Unraveling the weak hydrogen bonds of ethynylpyridines and ethynylbenzene with trimethylphosphate — A combined FT-Raman spectroscopic and quantum-chemical study

Journal of Molecular Liquids 218 (2016) 499–507

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Unraveling the weak hydrogen bonds of ethynylpyridines and ethynylbenzene with trimethylphosphate — A combined FT-Raman spectroscopic and quantum-chemical study Danijela Vojta a,⁎, Tomislav Horvat b, Snežana Miljanić c, Mario Vazdar a a b c

Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička 54, 10000 Zagreb, Croatia Biorobotics Laboratory, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Division of Analytical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 18 December 2015 Accepted 20 February 2016 Available online xxxx Keywords: Ethynyl compounds Hydrogen bonding Vibrational dephasing C`C moiety FT-Raman spectroscopy Quantum chemical calculations

a b s t r a c t The interplay of secondary hydrogen bonds of 2- and 3-ethynylpyridine or ethynylbenzene with trimethylphosphate in tetrachloroethene was elucidated using FT-Raman spectroscopy and MP2/6–311 + G(d,p) calculations. The direct participation of C`C moiety in the complex formation was demonstrated by the change in the shape of the C`C stretching band and further characterized in terms of vibrational dephasing of C`C stretching. With this aim, the complex band pattern in frequency domain was decomposed using analytical function, introduced by Egelstaff and Schofield and further disseminated by Kirillov, with analytical counterpart in the time domain. The amplitude of frequency fluctuations (M2), frequency modulation time (τω) and vibrational dephasing time (τν) were determined for both unassociated (C`C) and associated (C`C⋯) ethynyl moieties of 2- and 3-ethynylpyridine and ethynylbenzene. The differences in the dynamical parameters indicate broader distribution of the frequency fluctuations for C`C⋯ moiety (M2 ~1–2 ps−2) than for C`C moiety (M2 ≈ 0.5 ps−2), while the average time between perturbative events, as well as the time needed for the phase being completely lost, were shorter for C`C⋯ (τω ~0.2–0.7 ps, τν ≈ 1 ps) than for C`C moiety (τω ~1.4–1.7 ps, τν ≈2 ps). The shorter τω for C`C⋯ moiety of 2-ethynylpyridine (τω ≈ 0.23 ps), in comparison with analogous quantity of 3-ethynylpyridine and ethynylbenzene (τω ≈ 0.6 and 0.68 ps), is attributed to more frequent hindering of the C`C⋯HCH2 hydrogen bond by spatially close N-atom which competes for H-atom of CH3 group thus making the N⋯HCH2 hydrogen bond, as predicted by MP2 calculations. Additionally, a hydrogen bond between ortho H-atom of 3-ethynylpyridine and P\\O(CH3) group of trimethylphosphate is suggested from experimental FT-Raman spectra as well and also computationally verified. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Structural distinctions and dynamics related events in the systems whose assembly is driven by a hydrogen bond (HB) are tightly related to the HB affinities of protagonists in a HB complex and their ability to entail physically meaningful and reproducible response to imposed experimental conditions. For instance, ethynylbenzene (EB) is a multifunctional system with a comparably weak hydrogen bond (HB) donating and accepting capabilities and whose interaction pattern was shown to be highly unpredictable [1]. In revealing the structural attributes of gas phase HB complexes formed by EB and small molecules such as water, alcohol, ammonia or amines, UV-IR double resonance experiments supplemented by high-level ab initio calculations have been ⁎ Corresponding author at: Group of Environmental and Soil Chemistry, Institute for Environmental Chemistry, University of Koblenz-Landau, 76829 Landau, RheinlandPfalz, Germany. E-mail address: [email protected] (D. Vojta).

http://dx.doi.org/10.1016/j.molliq.2016.02.042 0167-7322/© 2016 Elsevier B.V. All rights reserved.

shown to be a successful recipe [2–4]. As the competition between electrostatic and dispersion contribution occasionally results with prevail of the latter, it is clear that dispersion interactions actually determine the final geometry of a HB complex where EB is participating. Raman spectroscopy, in spite of experimental limitations associated with it and inherently weaker sensitivity [5], has revealed a HB between EB and benzene (BEN) and acetonitrile (ACN), respectively, which formation includes indirectly a C`C moiety [6]. As the most conceivable structural arrangement the authors have assumed `C\\H⋯ π interaction in the former, while H⋯C`N interaction is dominant in the latter example (H refers to H-atoms of EB). Since the coupling of C`C stretching mode of EB with CH3 rotation of ACN, mediated by CC`N bending, appears to be extremely sensitive to the temperature change, this phenomenon was explained in terms of a structural reorganization and a hindered CH3 rotation in a frozen matrix [7]. Unfortunately, the authors have omitted the possibility of direct, although weak, interaction between C`C moiety and CH3 groups; its signal should also display temperature dependency primarily because the number density of HB

