Stability and molecular dynamics of chloroxylenol (API of antiseptics and drugs) in solid state studied by 35Cl-NQR spectroscopy and DFT calculations

Chemical Physics Letters 469 (2009) 201–206

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Stability and molecular dynamics of chloroxylenol (API of antiseptics and drugs) in solid state studied by 35Cl-NQR spectroscopy and DFT calculations J.N. Latosin´ska *, M.A. Tomczak, J. Kasprzak ´ , Poland Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan

a r t i c l e

i n f o

Article history: Received 26 November 2008 In final form 22 December 2008 Available online 30 December 2008

a b s t r a c t Thermal stability of 4-chloro-3,5-dimethyl-phenol (chloroxylenol) in solid state has been studied by 35ClNQR spectroscopy. Two NQR resonance lines at the frequencies 34.348 and 34.415 MHz at 77 K have been assigned to chlorine atoms from two crystallographically inequivalent molecules on the basis of the B3LYP/6-311++G** results. The temperature dependence of the resonance frequency and full width at half maximum suggest the occurrence of small-angle torsional oscillations of the mean activation energy of 3.83 kJ/mol and rotation of both methyl groups around their symmetry axis C3 with the activation energies 12.49 and 11.27 kJ/mol for CH3 in molecule A and B, respectively. B3LYP/6-311++G** method reproduced very well the activation energies of both motions. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The compound 4-chloro-3,5-dimethyl-phenol, p-chloro-m-xylenol, chloroxylenol, PCMX, Fig. 1a, is a phenolic derivative, the key halophenol used in many antiseptic or disinfectant formulations [1,2]. The bactericidal agent PCMX is an active pharmaceutical ingredient (API) of many therapeutic substances including the disinfecting fluid Desson (ICN Polfa), vaginal globules Sterovag (SecFarm) administered in the treatment of leucorrhoea, erosions, inflammations and infections, antiseptic agent Dettol (Reckitt Benckiser) for topical use on wounds and bedsores, in obstetrics and in veterinary preparations such as the disinfectant Bioval (Biowet-Drwalew) or antibacterial preparations Vetzyme, Zemol (Seven Seas Ltd.) [3–5]. PCMX is also an ingredient of cosmetic creams, deodorants, disinfecting agents, ECG paste, hair care products, pharmaceutical products, soap and air deodorants [6]. It has unique antiseptic properties and is very effective topical antimicrobial agent against the common infectious germs and the Streptococcus bacteria [7], however, the aerobic rod-shaped bacteria P. aeruginosa and many moulds are highly resistant to it [8]. Surprisingly, despite its widespread use over many years its mechanism of activity has been little studied. It is well known that PCMX works as a disruptor of the proton gradient of cell membrane necessary for the bacteria to produce adenosine triphosphate (ATP), whose deficiency leads to the death of bacteria from starvation. PCMX has some considerable advantages – low toxicity, low metal corrosivity (important when sterilizing medical instruments) and is ac* Corresponding author. Fax: +48 61 8257758. E-mail addresses: [email protected], [email protected] (J.N. Latosin´ska). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.076

