. The nitrogen spin-lattice relaxation rates can be determined from the Larmor frequency dependence of the proton spin-lattice relaxation rate. When vu> v. the proton spin-lattice relaxation rate is equal to WH while in resonance when vu = vo it is equal to
240
+~[w,+w+w_/(w++w_)].
J. Seliger et cil. / NQR in N-5-chlorosalicylideneaniline
(6)
Here E is twice the number of the chemically equivalent nitrogens per molecule divided by three times the number of protons per molecule and W+, W_ and W,, are the nitrogen spin-lattice relaxation rates corresponding to the NQR transitions with the frequencies v,, v_ and vo. When during the stay in the low magnetic field a rf magnetic field is applied with frequency v= v,, the nitrogen contribution to the proton spin-lattice relaxation rate in resonance changes to c( Wo+ W_). When v= v_ and vu= v. the nitrogen contribution to the proton spin-lattice relaxation rate becomes equal to e ( Wo+ W+ ). From the three nitrogen contributions to the proton spin-lattice relaxation rate on resonance one can in principle calculate the three nitrogen spin-lattice relaxation rates W,, W_ and W,.
IlitY*l N-5-
“N NQR IN CHLOROSALICYLIOENEANILINE
e 0 0
Z.!l-
0
2.8,_
0
2.1,_
> 0 0 0
3. Results and discussion The temperature dependences of the 14NNQR frequencies v, and v_ in N-5-chlorosalicylideneaniline are shown in fig. 1. The temperature dependences of the eigenvalues Vzz and VXXof the EFG tensor at the nitrogen site multiplied by eQ/h are shown in fig. 2. In contrast to the previously published data [ 13 ] only a single nitrogen site (site a) has been observed. Since the sensitivity and resolution of the present experimental technique are much higher than the sensitivity and resolution of the solid-effect technique applied in the earlier work, we believe that the additional lines (site b) were an experimental artefact. Another possibility is that the additional NQR lines are the consequence of the y irradiation of the sample which had been used in the earlier work in order to shorten the proton spin-lattice relaxation time T,. As seen from fig. 2, Vzz strongly decreases with increasing temperature while Vxx is nearly temperature independent. From the electric charge distribution around the nitrogen nuclei we may conclude that the Z axis of the EFG tensor lies within the molecular plane and is close to the intermediate molecular axis. Another principal axis X or Y is perpendicular to the molecular plane while the third principal axis Y or X
,2.6 _^^ -1LU
I _^
-IN
I ._
-4U
I
U
I ._
”
P
T[“C
1
Fig. 1. Temperaturedependences of the “N NQR frequencies v+ and v_ in N-5-chlorosalicylideneaniline.
is close to the long molecular axis. The analysis of the X-ray data in N-5chlorosalicylideneaniline [ 41 had shown the presence of two dominant molecular motions: the isotropic motion which does not change the EFG tensor at the nitrogen site and the librational motion around the long molecular axis which influences the principal values of the EFG tensor Vzz and Vxx or Vrr. A nearly linear temperature dependence of the corresponding two principal values of the EFG tensor is expected for this motion. Since Vxx is nearly temperature independent we may assume that the X axis is close to the long molecular axis and the Y axis is perpendicular to the molecular plane. The observed temperature dependence of V,, and Vzz is rather strong and nonlinear. The influence of the librational modes can be as well observed on the temperature dependence of the WI NQR frequency at the position 5 of the salicylaldehyde ring. Since this position is far from the 0-H...N bond, the influence
241
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
lLN N-5-
NQR
IN
CHLOROSALICYLIOENEANILINE
eWZ
h [MHz1
’
o
0
3.60 t -120
,
,
-80
- 40
,
,
o’
b
Ez - E, = 74 meV derived from P is in good agrcement with the thermal [ 1 ] and optical [2] data. The temperature dependence of the “0 NQR freqUenCieS v5/2,3/2 and V3/2,~/2iS shown in fig. 3. The corresponding quadrupole coupling constant and asymmetry parameter are plotted as a function of temperature in fig. 4. Since the temperature variation of the principal values of the EFG tensor is weak we may in eq. (2 ) consider only the part of the EFG tensor V2.which is diagonal in the eigenframe of the EFG tensor V, . The experimentally determined principal values of the EFG tensor at the “0 site can be again well described by eq. ( 2 ) ; eQr/zz/h=8035
kHz-Px2000
kHz ,
(8a)
- 5925 kHz+ Px 3900 kHz ,
eQr/uJh=
(8b)
eQvXX/h = - 2110kHz-Px1900kHz, q
ID
D,D
0:
-40
0
with the same probability Pas in the case of nitrogen.
