Intramolecular hydrogen bonding and electronic structure of thiadiazole annulated hemihexaphyrazine Текст научной статьи по специальности «Физика»

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http://macroheterocycles.isuct.ru

DOI: 10.6060/mhc180175o

Intramolecular Hydrogen Bonding and Electronic Structure of Thiadiazole Annulated Hemihexaphyrazine

Arseniy A. Otlyotov,@ Anton P. Merlyan, Vladimir V. Veretennikov, Alexander E. Pogonin, Evgeny N. Ivanov, Yana E. Filippova, Yuriy A. Zhabanov,@ and Mikhail K. Islyaikin@

Institute of Macroheterocycles, Ivanovo State University of Chemistry and Technology (ISUCT), 153000 Ivanovo, Russian Federation

@Corresponding authors E-mails: [email protected], [email protected], [email protected]

Different tautomeric structures of thiadiazole annelated hemihexaphyrazine (TDAHHp) were considered using DFT in B3LYP/pcseg-2 approximation. Energetics of the intramolecular hydrogen bonding was quantitatively estimated by means of NBO and AIM calculations and according to the NBO analysis results, the most energetically favorable tautomer of TDAHHp surprisingly turned out to possess the lowest total H-bond stabilization energy. The results are compared with that for the previously studied hemihexaphyrazine - 2,3,5,10,12,13,15,20,22,23,25,30-dodecaaza-hexaphyrin (C30H15N15S3). Electronic absorption and infrared spectra were simulated for the most favorable tautomer of TDAHHp.

Keywords: Hemihexaphyrazine, hydrogen bond, electronic spectrum, NBO, AIM.

Внутримолекулярные водородные связи и электронное строение тиадиазол-аннелированного гемигексафиразина

А. А. Отлётов,@ А. П. Мерлян, В. В. Веретенников, А. Е. Погонин, Е. Н. Иванов, Я. Е. Филиппова, Ю. А. Жабанов,@ М. К. Исляйкин@

Посвящается профессору Олегу Александровичу Голубчикову по случаю его 70-летнего юбилея

НИИМакрогетероциклических соединений, Ивановский государственный химико-технологический университет, 153000 Иваново, Россия

@E-mails: [email protected], [email protected], [email protected]

С помощью DFTрасчетов в приближении B3LYP/pcseg-2 изучены семь таутомеров тиадиазол-аннелирован-ного гемигексафиразина (TDAHHp). Энергия внутримолекулярных водородных связей в таутомерах оценивалась с использованием двух различных подходов: NBO анализа распределения электронной плотности и топологической теории AIM. На основе результатов NBO-расчетов сделано неожиданное заключение о том, что наиболее энергетически выгодный таутомер TDAHHp характеризуется минимальной суммарной энергией внутримолекулярных водородных связей. Результаты расчетов сопоставлены с результатами ранее изученного гемигексафиразина - 2,3,5,10,12,13,15,20,22,23,25,30-додекаазагексафирина (C30)H15N15S3). Для наиболее энергетически предпочтительного таутомера TDAHHp проведены TDDFTрасчеты электронных переходов. Смоделирован инфракрасный спектр исследуемого соединения на основе квантово-химических расчетов.

Dedicated to Professor Oleg Aleksandrovich Golubchikov on the occasion of his 70-th birthday

Ключевые слова: Гемигексафиразин, водородная связь, электронный спектр поглощения, NBO, AIM. Макрогетероциклы /Macroheterocycles 2018 11 (1) 67-72 © ISUCT Publishing

Introduction

Investigations of the molecular structure and tautomeric behaviour of macroheterocyclic systems have been the subject of the many recent studies.[1-6] Commonly used DFT calculations often allow to establish the most energetically favorable tautomer and then the preference is discussed in light of fundamental concepts of chemistry. Intramolecular hydrogen bonding is one of the most important issues among them for such objects. However, it is essential to put general considerations on the quantitative basis. One of the most popular approaches for quantitative estimation of the phenomenon of hydrogen bonding nowadays was developed in AIM theory and it is focused on the locating so-called "bond critical points" (BCPs) corresponding to hydrogen bonds. Besides, in the recent study of Vogt et al.,[7] the energy of a weak hydrogen bond was estimated based on the results of the NBO analysis. Both approaches are used in the present study for thiadiazoleannelated hemihexaphyrazine (TDAHHp) macroheterocycle (Figure 1). They are demonstrated to provide a different point of view on the observed tau-tometic behaviour as compared to the previously studied 2,3,5,10,12,13,15,20,22,23,25,30-dodecaazahexaphyrin

(C30H15NlA).[5]

s

Figure 1. Molecular model of TDAHHp (D3h point group) with atom labeling.