tive over pH range 4–9. The interest in PCMX stems from the fact that although it is weakly toxic to mammals, but it has both mutagenic and teratogenic effects, may have adverse reproductive effects, may cause skin irritation or produce allergic reactions and is also a suspected immunotoxicant and skin or sense organ toxicant as well as a gastrointestinal or liver toxicant [9,10]. According to the X-ray studies [11] PCMX crystallises in the monoclinic system P21/a with two molecules, whose rings’ planes make the angle of 89.2°, in the elementary cell and with the elementary cell parameters a = 8.537(4) Å, b = 13.936(2) Å, c = 12.839(3) Å, a = c = 90°, b = 92.92(3)°. As follows from the crystallographic data, thanks to the presence of the OH groups, each molecule of PCMX forms two intermolecular hydrogen bonds O–H  O with two neighbouring ones, Fig. 1b. The lengths of these bonds are not much different (2.781 and 2.775 Å) as are their OHO angles (172.5° and 163.37°); these lengths are slightly greater than the mean value for the bonds of this type of 2.72 Å. The melting point of PCMX has been reported to be 387–389 K. The Raman spectrum of PCMX in solid state in the range 50–3500 cm1 and IR spectrum in the range 400–4000 cm1 have been reported [12–14]. Although the 35Cl-NQR frequencies of PCMX at 77 K are known [15,16], but they have not been hitherto assigned to the specific positions and the molecular dynamics of the compound has not been studied yet. The advantage of using NQR to study the local structure is based on the fact that it provides detail information on the static and dynamic properties of the structure on the scale of the interatomic spacing. Therefore, the temperature variation of the NQR resonant lines has been measured to establish the thermal stability of PCMX and to get the information on the internal torsional frequencies, bond bending motions and phase transitions.

´ ska et al. / Chemical Physics Letters 469 (2009) 201–206 J.N. Latosin

202

OH

6 5

1 4

3. Calculation details

2 3

CH3

CH3 Cl

The quantum chemical calculations were carried out within the GAUSSIAN-03TM code [17] run on the CRAY supercomputer at the Supercomputer and Network Centre (PCSS) in Poznan. The full and partial geometry optimization, in which only the positions of the hydrogen atoms were allowed to relax while those of all other atoms remained frozen, the electric field gradient and IR and Raman spectra calculations were performed within the DFT with three-parameter hybrid functional of Becke B3LYP and the all electron 6-311++G** basis set. 4. Results and discussion The NQR spectrum of PCMX at 77 K reveals two Lorentzianshaped resonance lines at the frequencies 34.348 and 34.415 MHz and the full width at half maximum (FWHM) of about 7.3 kHz and 9.3 kHz and of slightly different intensities, Fig. 2. The line separation of 67 kHz between the two lines (0.2% of the resonant frequency) is well within the limits for crystallographic splitting and typical of 4-chlorophenol derivatives [18,19], Table 1. The temperature dependencies of the NQR frequencies and FWHM values in the range 77–353 K are presented in Figs. 2 and 3, respectively. The NQR spectrum of PCMX in the whole temperature range is composed of two lines of slightly different integral intensities; no anomalies in the line intensities or widths as well as no first-order phase transitions occur in the temperature range considered, Fig. 2, which suggests that under the effect of temperature, PCMX in solid state is very stable and does not decompose. The temperature behaviour of the FWHM plotted in Fig. 3, shows in general a total effect of static homogenous broadening associated with a slow narrowing to a minimum at about 300 K and a broadening at temperatures above 320 K. 5. Resonance lines assignment The assignment of the two NQR lines to chlorine atoms from the inequivalent molecules in the elementary cell of chloroxylenol was made on the basis of the results of B3LYP/6-311++G** computations, Table 1. The line of the higher frequency was assigned to the chlorine atom from molecule B (with a longer and more sym-

Fig. 1. (a) Structural formula of 4-chloro-3,5-dimethyl-phenol, chloroxylenol (PCMX). (b) The molecular arrangement of inequivalent molecules A (with a shorter and less symmetric C–Cl bond of 1.7493A; the \CCCl angles of 119.32 and 117.97) and B (with a longer and more symmetric C–Cl bond of 1.7526A; the \CCCl angles of 118.38 and 118.48) in the elementary cell (hydrogen bonds marked by dotted lines).

2. Experimental The material studied was polycrystalline (powder) sample of PCMX, purchased from Sigma–Aldrich and used without further purification. The 35Cl-NQR spectra of the compound were taken over the temperature range from 77 (liquid nitrogen temperature) to 353 K (temperature close to the melting point of the compound). NQR signals assigned to Cl nuclei were weak (S/N = 3 after 400 accumulations) and the resonant lines were very narrow (5– 9 kHz), therefore, the classical FID sequence could be applied in the entire temperature range studied. The optimized pulse length was about 24 ls, the repetition time was 120 ms. The accuracy of the 35Cl-NQR frequency determination was about 10 kHz. The temperature in the experiments was controlled using a homemade cryogenic system and stabilised with the accuracy to within 1°.