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complexes, dictated by the equilibrium constant of a HB complex and Boltzmann population distribution, is a direct function of temperature. Our recent findings on HB complexes formed between not only EB, but also 2- and 3-ethynylpyridines (2-EP and 3-EP) and trimethylphosphate (TMP) in C2Cl4, have revealed FT-IR spectroscopic signatures of both predicted C`C\\H⋯O interaction and surprisingly C`C⋯CH3 interaction [8]. A clearly distinguished band maxima of the stretching of unassociated (C`C) and a HB associated C`C moiety (C`C⋯) have unambiguously demonstrated that this secondary interaction is indeed strong enough and persistent to be captured and analyzed even with a low-resolution spectrometer at ambient conditions. The density functional calculations at the B3LYP/6-311++G(d,p) have predicted the additional N⋯HCH2 interaction in 2-EP⋯TMP complex, which presence could not be proven from solely FT-IR spectra because the stretching of unassociated (C = N and C\\H) and associated (C = N⋯ and C\\H⋯) oscillators involved in this HB type, is virtually impossible to distinguish. However, this obstacle might be circumvented and resolved by taking approach other than routine avenues in detection of a HB [9]. In particular, if the C`C ⋯ HCH2 HB is persistently perturbed by the N⋯HCH2 HB, then the latter one exerts the indirect impact on C`C⋯ stretching band of 2-EP⋯TMP complex not present in 3-EP⋯TMP and EB⋯TMP complexes. Therefore, the influence of the surroundings perturbation on spectroscopic signatures may be a promising way towards elucidation of a HB between N-atom of 2-EP and methyl group of TMP. One of the outstanding advantages of FT-Raman spectroscopy is ability to distinguish the vibrational and reorientational relaxation. Isotropic Raman spectrum contains the information only on vibrational relaxation, where vibrational dephasing and population relaxation, as its main events, are discriminated due to the different time scales [10, 11]. The most appropriate theoretical model for description of the vibrational dephasing of molecules in molecular liquids and binary mixtures is Kubo's model [12,13], in the frame of which the vibrational dephasing,τV, and frequency modulation times, τω, are determined; the former indicates the time during which the phase is completely lost and the latter the time between perturbative events. In the accomplishment of this goal, one is faced with several major obstacles: i) intensities in isotropic Raman spectra are FT of the vibrational correlation functions. Thus, for obtaining accurate spectral parameters, the functions that have analytical forms in both frequency and time domain should be utilized. Lorentzian and Gaussian, as the most frequently employed functions obeying this demand, are actually two limiting situations of Kubo's function which does not have an analytical counterpart in the frequency domain. In fact, in majority of situations the observed band profiles are their intermediates [14, 15]; ii) the aspiration that the analyzed band appears as a single strong symmetric band, undisturbed by the small satellite bands attributed to either hot bands or the bands originated due to different isotopic species in the sample, is rarely achievable. In spite of the mentioned drawbacks, majority of studies addressed to the analysis of isotropic vibrational band profiles is based either on the fitting on Gaussian, Lorentzian or Voigt profile [16] as their convolution, with occasional examples in which the vibrational correlation function of an asymmetric band is extracted only from the symmetric band-half [17]. However, the utility of Egelstaff and Schofield function [18], which was reanimated by Kirillov [19], enables the finding of time correlation functions and corresponding correlation times by fitting in the frequency domain without Fourier transformation, and is highly recommendable when one wants to analyze the overlapping bands [19–21]. The aim of this paper is to present a conclusive spectroscopic argument for N ⋯HCH2 HB formation in 2-EP⋯ TMP complex, as well as to identify other secondary interactions that were not detected with FT-IR spectra. With this respect, the vibrational dephasing of C`C stretching mode of 2- and 3-EP and EB, both free and associated via HB with CH3 group of TMP in C2Cl4, will be characterized in terms of vibrational dephasing parameters. As the overlapping bands are to be analyzed, the function recommended by Kirillov will be utilized. Experimental findings will be supported by high level ab initio calculations

at the MP2/6-311 + G(d,p) level of theory in order to further verify and confirm the additional N⋯HCH2 interaction in 2-EP⋯TMP complex, as well as other secondary interactions. 2. Methods 2.1. Description of the fitting model Time-dependent intermolecular interactions of investigated system with its surroundings, expressed in terms of frequency modulation time τω and vibrational dephasing time τV, are reflected in vibrational band shapes or, more precise, in the frequency distributions. As the theoretical models which describe vibrational dephasing are presented in numerous occasions [10–13,22,23], only the elementary assumptions and relationships needed for understanding of the quantities measured here will be briefly presented in this section. Isotropic Raman profile Iiso(ω) of totally symmetric normal modes, such as C`C stretching, is obtained by measuring the intensity of scattered radiation polarized along the same direction as incident radiation (VV or 0°), as well as in the direction perpendicular to it (VH or 90°) 4 Iiso ðωÞ ¼ IVV ðωÞ− IVH ðωÞ: 3

ð1Þ

Iiso(ω) is related with the vibrational time correlation function GV(t) by Fourier transformation. 1 Iiso ðωÞ ¼ 2π

þ Z∞

GV ðt Þ expð−iωtÞ dt

ð2Þ

−∞

where ω is radial frequency related to the vibrational wavenumber ~v by ~. ω ¼ 2πcv When the investigated oscillator is exposed to the weak, persistent and stochastic time-dependent interactions with its surroundings, its transition frequency will be changed as a function of time, resulting with the vibrational dephasing. Kubo's expression for vibrational time correlation function.      −t t −1 þ GV ðt Þ ¼ exp −M 2 τ2ω exp τω τω

ð3Þ

is found to be the most appropriate function that describes vibrational dephasing of molecular liquids and binary mixtures. It contains M2, þ∞

the vibrational second moment M2 ¼

∫ ω2 I iso ðωÞdω

−∞ þ∞

as a measure for

∫ I iso ðωÞdω

−∞

the distribution of possible frequency transitions, and τω, the frequency modulation time as the average time when the system resides at particular frequency. The utility of Eq. (3) is valid in Markovian regime (sometimes referred as Markovian–Gaussian regime due to randomness of environmental perturbations [15]), implying the short-time memory when instantaneous value of the frequency depends only on one previous value. The integral over GV(t) gives vibrational dephasing time τV. Z∞ τV ¼

GV ðt Þdt

ð4Þ

0

the quantity that indicates how long does it takes when the phase is completely lost. GV(t) is also possible to express by the function.      1 1 GV ðt Þ ¼ exp − t 2 þ τ21 2 −τ 1  τ2

ð5Þ

D. Vojta et al. / Journal of Molecular Liquids 218 (2016) 499–507

which has an analytical counterpart in the frequency domain.   2  τ1 K 1 ðxÞ τ1 : IðωÞ ¼ 2c exp τ2 τ2 x