Computational details

DFT calculations for possible NH-tautomers of TDAHHp

were performed with use of Gaussian 09 program package.[8]

The calculations were carried out using B3LYP[9-11] functional and pcseg-2[12] basis set (C,N,H,S) taken from the EMSL BSE library. Planar models of the tautomers (Figure 4) were optimized under D3h (TDAHHp), C3h (3PT), C2V (2PT-1, 2PT-2) and CS (1PT-1, 1PT-2, 2PT-3) symmetry constraints. Vibration frequencies were calculated in the harmonic approximation for all the optimized structures. In order to simulate the shape of the infrared (IR) spectrum of TDAHHp (Figure 7), the individual bands were described by Lorentz curves with a half width of 15 cm-1. The abbreviations of the tautomers reflect "proton(s) transfer"(PT) from the main TDAHHp structure. E.g., the structures 1PT-1 and 1PT-2 can be obtained from the main TDAHHp structure by "migration" of one hydrogen from atom N to Nt (see Figure 4), etc.

The natural bond orbital (NBO) analysis was performed as it is implemented in Gaussian 09.[13]

Quantitative QTAIM (Quantum Theory of Atoms in Molecules) analysis was performed using AIMAll software package.[14]

Electronic absorption spectrum of TDAHHp was simulated on the basis of TDDFT calculations in B3LYP/pcseg-2 approximation with use of Firefly QC package,[15] which is partially based on the GAMESS(US) source code.[16] Twelve low-lying electronic states for each irreducible representation of the point group D3h have been taken into account within TDDFT calculations. Molecular models and orbitals demonstrated in the paper were visualized by means of Chemcraft program.[17]

Results and Discussion

NH-Tautomerism and energetics of the intramolecular hydrogen bonding

According to the results of the calculations, none of the optimized planar structures of the tautomers possesses imaginary frequencies, except for tautomer 1PT-2, in which strong repulsion between the neighboring H atoms appears in the planar framework. Equilibrium structures of all six NH-tautomers are much higher in energy than the parent TDAHHp structure (see Table 1).

The lowest in energy parent TDAHHp structure contains six Nt...H contacts (or three three-centered contacts Nt.. .H.. ,Nt). If one contactbecomes broken due to "migration" of a hydrogen (TDAHHp^1PT-1), the energy of the whole molecule increases by 39.2 kJmol-1 (see Table 1). Further migration of another H atom (1PT-1^2PT-3) again leads to significant increase of energy.

The dependence of the relative energy of the most favorable NH-tautomers on the number of intramolecular H-bonds is almost linear (see Figure 2). It is possible to calculate the energy difference per one broken hydrogen bond (which should be approximately equal to the H-bond energy) by dividing the relative energy AE by the differ-

Table 1. Relative energies of NH-tautomers of TDAHHp and number of the intramolecular H-bonds.

Model TDAHHp 1PT-1 2PT-1 2PT-2 2PT-3 3PT

AE, kJmol-1 0 39.2 85.4 129.3 68.8 88.7

£sum(2)(LP(N)^a*(N-H), kJmol-1 92.6 106.1 - - 125.5 138.8

EHB,totаl, kJ'm°l4 107.7 104.6 - - 102.8 100.0

Number of H-bonds 6 5 4 4 4 3

Energy gain per one lost H-bond, kJmol-1 0 39.2 42.7 64.7 34.4 29.6

H-'-1-'-1-1-r

3 4 5 6

Number of hydrogen bonds

Figure 2. Relative energies of the most favorable NH-tautomers vs. number of the intramolecular H-bonds.

ence in the number of H-bonds between corresponding tautomers. For example, in the case of 1PT-1, this value is 39.2/(6-5)=39.2 kJmol-1.

One can estimate the energy of the intramolecular hydrogen bonding from the results of the NBO analysis. In terms of the orbital interactions, they can be treated as donor-acceptor interactions between the lone pair (LP) of the Nt atom and c*(N-H) antibonding orbital. The D3h-TDAHHp structure might have been considered to correspond to the minimum due to the largest number of the intramolecular Nt.. .H contacts. However, the results of the NBO calculations distinctly demonstrate the opposite trend within the set of the most favorable models (1PT-1, 2PT-3 and 3PT). Much shorter Nt...H contacts in these structures (ca. 1.95) as compared to the parent TDAHHp structure (ca. 2.20) stabilize the whole molecule in a greater degree (see Table 1). So, the formal largest number of Nt.H contacts apparently is not the reason of the pronounced stabilization of the D3h-TDAHHp structure. Note, that the migration of the inner hydrogen atoms leads to significant changes in the bond lengths and angles in the macrocycle (see Figure S1).

Another possible way to assess the energy of a hydrogen bond (EHB) is an approach developed by E. Espinosa et al.[18] in the framework of AIM theory. They proposed an equation, connecting this energy with the potential energy density V(rCP) calculated at a critical point: EHB=0.5V(rCp). We found the bond critical points (BCPs) corresponding to hydrogen bonds in the most favorable tautomers, calculated the values of EHB according to the above-mentioned formula, and estimated the total energy of intramolecular hydrogen bonds EHBtotal (see Table 1). Interestingly, the trend in EHB is opposite to that calculated using NBO approach (see Figure 3). AIM calculations underestimate the energies of short Nt.H contacts in the structures 1PT-1, 2PT-3 and 3PT as compared to the results of the NBO analysis and do not identify BCPs for the long-range Nt...H interactions (see Table S1). Despite the overall increasing tendency, small differences between the values of EHB for the tautomers as compared to AE (Table 1) cannot act as a main reason for stabilization of the parent TDAHHp structure.