Fig. 2. The temperature dependencies of the NQR frequencies and the fits obtained by assuming the Brown model. An inset shows the shape of the resonance lines assigned to the two chlorine atoms from A and B molecules.

´ ska et al. / Chemical Physics Letters 469 (2009) 201–206 J.N. Latosin Table 1 The experimental (at 77 K) and calculated by B3LYP/6-311++G** NQR frequencies, quadrupole coupling constants (QCC) and asymmetry parameters (g) for chloroxylenol and similar compounds. Chemical compound

Nucleus rC–Cl [Å]

NQR parameters Experimental Calculated B3LYP/ at 77 K m [MHz], 6-311++G** m [MHz], g [–], QCC [MHz] g [–], QCC [MHz] Opt

4-Chlorophenol

4-Chlorocresol (4-chloro-3methylphenol)

Cl-4a (molecule A; 34.700a 1.7416 Å) 0.071 68.18 a Cl-4 (molecule B; 34.945a 1.74334 Å) 0.096 68.59 Cl-4b (molecule A; 34.5569b 1.74271 Å) – – b Cl-4 (molecule B; 34.9355b 1.74407 Å) – – Cl-4 (molecule A; 1.74400 Å) Cl-4 (molecule B; 1.75096 Å)

4-Chloro-2,6xylenol (4-chloro-2, 6-dimethylphenol)

Cl-4

Chloroxylenol Cl-4 (molecule A; (4-chloro-3,51.7493 Å) dimethyl-phenol, PCMX) Cl-4 (molecule B; 1.7526 Å)

Partial X-ray opt.

34.784 34.260 33.914 69.455 68.370 0.078 0.071 0.081 67.690 34.284 33.938 0.081 0.078 68.418 67.738 34.269 33.915 0.080 0.078 68.393 67.692 34.273 33.917 0.081 0.077 68.396 67.701

34.535c 0.12 68.12 34.887c 0.07 68.64

34.327 33.812 33.417 0.096 0.111 0.103 68.444 67.347 66.717 34.360 33.562 0.204 0.096 67.779 66.919

34.572d – –

33.654 – 0.082 67.158

34.348 – – 34.415 – –

33.928 33.114 33.019 0.128 0.131 0.13 67.487 65.851 65.853 33.146 33.054 0.130 0.13 65.920 65.923

a,a

Form alpha Ref. [18]. Form beta Ref. [19]. c Ref. [20]. d Ref. [21].

b,b

metric C–Cl bond of 1.7526 Å; the \CCCl angles of 118.38 and 118.48) and the one of the lower frequency to the chlorine atom from molecule A (with a shorter and less symmetric C–Cl bond of 1.7493 Å; the \CCCl angles of 119.32 and 117.97), Fig. 1b. For the sake of comparison, the NQR parameters for 4-chlorophenol and a few its derivatives studied experimentally were calculated, Table 1. Although the NQR frequencies calculated by DFT on the basis of X-ray data (with optimized proton positions) or optimized geometry are in a good agreement with the experimental data (correlation coefficients are 0.81 and 0.75 and standard deviations 0.37 MHz and 0.29 MHz, respectively), the quadrupole coupling constants (QCC) are underestimated (by 1–4%), while the asymmetry parameters (g) are overestimated, Table 1.