ð6Þ 1

In the last expression,x ¼ τ 1 ½ðω−ωÞ2 þ τ12 2 , K1(x)is modified Bessel 2

~ 0 ) and τ1 and τ2 are function of second kind, ω0, is the band center (ν the empirical parameters used as inputs to find GV(t), τω and τV. At certain limiting conditions, briefly described in Supplementary Materials for the sake of completeness, τ1 is close to τω and, respectively, τ2 to τV. When discussing vibrational dephasing of C`C oscillator, only τω and τV, along with the distribution of possible frequency transition, M2, as relevant dynamics parameters will be further analyzed. 2.2. Calculations In order to verify experimental results and confirm the existence of a weak N ⋯ CH3 hydrogen bonds in 2-EP ⋯ TMP complex and other secondary interactions we performed advanced MP2/6-311 + G(d,p) calculations capable of proper description of dynamic electron correlation at the ab initio level. MP2 method is adequate for investigation of weak interactions driven by dispersion in contrast to the B3LYP functional used in the former study [8], due to the known fact that B3LYP is poor describing dispersion when a posteriori correction is not included [24]). Since TMP can exist in several conformations (C3, C1 and Cs), we used the C3 isomer of TMP which has been proven to be the most stable by 0.6 and 1.4 kcal mol−1 than C1 and Cs isomer, respectively, at the MP2/631G(d,p) level of theory [25]. Within this approach, we identified the 2-EP⋯ TMP, 3-EP ⋯TMP, and EB⋯TMP complexes. Subsequent vibrational analysis confirmed the stationary nature of minima at the potential energy surface. Calculated vibrational frequencies were scaled by a factor of 0.9523 in order to take into account anharmonicity effects [26]. 3. Experimental

501

ternary mixtures with 2-EP), 0.116 mol dm−3 ≤ c0(TMP) ≤ 0.902 mol dm−3 3 (for preparation of ternary mixtures with 3-EP) and 0.112 mol dm−3 ≤c0(TMP) ≤ 1.230 mol dm−3 (for preparation of ternary mixtures with EB). A set of 10 ternary mixtures was prepared from the corresponding binary mixture (TMP in C2Cl4) and 2-EP, 3-EP or EB, respectively. The concentration of EPs or EB in all ternary mixtures was held almost constant and was about c0 (2-EP) ≈ 0.05 mol dm− 3 (0.041 mol dm− 3 ≤ c0 (2EP) ≤ 0.050 mol dm − 3 ), c 0 (3-EP) ≈ 0.05 mol dm− 3 (0.044 mol dm−3 ≤ c0(3-EP) ≤ 0.057 mol dm−3) and c0 (EB) ≈ 0.06 mol dm−3 (0.063 mol dm−3 ≤ c0(EB) ≤ 0.066 mol dm−3). 3.3. Raman measurements FT-Raman spectra were measured at a room temperature (t = 26 ± 1 °C) on a Bruker Equinox 55 interferometer equipped with the FRA 106/S Raman module using Nd-YAG laser excitation at 1064 nm and the laser power of 500 mW. Scattered radiation was collected using 180° scattering arrangement. A quartz cuvette with backscattering mirror was used for handling liquid samples. Spectra were taken in the 3500 to − 2000 cm−1 range at the spectral resolution of 2 cm−1. The Blackman–Harris 4-Term function was applied for interferogram apodization. To obtain a good spectral definition of ternary mixtures and binary mixtures of EPs or EB in C2Cl4, 1024 scans were averaged for a spectrum. The spectra of binary mixtures of TMP in C2Cl4 were averaged over 128 scans; this was found to be sufficient for verifying that TMP does not produce any signal in investigated spectral range (C`C stretching). All solutions were recorded once when scattered light is polarized in a direction parallel (VV) and perpendicular to that of incident light (VH). The exception was EB in C2Cl4 mixture; this solution was recorded three times at both VV and VH conditions in order to estimate the instrumental uncertainty in the subsequent band profile analysis. In order to verify that the spectral acquisition time, during which the sample is heated by the laser radiation, does not affect the shape of the feature attributed to the C`C and C`C⋯ stretching, FT-Raman spectra of liquid CH were taken under laser power of 500 mW and averaged with, respectively, 64, 128, 512 and 1024 scans (Fig. S10 in Supplementary Materials).

3.1. Chemicals 3.4. Spectral analysis 2-Ethynylpyridine (2-EP; dark brown liquid at room temperature, b.p. = 85 °C at 12 mm Hg, 99% purity), 3-ethynylpyridine (3-EP; pale brown solid at room temperature, m.p. = 39–40 °C, 98% purity) and trimethylphosphate (TMP; colorless liquid at room temperature, b.p. = 192–194 °C, 99 + % purity) are purchased from Sigma Aldrich. Ethynylbenzene (EB; dark yellow liquid at room temperature, b.p. = 142–144 °C, 99% purity) and tetrachloroethene (C2Cl4; colorless liquid at room temperature, b.p. = 122 °C, spectroscopic grade) are purchased from Alfa Aesar and Acros Organics, respectively. Cyclohexane (CH; colorless liquid at room temperature, b.p. = 81 °C, ACS purity), used for testing the influence of spectral acquisition time on the band shape, is purchased from Fluka. All chemicals were used as received. Densities of the studied liquids were determined by a densitometer DMA 5000-Anton Paar. 3.2. Solutions preparation 3.2.1. Mixtures of EPs or EB in C2Cl4 Binary mixtures were prepared by dissolving appropriate amount of 3-EP in C2Cl4 and by mixing appropriate volume of 2-EP or EB with C2Cl4, resulting in final concentrations: c0(2-EP) = 0.044 mol dm−3, c0(3-EP) = 0.055 mol dm−3 and c0(EB) = 0.061 mol dm−3. 3.2.2. Mixtures of EPs or EB with TMP in C2Cl4 A set of 10 binary mixtures was prepared by mixing appropriate volume of TMP with C 2Cl4 . The concentrations of TMP were: 0.111 mol dm − 3 ≤ c 0(TMP) ≤ 0.839 mol dm − 3 (for preparation of