3 4 5 6

Number of H-bonds

Figure 3. The dependence of the total H-bond stabilization energy on the number of hydrogen bonds for the tautomers of TDAHHp calculated using NBO and AIM approaches.

Electronic absorption spectrum

Calculations of the electronic absorption spectrum (Figure 5) predict the most intensive peak with oscillator strength off=0.68 to correspond to the excitation to the lowest 1E' state originating from HOMO^LUMO (2e"^1e"*) transition. The next intensive peak at X=336 nm corresponds to transition 1a1"^1e"*. Detailed data are given in Table 2. The MO level diagram for TDAHHp is shown in Figure 6 and the energies of the MOs are given in Table S2.

IR Spectrum

TDAHHp molecule possesses 138 normal modes of vibration. The vibrational representation for TDAHHp in D3h symmetry is r=16A1+15A2+31E'+6AM1+9A"2+ 15E''. Assignment of the infrared active modes was carried out by the potential energy distribution (PED) analysis among internal coordinates using the VibModule program. [19] The N-H stretching vibration is predicted at 3444 cm-1 (E'). The peak at ra=787 cm-1 can be assigned to the movement of hydrogen atoms out of the plane of the macrocycle (A''2). It should be noted that the composition of most vibrational modes is complicated. Thus, the mode at ra=1242 cm-1 (E') corresponds to the stretching of St-Ct (contribution ~15 %), Nm-Ct (~12 %), Nt-Ct (~23 %) bonds and in-plane bending of N-H (~15 %). The vibration mode at ra =1444 cm-1 (E') corresponds mostly to the stretching of Ca-Nm (~30 %) and Ct-Nt (~30 %) bonds. The main contribution (~70 %) to the modes at 1680 cm-1 (E') and 1686 cm-1 (E') stems from the stretching of Ca-Nm bond. Note, that the region from 1800 to 3400 cm-1 contains no peaks.

Conclusions

The tautomeric behaviour of thiadiazole annelated hemihexaphyrazine (TDAHHp) was studied for the first time by means of DFT calculations. The parent TDAHHp

Figure 4. Models of possible tautomers of TDAHHp.

Figure 5. Calculated electronic spectrum of TDAHHp.

structure corresponds to the minimum of energy, while the other NH-tautomers are much (39.2 kJmol-1 and more) higher in energy. Thus, the conformational behaviour of TDAHHp exhibits both qualitative and quantitative similarity with that of the previously studied C30H15N15S3 macrocycle.[5] However, NBO and AIM calculations performed in the present study provide another point of view on the pronounced stability of the main D3h-structure of TDAHHp as compared to[5]. NBO calculations of energetics of intramolecular hydrogen bonding indicate that several short N...H bonds in the tautomers stabilize the whole structure in a greater degree as compared to six long Nt. H contacts in the parent TDAHHp structure. AIM calculations demonstrate the opposite trend in changing of the total hydrogen bonding energy within the series of the tautomers, but nevertheless, the differences

Figure 6. MO level diagram for TDAHHp.

between the values of are about 10 times less as

HB,total

compared to the corresponding AE magnitudes. Therefore, intramolecular hydrogen bonding is not an interaction stabilizing the main D3h-structure of TDAHHp as might have been thought from qualitative point of view. Investigations of the similar macroheterocyclic systems can shed further light on the true reason of tautomeric preference.

Acknowledgements. We acknowledge Russian Science Foundation (grant № 17-73-10198) for financial support. We also thank Dr. Oleg A. Pimenov for the valuable suggestions concerning AIM calculations.

Table 2. Calculated composition of the lowest excited E' states, excitation energies and oscillator strengths for TDAHHp.

State

Composition (%)

Excitation energy

E, eV

1 ,, nm

calc'

Oscillator strength

1E' 2e"^1e"* (96) 2.9 424 0.68

2E' 2e"^1a2"* (84) 2e"^2e"* (10) 3.3 380 0.12

4E' 1a/'^1e"* (7) 2e"^1a/'* (68) 2e"^2e"* (19) 3.5 349 0.17

5E' 1a/'^1e"* (87) 3.7 336 0.44

6E' 1e"^1e"* (33) 1a2"^1e"* (39) 2a2"^1e"* (20) 4.0 312 0.06

7E' 1e''* (14) 1a2"^1e"* (7) 2a2"^1e"* (70) 4.1 305 0.19

12E' 1a2''* (28) 1a2"^2e"*(6) 2e"^ 2a2''* (43) 4.9 254 0.34

"Only the excited states with an oscillator strength larger than 0.05 are reported. Макрогетер0циmbl /Macroheterocycles 2018 11(1) 67-72

Figure 7. Simulated IR spectrum of TDAHHp.

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Received 26.01.2018 Accepted 06.02.2018