203

on the electric field gradient (EFG) caused mainly by the small amplitude torsional oscillations. The variation in NQR frequency with temperature in the high temperature region can be fitted with the standard models available in literature [22–24], which take into account the standard contribution of the rotary optic librational modes proposed by Bayer [22] and Kushida [23]

mQ ðTÞ ¼ m0 

    c  3 X hm0 hxi coth ; ¼ a 1  b coth 4 i I i xi T 2kB T

ð1Þ

where xi is the ith torsional frequency of the ith mode, Ii is a moment of inertia associated with that mode and m0 is the NQR frequency in a stationary state or modified by Brown’s introduction of anharmonicities [24], which have a general form of a polynomial series:

mQ ðTÞ ¼

X

ai T i ;

ð2Þ

i

where i changes from 1 to 3 depending on the specific model, Table 2. Table 3 lists the values of the parameters a0, a1, a2, a3, b, c and a1, obtained from the fits. Although qualitatively good fits have been obtained assuming the Brown models (Fig. 2c and d), they are physically unacceptable. The plot in Fig. 2 presents the best result of the physically acceptable fit performed by assuming the theoretical model of Brown. The parameters characterising the PCMX dynamics, i.e. the frequency of vibrations of the crystalline lattice x, coefficient of vibration anharmonicity g, moments of inertia NQR at 0 K mQ(0) along with the correlation coefficients (r) and standard deviations (s), are collected in Table 4. The average moments of inertia calculated on the basis of the parameters of the fit by Eq. (1) equal 83  1047 kg m2 and 107  1047 kg m2 or assuming Eq. (1) simplified by Bayer 274  1047 kg m2 and 436  1047 kg m2 for molecules A and B, respectively. These values are close to those calculated theoretically on the basis of the X-ray structure of PCMX [11]: Ixx = 284.8  1047, Iyy = 545.7  1047, Izz = 825.4  1047 kg m2 for molecule A and Ixx = 286.5  1047, Iyy = 545.9  1047, Izz = 827.1  1047 kg m2 for molecule B. The vibrational frequencies calculated by DFT on the basis of X-ray data are higher than those following from the fit, which suggests that the principal axes of the moment of the inertia tensor and the principal EFG axes do not coincide, Table 3. The angles estimated from the extended Bayer’s model for the temperature dependence of NQR frequency,

6. Molecular dynamics The 35Cl-NQR frequencies of both chlorine atoms at inequivalent molecular sites slowly decrease with temperature. In the temperature range studied, the temperature coefficients 1/mQ(dmQ/dT) take the mean values of 5.05  105 1/K (at 77 K) and 9.75  105 1/K (at 353 K) and 4.41  105 1/K (at 77 K) and 9.31  105 1/K (at 353 K) for the chlorine atoms from molecules A and B, respectively, which suggests a slightly different temperature sensitivity. The negative temperature coefficient of NQR frequencies may be interpreted as related to the averaging effect

Fig. 3. The temperature dependencies of full width at half maximum. Solid line is a fit to the theoretical model based on Eq. (7).

´ ska et al. / Chemical Physics Letters 469 (2009) 201–206 J.N. Latosin

204 Table 2 The theoretical models describing mQ(T) dependence. Equation 2a 2b 2c 2d 2e

Formula

Table 4 The physical parameters obtained from the fit by equations (1), (2a), (2b). Model

1

mQ(T) = a0 + a1T + a1T mQ(T) = a0 + a1T + a2T2 mQ(T) = a0 + a1T + a2T2 + a1T1 mQ(T) = a0 + a1T + a2T2 + a3T3 mQ(T) = a0 + a1T + a2T2 + a3T3 + a1T1