The stretching of C`C moieties in isotropic Raman spectra, both associated and unassociated, where analyzed by using Matlab R2014a in the following spectral ranges: 2-EP (2180–2060 cm−1), 3-EP (2177– 2157 cm−1) and EB (2174–2054 cm−1), comprising of 125 points with the absorption maximum in the center. The curve fitting was formulated as an optimization problem which was then solved by a mutation selection algorithm with a simulated annealing [27]. The signals generated by of P\\O\\(CH3) moieties for all three complexes were qualitatively analyzed in spectral range 1100–1060 cm−1 (53 points). FT-Raman spectra of liquid CH collected with different numbers of scans were analyzed in spectral range 1320–1216 cm−1 (109 points). The band at 1267 cm−1 is used as a calibration tool because its bandwidth is considered to be invariant to the instrumental conditions [28]. The details of its analysis, along with the confirmation that the heating of the sample during spectral acquisition is not an issue in the present study, are exhibited in Supplementary Materials. 4. Results and discussion 4.1. Prediction of geometries and vibrational frequencies of HB complexes between ethynyl compounds and trimethylphosphate Fig. 1 shows optimized geometries of complexes 2-EP and TMP (a), 3EP and TMP (b), and EB and TMP (c) obtained at the MP2/6–311 + G(d,p) level of theory.

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In the case of 2-EP ⋯ TMP complex, we see a hydrogen bond (HB) formed between 2-EP and TMP methyl group, as indicated by the distance of 2.451 Å between 2-EP nitrogen atom and hydrogen atom of the methyl group. Also, a HB is simultaneously formed between C`C triple bond and methyl group of TMP with the distance of 2.953 Å. A HB between`C\\H group and oxygen atom of TMP is present as well but the distance between hydrogen atom and oxygen atom is slightly longer being 3.294 Å (Fig. 1a). In contrast, in 3-EP ⋯ TMP complex, a HB between 3-EP and TMP does not exist due to simple geometrical reasons — it is not possible that TMP methyl group interacts simultaneously with 3-EP nitrogen atom and C`C triple bond. On the other hand, a HB between C`C triple bond and TMP methyl group is witnessed by the distance of 2.906 Å (Fig. 1b), which is slightly shorter than in 2EP⋯TMP complex. Interestingly, a HB between `C\\H group and oxygen atom of TMP is now significantly shorter than in 2-EP⋯TMP complex (2.652 Å vs. 3.294 Å, respectively). In this complex, a weak HB is also formed between TMP methyl group and pyridine hydrogen atom, as indicated by the distance of 2.716 Å. Finally, Fig. 1c shows EB⋯TMP complex where no HBs containing nitrogen atom are possible. However, a HB bond between C`C triple bond and TMP methyl group still exists, with the corresponding distance of 2.830 Å. In this case, a HB between `C\\H group and oxygen atom of TMP is present too with the distance of 2.738 Å. Taken together, a short analysis of HB geometrical parameters shows that several types of HB are possible, as nicely illustrated in the 2-EP ⋯ TMP complex which possess three distinct HBs. In the case of 3-EP⋯TMP complex and EB⋯TMP complex, respectively, a HB between pyridine nitrogen atom is not present anymore indicating that this HB will exist if geometrical orientation between HB donor and acceptor is favorable, similar to previously studied systems in our group [8,29,30]. We should stress that present calculations are obtained at the MP2 level of theory which includes implicit description of electron correlation, and in turn is capable of describing dispersion interactions from first principles. In particular, these calculations predict that a weak HB between TMP methyl group and 2-EP indeed exists thus allowing a detailed analysis by FT-Raman spectroscopy. Additionally, MP2 calculations predict a HB between TMP methyl group and pyridine hydrogen atom in 3-EP⋯TMP complex, which is also experimentally analyzed in detail in the subsequent sections. The calculated and experimentally observed frequencies of the most interesting FT-Raman bands, i.e. of those

originated from the stretching of C`C and P\\O\\CH3 groups, both unassociated and HB associated, are presented in Table 1.

4.2. The analysis of relevant FT Raman bands 4.2.1. C`C and C`C⋯ stretching bands Spectral signatures of the C`C⋯H(CH2) HB in isotropic FT-Raman spectra are delicate to deal with because C`C and C`C ⋯ stretching generate one broad contour with emphasized asymmetry on the lowwavenumber side (Fig. 2). A simultaneous rise in the band asymmetry on low-wavenumber side and a slight decrease in maximum band intensity along its low-frequency shift by 1–3 cm−1, accompanied with TMP concentration increase, could be easily overlooked if absorption of C`C and C`C⋯ stretching bands would not be relatively easy to distinguish from FT-IR spectra [8] (Fig. S1 in Supplementary Materials). Since an eventual contribution of any other chemical species (TMP and/or C2Cl4) in this spectral region in FT-Raman spectra is eliminated (Fig. S8a in Supplementary Materials), the form of studied envelope is caused by the proximity of the absorptions maxima of the C`C and C`C ⋯ stretching bands, along with their intensity ratio, apparent widths and especially their shapes. Fortunately, due to significantly 2

dμ Þj for C`C⋯ than for C`C oscillator (at least when EPs are higher jðdQ

concerned), the estimation of their absorption frequency differences ~ = 9 cm−1 (2-EP), 9 cm−1 (3-EP) and 7 cm−1 from FT-IR spectra (Δv (EB)) is relatively straightforward. In contrast, intensities of C`C and C`C⋯ stretching bands in FT-Raman spectra are distributed in opposite 2

dα sense due to greater jðdQ Þj for the former than the latter band.

