Hydrogen bond strengthening between o-nitroaniline and formaldehyde in electronic excited states: A theoretical study

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 199 (2018) 194–201

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Review Article

Hydrogen bond strengthening between o-nitroaniline and formaldehyde in electronic excited states: A theoretical study Juan Yang, An Yong Li ⁎ School of Chemistry and Chemical Engineering, Southwest University, Tiansheng Road No. 1, Chongqing 400715, People's Republic of China

a r t i c l e

i n f o

Article history: Received 29 January 2018 Received in revised form 16 March 2018 Accepted 23 March 2018 Available online 24 March 2018 Keywords: Hydrogen bonds TD DFT Vibrational spectra Blue shifted Excited states

a b s t r a c t To study the hydrogen bonds upon photoexcited, the time dependent density function method (TD DFT) was performed to investigate the excited state hydrogen bond properties of between o-nitroaniline (ONA) and formaldehyde (CH2O). The optimized structures of the complex and the monomers both in the ground state and the electronically excited states are calculated using DFT and TD DFT method respectively. Quantum chemical calculations of the electronic and vibrational absorption spectra are also carried out by TD DFT method at the different level. The complex ONA⋯CH2O forms the intramolecular hydrogen bond and intermolecular hydrogen bonds. Since the strength of hydrogen bonds can be measured by studying the vibrational absorption spectra of the characteristic groups on the hydrogen bonding acceptor and donor, it evidently confirms that the hydrogen bonds is strengthened in the S1/S2/T1 excited states upon photoexcitation. As a result, the hydrogen bonds cause that the CH stretch frequency of the proton donor CH2O has a blue shift, and the electron excitations leads to a frequency red shift of N_O and N\\H stretch modes in the o-nitroaniline(ONA) and a small frequency blue shift of CH stretch mode in the formaldehyde(CH2O) in the S1 and S2 excited states. The excited states S1, S2 and T1 are locally excited states where only the ONA moiety is excited, but the CH2O moiety remains in its ground state. © 2018 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . Computational Details . . . . . . . . . . . . . . . . . Results and Discussion . . . . . . . . . . . . . . . . . 3.1. Geometric Structures in the S0, S1, S2 and T1 States . 3.2. Electronic Spectra . . . . . . . . . . . . . . . 3.3. Frontier Molecular Orbitals . . . . . . . . . . . 3.4. Natural Bond Orbital and AIM Analyses . . . . . . 3.5. Vibrational Absorption Spectra . . . . . . . . . . 3.6. Hydrogen Bond Strengthening . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary Data . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Noncovalent interactions play a fundamental role in study of the physical and chemical properties of the complex systems. It still currently attracts a considerable attention. Among the noncovalent ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (A.Y. Li).

https://doi.org/10.1016/j.saa.2018.03.062 1386-1425/© 2018 Elsevier B.V. All rights reserved.

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194 195 196 196 196 197 197 198 199 200 200 200

interactions, hydrogen bonding acts as an important factor in several fields, ranging from chemistry to biochemistry and biophysics, and plays a critical role in the configurations of proteins, DNA and RNA [1–11]. Hydrogen bonding is composed of the proton donor and acceptor, and is also a fundamental type of solute-solvent interaction [9–13]. Up to now, the nature of hydrogen bonds in the ground state and excited states is a subject of intense contemporary research interest [4–17].

J. Yang, A.Y. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 199 (2018) 194–201

Upon photoexcitation of hydrogen-bonded systems, the hydrogen bonds are easy to change. The hydrogen donor and acceptor molecules need to reorganize because of the difference in the charge distribution of electronic states. This process is considered as electronic excitedstate hydrogen bonding dynamics (ESHBD) [4,14,17,18]. In this process either hydrogen donor or acceptor, or the whole hydrogen bonding complex is photoexcited to its electronic excited state [14,17]. ESHBD is mainly studied by vibrational properties of hydrogen-bonding acceptor and donor groups. The formation of hydrogen bonds can mainly lead to spectral shifts of some hydrogen-bonded groups [19,20]. Various electronic structure methods have been explored to study the nature of the electronic excited state. Time-dependent density functional theory (TD DFT) is the most widely used alternative to explore the excited states of molecules. Since TD DFT is low computational cost and moderate efficiency a great many of excited state properties based on the TD DFT method have been performed. TD DFT method can obtain reasonable absorption spectra. Consequently, TD DFT has been successfully applied to study the spectra and the transition natures of the excited states [19,21]. There has been significant effort in past decade to study the excited-state for understanding many physical, chemical, and biological phenomena both experimentally and computationally, such as hydrogen-bonded water or alcohol networks, organic compounds in solution, hydrogen band DNA and proteins [22,23]. A detailed understanding of the properties and dynamics of the excited states is very important because of their scientific significance. The compounds studied in the excited states are characterized by the possession of electron acceptor and electron donor substituents connected by a π-system [24]. The studies were already performed for some nitroaromatic and nitro-polyaromatic compounds including o-nitroaniline (oNA), m-nitroaniline (mNA) and p-nitroaniline (pNA) [24–26]. Nitroaniline, served as a simple model compound of important nitroaromatics is significant to explore the nature of the excited states. Nitroaniline as a desired chromophore has some characteristically spectroscopic properties. The active N_O group involved in nitroaniline plays a key role in the spectroscopy and the excited states dynamics. So far, for the individual nitroaniline, vertical and adiabatic excitation energies, conical intersection structures, and relaxation pathways have been calculated at various levels of theories and experiments, and some excited-state simulations have been performed as well [31–35]. Meanwhile, the hydrogenbonding complexes of nitroaniline with several water molecules have been represented for studying the effect of hydrogen-bonding interactions [27–30]. Aromatic amines are very import in biology and chemical industry. Among many molecules, o-nitroaniline (oNA) is an ideal model system for the study of the nature of the electronic properties since it is disubstituted benzenes and contains the proton donor amine group (\\NH2) and the acceptor the nitro group (\\NO2) [30–32]. The excited state is associated with an intramolecular charge transfer from the amino group to the nitro group across the phenyl ring. o-Nitroaniline (ONA) is an important donor-π-acceptor chromophore and has been the subject of many theoretical and experimental studies. Donor-π-acceptor molecules are stabilized in polar solvents through solvent interactions such as the hydrogen bonding [24,28,32–34]. ONA in the excited states is dynamically quenched through nonradiative deactivation, and it progresses very fast by intersystem crossing (ISC) from the singlet excited state to triple excited state and by internal conversion (IC) from the fluorescent state to the ground state. The intersystem crossing (ISC) is found to depend strongly on the protic solvents. The nitroaromatic compounds have significantly different ISC behaviors in different protic solvents. Instead, a characteristic feature of ONA is the absence of any measurable fluorescence [25,27,35].Recently theoretical investigations of ONA in different solvents have been reported [32,34,35].The intramolecular hydrogen transfer of ONA was already studied by Zhang et al. using infrared spectroscopic and quantum chemistry methods [25,36].