KBB Brown Brown Brown Brown

Ref. [23] [24] [24] [24] [24]

are: 54 and 77 deg for Cl-4 (molecule A), Cl-4 (molecule B), respectively and are in a good agreement with those estimated on the basis of structural data [11]. Usually, the frequency (or wavenumber) of the crystal lattice vibrations obtained from NQR data is compared with the FT IR/Raman data. For PCMX a good agreement was obtained between the theoretical vibrational wavenumbers calculated at the B3LYP/6311++G** level and the experimental FT IR/Raman data available. Generally, the wavenumbers of the torsional vibrations of the molecules are expected to be in the range from 20 to 100 cm1 [25], for PCMX these wavenumbers are higher, see Table 4. The wavenumbers of the crystal lattice vibrations obtained from the NQR frequency temperature dependence: 214, 345 and 176 cm1 Eq. (1), (1) simplified by Bayer or (2a), respectively) are in a good agreement with those following from the experimental Raman spectra 261, 346, 396 cm1 [14] as well as with those following from the calculations within the DFT at the B3LYP/6-311++G** level, performed with the aid of normal coordinate analysis which revealed low frequency modes at 98, 149, 150 (hindered rotation of CH3), 187(only active in IR spectrum), 228, 250, 288, 291 (hindered rotation of OH, the strongest), 312, 328, 386, 514 cm1 of the 51 normal modes of vibrations for the PCMX molecule. The average difference in the wavenumbers of lattice vibrations for A and B molecules does not exceed 2.5 cm1. The generally higher vibrational frequencies obtained from the temperature dependencies of the NQR resonance frequencies for PCMX (Table 3) in comparison to 2,5-dichlorophenol (60 and 62 cm1) [25,26], 2,6-dichlorophenol (38 and 40 cm1) [27], 3,5dichlorophenol (65 and 66 cm1) [28] or 3,4-dichorophenol [29] can be interpreted as a consequence of the greater binding energy in PCMX than in the other compounds from the group of chlorophenols (2,5-dichlorophenol, 2,6-dichlorophenol or 3,5-dichlorophenol). The average activation energy of torsional oscillations (librations) obtained from the NQR data as 3.83 kJ/mol is in a good agreement with 4.26 kJ/mol following from the calculations within the density functional theory (DFT) at the B3LYP/6-311++G** level. The activation energy of librations calculated from the temperature dependence of the NQR frequency assigned to molecule A is by

Equation

Nucleus

I  1047 [kg m2]

g  103 [K1]

m  1011

m

[MHz]

[Hz]

[cm1]

Ea [kJ/ mol]

mQ ð0Þ  mSQ

1

ClmolA ClmolB

34.821 34.862

214 170

– –

128 146

356 283

4.27 3.39

2a

ClmolA ClmolB

34.751 34.743

– –

– –

56 50

187 167

2.24 2.00

2b

ClmolA ClmolB

34.429 34.498

– –

1.76 1.14

– –

– –

– –

0.87 kJ/mol lower than that calculated for molecule B, Table 3, which means that molecule A is more rigid than B. With increasing temperature, the difference between the resonance frequencies assigned to the chlorine atoms in inequivalent molecules A and B decreases to reach a minimum at 140K and then increases, Fig. 4. The frequency difference in PCMX can be the best approximated by the equation:

DmQ ðTÞ ¼ Da0 þ a1 T þ Da1 =T

ð3Þ

derived from the Kushida model Eq. (2) where

Da0 ¼ Dm0 ; " # 3 mA MAA mB MAB Da1 ¼ aA1  aB1 ¼  k 0 2  0 2 ; 2 xA xB

Da1 ¼ aA1  aB1 ¼ 

ð4Þ

i h h A m MAA  mB0 MAB 8k 0

and k is Boltzmann constant, xi = vi(1  giT) is the frequency of the mode at the temperature T, MA is a quantity related to the inverse of the moment of inertia, m0 is the limiting static value of the NQR frequency and the indices B and A refer to the high- and low-frequency lines, assigned to B and A molecules respectively. The correlation coefficient of this fit is acceptably high 0.9847 and the standard deviation is reasonably small 0.00124 MHz. The first and third parameters Da0 and Da1 describe changes in the molecular packing; the second Da1 informs about the differences in the molecular dynamics of A and B molecules. The low value of Da0 = 0.00751 MHz and large Da1 = 4.2866 MHz K suggests small (but not negligible) differences between the crystallographically inequivalent molecules A and B, however, the value of a1 = 0.00019446 MHz K1 suggests differences in the rigidity of the two molecules. It means that the intermolecular interactions have different stiffening effect on molecules A and B.