The effect preventing an unambiguous deconvolution of C`C and C`C⋯ stretching bands is intrinsic low-frequency asymmetry of C`C oscillator. As seen from Fig. 2, (yellow curve in FT-Raman spectra of all ethynyl compounds), an apparent C`C stretching band exhibits asymmetry even in the absence of TMP. Along with the natural lifetime of an oscillator, environmental fluctuations and vibrational resonant energy transfer [31], an additional mechanism needs to be introduced to unravel the physical cause that generates the asymmetric band shape. Since for EB in binary mixtures this phenomenon is already documented, two of the most frequent and/or plausible interpretations are presented here. Abramczyk et al. have presumed that the changes in structural and

Fig. 1. HB complexes of: a) 2-EP and TMP; b) 3-EP and TMP; c) EB and TMP obtained by MP2/6–311 + G(d,p) calculations. All distances are given in Å.

D. Vojta et al. / Journal of Molecular Liquids 218 (2016) 499–507

503

Table 1 Comparison of experimental (FT-IR; FT-Raman) and calculated (MP2/6–311 + G(d,p)) frequencies of ethynyl compounds and TMP. The solutions with the highest analytical concentrations of TMP, in all three complexes, are used for determination of the position of HB associated bands. ~a ν C`Cb,c

C`C⋯b,c

System 2-EP 3-EP EB TMP 2-EP⋯TMP 3-EP⋯TMP EB⋯TMP a b c d

POCH3 & POCH3⋯d

POCH3

Exp

Calc

Exp

Calc

Exp

Calc

2120, 2122 2116, 2118 2113, 2114 – 2119, 2120 2115, 2116 2112, 2113

2022 2027 2020 – – – –

– – – – 2111, overlapped 2107, overlapped 2106, overlapped

– – – – 2017 2018 2011

– – – 1093, 1087, 1081, 1078 1093, 1087, 1084 1097, 1095, 1091, 1086, 1082 1094, 1089, 1084, 1079

– – – 1075, 1043, 1043 – – –

POCH3⋯

– – – – 1073, 1041, 1037 1074, 1043, 1041 1074, 1041, 1039

In cm−1. Taken from FT-IR spectra [8]. From FT-Raman spectra presented in this paper. Associated and unassociated oscillators are rather undistinguishable in FT-Raman spectra.

orientation order of EB, driven by primarily hard-sphere repulsive collisions, produce the observed band pattern [32,33]. Major objection to their hypothesis is ignoring the possibility that HB between EB molecules themselves (C`C ⋯ H\\C`C) produces a signature that might give a certain contribution to the observed band shape [34]. The second interpretation of the asymmetry of C`C stretching band of EB in CCl4 solution, in both IR and Raman spectra, is addressed to the hot band sequence (HBS) arisen due to the anharmonic coupling between the fundamental transition of C`C stretching and likely at least one of the bendings in EB molecule [16,17,35,36–38]. Analogously, we assume that the HBS is the reason for the appearance of the asymmetry of EPs C`C stretching in C2Cl4 solution. The characterization of both C`C and C`C⋯ oscillator stretching in vibrational dephasing terms is therefore extremely difficult. The major obstacle in the fitting procedure, by employing Eq. (6), is the estimation of the number of bands lying underneath the observed band profile. This is a serious drawback when considering EPs or EB in C2Cl4 mixtures, while in ternary mixtures at least one more unknown term should be added. In situations like these, when unknown number of closely overlapped bands constitutes a specific band pattern, statistical indicators such as χ2 usually cannot produce the significant differences between the fitting results which differ by the number of bands [39]. The dilemma on the number of the bands that produce the observed isotropic band (Fig. 2) was resolved by applying derivative spectroscopy in both FT-Raman and FT-IR spectra. The details on the performed procedure are given in Supplementary Materials, and only the determined number of bands will be presented here. It is presumed that in binary mixtures,

EP or EB in C2Cl4, two bands are overlapped: one is attributed to the fundamental transition of C`C stretching, while another to the HBS produced by the coupling of the fundamental transition of C`C stretching and undefined linear bending(s). Consequently, in ternary mixtures (the analysis is also given in Supplementary Materials), the number of the bands is assumed to be three, where the additional band is attributed to the fundamental transition of the stretching of C`C⋯ oscillator (Figs. S2–S4 in Supplementary Materials). For binary and two ternary mixtures of all three systems (with the lowest and the highest analytical concentrations of TMP given in Fig. 3 caption), the fitting outcome is exhibited in Fig. 3. In turn, when expression (6), with presumed 2 and 3 bands, respectively, is used for the fitting of isotropic Raman band profiles (Fig. 2), the estimation of vibrational dephasing parameters (Table 2) and, the influence of HB on the frequency modulation events on C`C stretching is enabled. Before any discussion on dynamical parameters of C`C stretching in ternary mixtures (both associated and unassociated), we should note that the relevant quantities are determined as an average value of the measurements obtained in the whole concentration range of EPs or EB with TMP in C2Cl4 mixtures. The uncertainties associated with the instrumental response are estimated by collecting FT-Raman spectra of EB in C2Cl4 mixtures three times in VV and VH polarizations of the incident and scattered light. They are most probably the same for both EPs in C2Cl4 and EB in C2Cl4 binary mixtures. The instrumental uncertainties attributed to the HBS and C`C⋯ stretching band are assumed to be the same (in either case, larger than for C`C stretching band). With several

Fig. 2. Isotropic C`C stretching band profile of: a) 2-EP, b) 3-EP and c) EB. The lowest spectrum (yellow) is C`C stretching band of EPs or EB in C2Cl4, while the other spectra (red, black (3 of them) and blue) are the C`C stretching bands of EPs or EB in presence of TMP in C2Cl4. The analytical concentrations of TMP in given spectra are in range 0.184 mol dm−3 ≤ c0(TMP) ≤ 0.839 mol dm−3 for mixtures of 2-EP with TMP in C2Cl4, 0.212 mol dm−3 ≤ c0(TMP) ≤ 0.902 mol dm−3 for mixtures of 3-EP with TMP in C2Cl4 and 0.214 mol dm−3 ≤ c0(TMP) ≤ 1.230 mol dm−3 for mixtures of EB with TMP in C2Cl4. The observed band profiles were normalized with respect to the concentrations of EPs (c0(EPs) = 0.05 mol dm−3) and EB (c0(EB) = 0.06 mol dm−3) in order to better distinguish changes in band intensity.