04,0:;
170 - 120
-80
40
l[‘Cl
Fig. 2. Temperaturedependences of the “N eQV,/h and eQV,l h in N-5-chlorosalicylideneaniline.
of the proton transfer to the 35C1NQR frequency is expected to be small. The experimental results of ref. [ 81 show that in the temperature interval between - 100°C and 50°C the relative change Au/v of the “Cl NQR frequency is approximately 0.01 while in the same temperature interval the relative change of the i4N and “0 NQR frequencies is approximately 0.06 which is much more than in the case of chlorine. Therefore we can safely assume that the temperature dependences of the 14Nand 0” NQR frequencies are mainly due to the proton transfer. The temperature dependence of V,, V,, and VZZ at the nitrogen site can be well described by eq. (2). In this case we have: eQVz-/h=3767
Wz-Px24OOkHz,
eQVrr/h=
-2152 kHz+P~23OOkHz,
eQI’&h=
- 1615 kHz+Px
1OOkHz ,
(8~)
(7a) (7b) (7c)
where P is given by eq. ( 3 ). The energy difference
NOR
IN
N-5-CHLOROSALICYLIOENEANILINE 2.35
0
0
0 0
2.30
0 0 0
1.50
1.45
0
1.40
I
-100
I
-60
I
-20
I
2o
I
TVCI
Fig. 3. Temperature dependences of the “0 NQR frequencies us,2,3,2and v,,~.,,~ in N-S-chlorosalicylideneaniline.
242
J. Seliger et al. / NQR in N-khlorosalicylideneaniline
170
NQR IN
‘H -“O NQOR LINESHAPES IN N- 5- CHLOROSALlCYLlDENEANltlNE
N-5-CHLOROSALICYLIOENEANltlNE
T =23”C
7.95 7.9C
0
I
-100
I
-60
I
-20
I
2o
T[“Cl
T 0.48
.
G
0
z
0
0
0
I
-100
2.25
E
0.46
I
-60
I
-20
2.30
2.35 v#lHzl
I
2o
11°C
I
Fig. 4. Temperature dependences of the “0 quadrupole coupling constant eQV&h and the asymmetry parameter q in N-5chlorosalicylideneaniline.
In order to find the O-H distance, the sign of V,,and the orientation of the principal axes of the EFG tensor in the molecular frame we performed the measurement of the dipolar structure of the “0 NQR lines at 296 and 145 K. The experimental results at 296 K are shown in fig. 5. The quadrupole coupling constants, asymmetry parameters, O-H distances and the angles 19and @are presented in table 1. As we see from table 1 at both low and high temperature the Z axis lies in the molecular plane and forms an angle rY= 57’ with the direction of the O-H bond. The Y axis is perpendicular to the molecular plane. On going from the room temperature to 145 K the dipolar structure of the “0 NQR lines does not change while the width increases by approximately 5%. The experimentally determined O-H distance is thus at room temperature approximately 2% longer than at low temperature. So if we set the probability of finding the molecule in the excited state at room temperature equal to P=O.O5 then the above results
3.65
3.70
3.75
v2[MHzl
Fig. 5. Dipolar structures of the “0 NQR lines in N-S-chlorosalicylideneaniline at 23°C as obtained with the help of the twofrequency irradiation technique. The arrows indicate the fixed irradiation frequencies v,. The positions of the resonance lines calculated with the parameters given in table 1 are indicated on the frequency scales.
are consistent
with
eq.