Table 3 The parameters obtained from the fit of the equations (1), (2a)–(2e). Equation

Nucleus

a, a0 [MHz]

a1  103 [MHz/K]

a2  106 [MHz/K2]

a3  1010 [MHz/K3]

a1 [MHz K]

b [MHz]

c [K]

1

ClmolA ClmolB

34.821 34.862

– –

– –

– –

– –

0.556 0.695

154 171

2a

ClmolA ClmolB

34.751 34.743

2.7 2.5

– –

– –

16.34 12.05

– –

2b

ClmolA ClmolB

34.429 34.498

0.9 1.1

3.16 2.52

– –

– –

2c

ClmolA ClmolB

34.333 34.307

0.4 0.07

3.99 4.16

– –

2d

ClmolA ClmolB

34.470 34.570

1.6 2.3

0.42 3.67

55.30 95.38

2e

ClmolA ClmolB

34.517 34.426

2.0 1.1

1.76 0.46

71.31 45.90

5.28 10.49 – – 1.90 5.86

s [kHz]

r [–]

7.9 7.9

0.9991 0.9991

– –

10.0 10.4

0.9987 0.9985

– –

– –

2.8 4.8

0.9999 0.9997

– –

– –

1.7 1.9

0.9999 0.9999

– –

– –

1.2 2.0

0.9999 0.9999

– –

– –

1.2 1.8

0.9999 0.9999

´ ska et al. / Chemical Physics Letters 469 (2009) 201–206 J.N. Latosin

The quadrupole resonance line gets narrower when the rate of reorientation becomes too fast for effective direct relaxation and gets broadened due to relaxation effects and randomization of field gradients by disordering processes. The temperature behaviour of FWHM plotted in Fig. 3, shows in general the total effect of static homogenous broadening associated with some kind of motions: (1) at 77 K–110 K – the major contribution from the molecular torsional oscillations, (2) at 110 K–300 K – a slow narrowing, probably a motional one, (3) at 300 K–320 K – a flat, wide modulation minimum of about 3.5–4.0 kHz, (4) above 320 K – the resonance line broadening because of the fast motion; the effect of the thermally activated contribution to FWHM becomes remarkable. All the contributions to the FWHM:

dmQ ðTÞ ¼ dmad þ dmlib ðTÞ þ dmrot ðTÞ þ dmmod ðTÞ

ð5Þ

can be considered as coming from independent processes: librations, rotations of the whole molecules, CH3 groups rotation. For the contributions to relaxation arising from movements of the methyl group, Woessner and Gutowsky [30] developed a theory of the quadrupolar nuclear relaxation originated from the fluctuations of the EFG in the nuclear site as a consequence of the movements of a nearby moving group. According to their formula simplified by Kyuntsel [31] adopted to FWHM:

Dmjmin ¼

 2

pm q 0 3

q

ð6Þ

where q0 /q is the fluctuating fraction of the EFG, and m is the resonance frequency. The fluctuating fraction of EFG calculated from (6) is equal 1.6  103 for molecule B higher than 1.1  103 for molecule A, both are reasonable values in consistence with the values usually obtained for this type of relaxation mechanism from T1 data. Fitting our experimental data by means of expression (5) leads to unrealistic values of s0. The best fit of our experimental data, shown as a solid line in Fig. 3 is obtained with the expression:

dmQ ðTÞ ¼ dmad þ dmlib ðTÞ þ dmmod ðTÞ;

ð7Þ

where Dmad is the adiabatic contribution, dmlib ðTÞ is a contribution from torsional oscillations (librations)