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Fig. 3. The fitting outcome, by using (6), of FT-Raman spectra of: a) 2-EP in C2Cl4 (c0(2-EP) = 0.044 mol dm−3) and 2-EP with TMP in C2Cl4 mixtures (S1: c0(2-EP) = 0.050 mol dm−3, c0(TMP) = 0.111 mol dm−3 and S10: c0(2-EP) = 0.044 mol dm−3, c0(TMP) = 0.839 mol dm−3); b) 3-EP in C2Cl4 (c0(3-EP) = 0.055 mol dm−3) and 3-EP with TMP in C2Cl4 mixtures (S1: c0(3-EP) = 0.057 mol dm−3, c0(TMP) = 0.116 mol dm−3 and S10: c0(3-EP) = 0.054 mol dm−3, c0(TMP) = 0.902 mol dm−3); c) EB in C2Cl4 (c0(EB) = 0.061 mol dm−3) and EB with TMP in C2Cl4 mixtures (S1: c0(EB) = 0.065 mol dm−3, c0(TMP) = 0.112 mol dm−3 and S10: c0(EB) = 0.066 mol dm−3, c0(TMP) = 1.230 mol dm−3).

Table 2 Relevant dynamics parameters related to the C`C stretching, associated and unassociated, of EPs and EB. System 2-EP 3-EP EB 2-EP⋯TMP

3-EP⋯TMP

EB⋯TMP a b c d

~a ν

M2b

τVc

τ ωc

2122 ± 1 2117 ± 1 2118 ± 1 2113 ± 1 2114 ± 1 2110 ± 1 2121 ± 1 2118 ± 1 2112 ± 2 2118 ± 1 2114 ± 2 2110 ± 1 2114 ± 1 2110 ± 1 2110

0.56 ± 0.01 6.1 ± 0.8 0.52 ± 0.01 2.4 ± 0.8 0.48 ± 0.01 3.3 ± 0.8 0.57 ± 0.05 6±2 1.1 ± 0.3 0.52 ± 0.06 3±1 2±2 0.5 ± 0.2 2.2 ± 0.8 2.5 ± 0.2

2.20 ± 0.03 1.08 ± 0.03 2.30 ± 0.03 1.16 ± 0.03 2.37 ± 0.03 1.23 ± 0.03 2.21 ± 0.09 1.7 ± 0.2 0.96 ± 0.09 2.4 ± 0.2 1.6 ± 0.2 1.2 ± 0.2 2.5 ± 0.2 1.3 ± 0.1 1.06 ± 0.06

1.51 ± 0.03 1.8 ± 0.1 1.53 ± 0.03 0.6 ± 0.1 1.67 ± 0.03 0.3 ± 0.1 1.5 ± 0.1 1.0 ± 0.3 0.23 ± 0.07 1.4 ± 0.1 0.5 ± 0.3 0.6 ± 0.2 1.6 ± 0.4 0.6 ± 0.2 0.68 ± 0.07

In cm−1. In ps−2. In ps. The interval determined from three isotropic FT-Raman spectra.

χ2 0.047 0.026 0.022–0.035d 0.021–0.051

0.023–0.075

0.035–0.086

exceptions addressed to the M2 or τω values of HBS and C`C ⋯ stretching band, they are smaller than the uncertainties coming out after taking the average value of expected ten measurements. As the exceptions are concerned, the deviations in two types of uncertainties are estimated to be below 10%. According to the data in Table 2, 1 ← 0 transition of the C`C stretching appears at 2122 cm−1, 2118 cm−1 and 2114 cm−1 for 2-EP, 3-EP and EB in C2Cl4, respectively, while in ternary mixtures their position remain unchanged within the limits of the uncertainty. The band attributed to the HBS is, both in the absence and the presence of HB in all systems, shifted by 4–5 cm−1 to the lower frequencies with respect to the fundamental transition. The band attributed to the C`C ⋯ stretching, with respect to the C`C stretching band, in EPs⋯TMP HB complex is downshifted for 8 cm−1, while in EB⋯TMP system its position is estimated to be at 2110 cm−1, just like the HBS. The wavenumbers of the C`C⋯ stretching bands, except for EB⋯TMP, coincide with the corresponding values obtained from IR spectra [8]. Regarding the latter, even from FT-IR spectra it was very difficult to make an estimation of the position of C`C ⋯ stretching band maximum. The calculations at the MP2/6-311 + G(d,p) level of theory predict downshift of 5 cm−1, 9 cm−1 and 9 cm−1, for complexes 2-EP⋯TMP, 3-EP⋯TMP and EB⋯TMP, respectively, which is in an excellent agreement with the experimental values (Table 1).