(5)
if we assume
that that the proton is jumping in the 0-H...N hydrogen bond between two inequivalent equilibrium sites so that in the ground state it is close to the oxygen while in the excited state it is close to the nitrogen. The jump rate may be in principle determined from the proton or nitrogen spin-lattice relaxation rate or from the temperature dependence of the width of the 14N NQR lines. The width of the 14N NQR lines is within the experimental resolution equal to 5 kHz and temperature independent. It is mainly determined by the resonant proton-nitrogen dipolar coupling. The proton spin-lattice relaxation time T, at 32 MHz is equal to 40 s and is nearly temperature independent. In the low frequency region between 1 and 2 MHz, T, is equal to 15 s and is nearly temperature indepen-
R 3( O...H ) 3 R 3(O-H ) . We may thus conclude
243
J. Seliger et al. / NQR in N-5-chlorosakylideneanilidene
Table 1 Quadrupole coupling constants, asymmetry parameters, O-H distances and the angles 6 and 4 from the dipolar structureof the “0 NQR lines Temperature (K)
eQVz.zlh &Hz)
1
R(O-H) (nm)
6 (deg)
Q (de@
145 296
8030 7940
0.413 0.445
0.100 0.102
57f5 57f5
Ok20 Of20
SPIN- LATTICE
RELAXATION
dent below room temperature. No nitrogen contribution to T, at vu = v. has been observed at any temperature. Since the fluctuations of the 14NEFG tensor are not small we may conclude that the absence of the nitrogen contribution to the proton T, is due to the fact that the lifetime of the excited state is rather short, presumably shorter than 10-‘” s. Above room temperature, a frequency dependence of the proton T, has been observed at low Larmor frequencies. The Larmor frequency dependence of the proton T, at 50 and 90°C is shown in fig. 6. At 50°C the correlation time of the fluctuations which relax the proton system is equal to 0.12 us and it decreases to 0.06 us at 90°C. The extrapolation of the spin-lattice relaxation rate which is due to the slow motion to zero frequency is nearly temperature independent. Such a behavior of the proton spin-lattice relaxation rate is expected when an exchange between the ground state and one or more excited states takes place [ 14 1. This slow motion does not affect the t4N NQR frequencies and the spin-lattice relaxation. One possible assumption is that the aniline ring rotates for a small angle about the C-N bond with respect to the rest of the molecule. Further experiments are needed to prove this assumption.
4. Conclusion The temperature dependences of the 14N and “0 NQR frequencies show a fast proton exchange between the ground and excited states in the asymmetric 0-H...N double minimum H-bond potential of NS-chlorosalicylideneaniline. The energy difference of the two states is 74 meV, consistent with the value determined previously from other experiments. The exchange is fast on the NQR time scale and does not influence the width of the NQR lines. The nitrogen spin-lattice relaxation rates are lower than 1 s-r,
PROTON N-5-
IN
CHLOROSALlCYLlOENEANltlNE
A-
0
o
00
0
0 0
12 -
0
10 -
0 a6-
0
0
0
0
0
T=50”C
o” I
0
I
I
I
vL [ MHz1
2
1
Is1TI -
0 0
12 -
0 0
10 -
II-
66 0
I
I
0
T =9o”C
0 0 0
1
0
0
0
0 0
0
2
vJMHz1
Fig. 6. Larmor frequency dependence of T, in the N-5chlorosalicylideneaniline.
which is consistent with the fast exchange limit. The time averaged O-H distance as determined from the dipolar structure of the “0 NQR lines decreases with decreasing temperature, directly demonstrating that the proton is moving rapidly in the 0-H...N hydrogen bond. At room temperature the proton spends about 5% of its time in the O...H-N configuration and 95% in the 0-H...N configuration.
244
J. Seliger et al. / NQR in N-5-chlorosalicylideneaniline
References [ 1 ] M.D. Cohen and G.M.J. Schmidt, J. Phys. Chem. 66 ( 1962) 2442. [ 21 E. Hadjoudis, M. Vittorakis and I. Moustakali-Mavridis, Tetrahedron 43 ( 1987) 1345. [ 31 M.D. Cohen, G.M.J. Schmidt and S. Flavian, J. Chem. Sot. (1964) 2041. [4] J. Bregman, L. Leiserowitz and G.M.J. Schmidt, J. Chem. Sot. ( 1964) 2068. [ 5] J. Bregman, L. Leiserowitx and K. Osaki, J. Chem. Sot. ( 1964) 2086. [ 61 A. Senier, F.G. Shepheard and R. Clarke, J. Chem. Sot. 101 (1952) 1912.
[ 71 G.M.J. Schmidt, in: Reactivity of the Photoexcited Organic Molecules (Interscience, New York, 1967) p. 227. [ 81 F. Milia, E. Hadjoudis and J. Seliger, J. Mol. Struct. 177 (1988) 191. [ 91 A. Abragam, The Principles of Nuclear Magnetism (Clarendon Press, Oxford, 196 1). [ 1OlS.G.P. Brosnan and D.T. Edmonds, J. Magn. Reson. 38 (1980) 47. [ 111 J. Seliger, V. Zagar, R. Blinc and A. Novak, J. Chem. Phys. 84 (1986) 5857. [ 121 S.G.P. Brosnan and D.T. Edmonds, Phys. Letters A 81 (1981) 243. [ 131 E. Hadjoudis, F. Milia, J. Seliger, R. Blinc and V. Zagar, Chem. Phys. 47 (1980) 105. [ 141 D.C. Look and I.J. Lowe, J. Chem. Phys. 44 (1966) 3437.