Fig. 4. The temperature dependence of the difference in the resonance frequencies assigned to the two chlorine atoms from A and B molecules.

dmlib ðTÞ ¼ aT k

205

ð8Þ

and the dmmod(T) is a contribution from movements of the methyl groups

dmmod ðTÞ ¼

  1 q0 x20 s0 expðU=RTÞ ; 3 q 1 þ x20 s20 expð2U=RTÞ

ð9Þ

where q0 /q is the fluctuating fraction of the EFG tensor, s = s0exp(U/ RT) is the correlation time of the C3 reorientation with the activation energy U. The activation energies of methyl groups rotation obtained from the fit of Eq. (7) are about 12.49 kJ/mol and 11.27 kJ/mol for CH3 groups in molecules A and B, respectively. The correlation coefficients are quite high 0.962 and 0.943 and standard deviations are reasonably small 0.4 kHz and 0.7 kHz. The activation energies of the methyl group rotation calculated by DFT assuming the crystallographic structure with optimized proton positions equal to 14.47 kJ/mol (molecule A) and 13.38 kJ/mol (molecule B) and are in a good agreement with the values obtained from the temperature dependence of the FWHM of the NQR resonance line. The small difference in the activation energies 1.22 kJ/mol of CH3 groups rotation in molecules A and B is in good agreement with the DFT result 1.09 kJ/mol and confirms that molecule A is more rigid than B. Generally, DFT overestimates the barriers but it is well known that in most of the real crystals the barrier is reduced and the rotation is accelerated due to an increase in density. The changes in NQR frequency caused by CH3 rotation around their symmetry axis C3 calculated by DFT do not exceed 0.20 MHz and 0.15 MHz for chlorine nucleus from molecules A and B, Fig. 5, and the difference between them is in a good agreement with the observed differences in the slopes of m(T). The differences in the asymmetry parameter caused by rotation of CH3 group are negligibly small (0.1%), which means that the rotation causes significant changes only in the component of EFG tensor lying in the direction of C–Cl. Moreover, the lowest NQR frequency corresponds to the most stable conformation of the CH3 group, because the positions of the minima on the plot of the 35Cl-NQR frequency versus the torsion angle describing the methyl group rotation around their symmetry axis C3 (revealed by DFT) coincide with the positions of the minima on the plot of the potential energy versus the same torsion angle, Fig. 5.

Fig. 5. The potential and 35Cl-NQR frequencies versus torsional angle describing methyl groups rotation around their symmetry axis C3, revealed by DFT calculations.

206

´ ska et al. / Chemical Physics Letters 469 (2009) 201–206 J.N. Latosin

7. Conclusions (1) PCMX in solid state has been found to be thermally stable; no release of chlorine or evidence of PCMX decomposition has been detectable. (2) The small-angle torsional oscillations around the molecular axis different from that along the C–Cl bond and of the mean activation energy of 3.83 kJ/mol have been manifested on the temperature dependence of the NQR frequency. The activation energy of librations calculated from the temperature dependence of the NQR frequency assigned to molecule A is by 0.87 kJ/mol lower than that calculated for molecule B, which means that molecule A is more rigid than B. (3) For T < 110 K the major contribution to FWHM comes from the molecular torsional oscillations (librations). For 110– 300 K, a slow narrowing of FWHM, probably a motional one has been noted followed by a broadening in higher temperatures, above 320 K. The flat, wide modulation minimum in the temperature dependence of the FWHM has revealed the activation processes: the rotation of the both methyl groups around their symmetry axis C3 with the activation energies 12.49 kJ/mol and 11.27 kJ/mol for CH3 in molecule A and B, respectively. These energy activations are close to those revealed by DFT calculations: 14.47 kJ/mol and 13.38 kJ/mol for CH3 from molecules A and B, respectively.

Acknowledgements Generous allotment of computer time from the Poznan´ Supercomputer Centre (PCSS) in Poznan´ is gratefully acknowledged.

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