D. Vojta et al. / Journal of Molecular Liquids 218 (2016) 499–507

The amplitude of the frequency fluctuations, expressed as M2, is almost the same for the fundamental transition of C`C stretching in all systems and is about 0.5 ps−2. This quantity is greater for the band attributed to the HBS for an order of magnitude; the greatest is for 2-EP in C2Cl4, in mixture with and without TMP (about 6 ps−2). For 3-EP and EB, both binary and ternary mixtures, the evaluated M2 value is between 2 and 3 ps−2. In each case, M2 value for HBS band coincide when formation and disruption of a HB is in dynamic equilibrium. For C`C⋯ stretching in 2-EP ⋯ TMP HB complex M2 is about 1 ps−2, while in 3EP⋯TMP and EB⋯TMP HB complex this value is twice as large. However, when taking into account the uncertainty associated with the latter value measured for 3-EP⋯TMP HB complex (2 ± 2 ps−2), it cannot be stated that parameters for C`C⋯ stretching of EPs, are indeed different. Overall, frequency fluctuations exhibit broader distribution for C`C group involved in a HB complex. τν values, i.e. the time needed for the vibrational phase being completely lost, regardless of the HB formation, are almost the same for both C`C stretching and HBS band in all systems; in the former it is about 2 ps, while in the latter it is roughly between 1 and 2 ps. For the C`C⋯ stretching, in all systems of EPs or EB with TMP in C2Cl4, it is about 1 ps. Frequency modulation times, τω, for binary and ternary mixtures that contain 2-EP are between 1.0 and 1.8 ps for C`C stretching and HBS bands, while for the mixtures composed from 3-EP and EB these values are somewhat smaller (0.5–1.6 ps). However, more distinguished differences are obtained for frequency modulation times of the C`C⋯ stretching; τω is for 2-EP⋯TMP HB complex 0.23 ps, while for 3-EP ⋯ TMP and EB ⋯ TMP HB complexes is 0.6–0.7 ps. In general, the comparison of τν and τω for C`C and C`C ⋯ stretching in all HB complexes suggests that the dephasing of the latter one occurs at a shorter time scale than the former one which is the consequence of greater sensitivity of the latter on the perturbative events by the surroundings. Overall, vibrational dephasing parameters in ternary mixtures containing different ethynyl compound reveal modest, but relevant differences. The most distinguished variations are addressed to the: i) M2 values of HBS band which are about 6 ps−2 in 2-EP (+ TMP) in C2Cl4 mixtures and 2–3 ps−2, respectively, in 3-EP (+TMP) in C2Cl4 mixtures and EB (+TMP) in C2Cl4 mixtures; ii) τω for C`C⋯ stretching band in 2-EP ⋯ TMP HB system (0.23 ± 0.07 ps) is about three times smaller than the analogous quantity obtained for 3-EP ⋯ TMP (0.6 ± 0.2 ps) and for EB ⋯ TMP (0.68 ± 0.07 ps) HB systems. At present, we can make suggestions related to the proximity of N-atom to the ethynyl group in 2-EP, in contrast to 3-EP and EB as suggested by MP2 calculations: i) regardless of the presence of TMP, N-atom in 2-EP can increase electrical and/or mechanical anharmonicity that naturally leads to the coupling of C`C stretching mode with bending mode(s) in all studied ethynyl compounds. In turn, the amplitude of frequency fluctuations for 2-EP (+ TMP) in C2Cl4 mixtures is greater than for, respectively, 3-EP (+ TMP) in C2Cl4 and EB (+ TMP) in C2Cl4 mixtures; ii) in the former complex, in contrast to the latter ones, the vicinity of N-atom disturbs HB formed between C`C (2-EP) and CH3 (TMP) group (Fig. 1a). In 3-EP⋯TMP HB complex this is not possible due to spatial relationship between N-atom and C`C group (Fig. 1b), while EB does not contain N-atom, or any other relatively electronegative atom which would interfere and consequently induce faster decay of vibrational phase relaxation of C`C⋯ stretching mode (Fig. 1c). 4.2.2. P\\O\\(CH3) stretching bands The calculations at MP2/6–311 + G(d,p) level of theory predict that the stretching of P\\O\\(CH3) moieties produces three distinguished bands that originate from stretching of P\\O\\(CH3) moieties (Table 1). Unfortunately, the signal produced by P\\O\\(CH3) stretching of TMP in FT-Raman spectra is inherently low (Fig. S9c in Supplementary Materials). In the presence of all ethynyl compounds (Fig. 4) the signal is seriously compromised with noise and to a certain extent, also with weak but

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interfering signals of ethynyl compounds itself. The signals of different conformers may be discerned analogously to P = O stretching signals, although isomers other than the C3 (the strongest bands) are barely differentiated from the noise. Even though the signals are of rather low quality and of poor intensity, we focus our efforts in making comparative analysis of P\\O\\(CH3) stretching bands in different complexes. Furthermore, in all examples we discuss spectrum with the greatest analytical concentration of TMP (c0 ~1 mol dm−3) and count only band maxima generated by the stretching of P\\O\\(CH3) in the C3 isomer. In 2-EP⋯TMP complex the bands maxima 1093 cm−1, 1087 cm−1 and 1084 cm−1; very similar, in EB⋯TMP HB complex there is registered only one more band maximum: 1094 cm−1, 1089 cm−1, 1084 cm−1 and 1079 cm−1, but overall shape resembles the band pattern in 2-EP ⋯TMP complex. The band structure in 3-EP⋯TMP complex significantly differs from other two — not only there are more band maxima of differently distributed intensities 1097 cm−1, 1095 cm−1, 1091 cm−1, 1086 cm−1, and 1082 cm−1, but also one has to take in consideration the shift of the most prominent band maxima as the analytical concentration of TMP gets higher. In particular, for c0(TMP) ~0.3 mol dm−3 one of the maxima is found at 1100 cm−1 and another at 1089 cm−1, while for c0(TMP) ~1 mol dm−3, the same maxima are shifted to low-frequency range and are registered at 1097 cm− 1 and at 1082 cm− 1 , respectively. Their shift to lowfrequency range suggests the lengthening and shortening of the P\\O\\(CH3) bond when TMP is in a HB complex with 3-EP. Due to greater acidity of ortho H-atom on aromatic ring substituted with an electron accepting group such as ethynyl group [40], the most straightforward explanation for this case is in involvement of P\\O\\(CH3) moiety in the formation of a HB with H-atom of 3-EP placed between N-atom and ethynyl moiety, as indicated by the MP2 calculations (Fig. 1b). 4.2.3. C`C\\H and C`C\\H⋯ stretching bands C`C\\H⋯O interaction is considered to be the main driving force for EPs or EB and TMP complexes formation. Unfortunately, this study does not provide any additional information in vibrational dephasing of C`C\\H oscillators because the signals of neither unassociated nor associated oscillators are observed in the FT-Raman spectra (Fig. S5 in Supplementary Materials). Calculated shifts of C`C\\H ⋯ frequencies are downshifted to low-frequency range by 3 cm− 1, 13 cm− 1 and 12 cm−1 in complexes 2-EP⋯TMP, 3-EP⋯TMP and EB⋯TMP, respectively, as obtained by MP2/6-311 + G(d,p) calculated level of theory. However, the calculated downshift is smaller (in the order of ten of cm−1), as compared to the experimental one which is larger, being in the order of hundred cm−1 (Table S2 in Supplementary Materials). 4.2.4. CH3 stretching bands The stretching of CH3 groups, even though completely observable in FT-Raman spectra for all investigated HB systems, does not provide an information which would enable the discrimination of CH3 ⋯ N from CH3 ⋯ C`C HB in 2-EP ⋯ TMP HB complex (Fig. S6 in Supplementary Materials). 4.2.5. P_O stretching bands As the strongest HB accepting center in explored HB complexes, P_O moiety exhibits some changes (Fig. S7 in Supplementary Materials). Major obstacle in their deeper analysis is the fact that P_O stretching, even in absence HB, generates several bands (Fig. S9b in Supplementary Materials). A more detail description of experimentally obtained FT-Raman spectra is given in Supplementary Materials. 5. Conclusions The secondary interaction pattern between 2-ethynylpyridine (2-EP), 3-ethynylpyridine (3-EP) and ethynylbenzene (EB) with trimethylphosphate (TMP) in C2Cl4 was explored with FT-Raman spectroscopy and calculations at the MP2/6-311 + G(d,p) level of theory. The stretching of ethynyl moiety of EPs and EB, both unassociated

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D. Vojta et al. / Journal of Molecular Liquids 218 (2016) 499–507

Fig. 4. Isotropic P\ \O\ \(CH3) stretching bands of TMP in C2Cl4 when in complex with: a) 2-EP, b) 3-EP, c) EB. The lowest spectrum (yellow) is of EPs or EB in C2Cl4, while the other spectra (red, black (3 of them) and blue) are the P = O stretching bands of TMP in C2Cl4 in presence of EPs or EB. The analytical concentrations of TMP in given spectra are in range 0.184 mol dm−3 ≤ c0(TMP) ≤ 0.839 mol dm−3 for mixtures of 2-EP with TMP in C2Cl4, 0.212 mol dm−3 ≤ c0(TMP) ≤ 0.902 mol dm−3 for mixtures of 3-EP with TMP in C2Cl4 and 0.214 mol dm−3 ≤ c0(TMP) ≤ 1.230 mol dm−3 for mixtures of EB with TMP in C2Cl4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(C`C) and associated (C`C ⋯), was shown to be the most sensitive probe for characterization of C`C ⋯ CH3 hydrogen bond (HB) in the frame of vibrational dephasing. The complex band feature, composed from closely overlapped bands originating mostly from the stretching of C`C and C`C ⋯ groups, was resolved with the utility of Egelstaff & Schofield function. The frequency modulation time τω of C`C ⋯ stretching for 2-EP ⋯ TMP HB was found to be approximately 1/3 of the frequency modulation time values evaluated for analogous quantities for 3-EP ⋯ TMP and EB ⋯ TMP HB. This difference was attributed to the additional N ⋯ HCH2 HB in 2-EP ⋯ TMP HB complex which, in contrast to the molecular structure of 3-EP and EB, disturbs the HB formed between C`C and CH 3 groups as confirmed by MP2 calculations. Finally, based on the concentration-dependent shift of P\\O\\(CH3) stretching bands when TMP is in the HB complex with 3-EP, the weak P\\O\\(CH3)⋯ H\\C = interaction is present in 3-EP ⋯ TMP HB complex. This interaction is absent in other two HB complexes due to spatial arrangement as suggested by MP2 calculations. Acknowledgments This work was supported by grants Nos. 0982904-2927 and 1191191342-2959 from Ministry of Science, Education and Sports of the Republic of Croatia. D. V. thanks J. Alerić and T. Parlić-Risović from Croatian Metrology Institute for measuring the densities of studied liquids. M. V. thanks Croatian Academy of Sciences and Arts for support. The constructive comments and advices of Prof. Sviatoslav A. Kirillov are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2016.02.042. References [1] S. Maity, M. Guin, P. Chandra Singh, G. Naresh Patwari, Phenylacetylene: a hydrogen bonding chameleon, ChemPhysChem. 12 (2011) 26–46. [2] P. Chandra Singh, G. Naresh Patwari, IR-UV double resonance spectroscopic investigation of phenylacetylene-alcohol complexes. alkyl group induced hydrogen bond switching, J. Phys. Chem. A 112 (2008) 5121–5125. [3] R. Sedlak, P. Hobza, G. Naresh Patwari, Hydrogen-bonded complexes of phenylacetylene with water, methanol, ammonia, and methylamine. the origin of methyl group-induced hydrogen bond switching, J. Phys. Chem. A 113 (2009) 6620–6625.

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