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And, the behavior of the excited states has been addressed for ONA interacting with several water molecules. In this work, we study the intramolecular and the intermolecular hydrogen bonds in the complex ONA⋯CH2O between ONA and CHO. It is an ideal modal that we choose the N_O (ONA) group and C_O (CH2O)group as the electron acceptors. Meanwhile, the N\\H (ONA) group and C\\H (CH2O) group are the electron donors. Nitroaniline is an important donor-π-acceptor chromophore and has been the subject of many theoretical and experimental studies. The availability of the carefully assigned experimental and theoretical frequencies for isolated ONA-water complexes in the spectral range of NH and NO stretching vibrational frequencies allows one to assess the influence of excited states and the formed hydrogen bond [32–37]. The NH and NO stretches in ONA are sensitive vibrational modes to monitor ESHBD with frequencies red shift in H-bonds. It is beneficial to the formation of hydrogen bonds. Moreover, it is worth noting that the CH bond in formaldehyde can also participate in a hydrogen bond X⋯H\\C and its stretch frequency is usually blue shifted as the proton acceptor is not very strong, since in CH2O there is a strong intramolecular hyperconjugation n(O) → σ*(CH) which couples with the intermolecular hyperconjugation n(X) → σ*(CH) [17,38]. Therefore, we observed for the first time the ONA⋯CH2O system. It attracts us a lot of interest for forming a large conjugated system including intramolecular and intermolecular hydrogen bonds. ONA can serve as the proton acceptor using the N_O group in one hydrogen bond and as the proton donor using its NH group in the other intermolecular hydrogen bond. From the computational point of view, these systems are small enough to allow the use of accurate quantum mechanical (QM) methods for studying the effect of hydrogen-bonding interactions and excited states on physicochemical properties. It is quite remarkable effect of the intramolecular and intermolecular hydrogen bonds and excited states on the nature of ONA⋯CH2O system. This work is organized as follows. We aim to perform a systematic study on the hydrogen bond ONA⋯CH2O in the excited states using TD DFT method theoretically. We calculated the geometric configurations and the molecular properties including the vibrational frequencies and IR intensities, the vertical transition energies in some excited states, the adiabatic transition energies and adiabatic geometry in the electronically excited states S1, S2 and T1 for the isolated ONA, formaldehyde and their complexes. Similar to the monomers ONA and CH2O, the hydrogen bonding complexes both in the ground state and the T1 excited state have planar geometry, but ONA has a nonplanar structure in the S1 and S2 states, as shown in Fig. 1. The hydrogen bond energies in different electronic states were calculated, we obtain that the hydrogen bonds are strengthened in the excited states S1, S2 and T1. The details of computations and results are presented below.

2. Computational Details Electronic excitation is studied with the time-dependent DFT (TD DFT) method, which is an efficient tool to offer the vibrational spectra for an understanding of the electronic excitation process. Time-dependent density functional method has been widely used in theoretical chemistry for studying electronically excited states and evaluating vibrational spectra in electronically excited states [39–41]. In this work, geometry optimization and the calculations of the properties including the vertical and adiabatic transition energies, vibrational spectra, and electronic absorption in different excited states were carried out at the B3LYP/aug-cc-pVDZ level of theory [42,43]. Natural bond orbital (NBO) analyses were performed using NBO5.0 program [44,45]. The electron density topological analysis in the theory of Atom in Molecule (AIM) was performed using the AIMALL program [46], which presents useful information about intermolecular interactions and characterization of bonds. These calculations were carried out employing the Gaussian 09 package in the gas phase [47].

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Fig. 1. Optimized geometric structures and bond lengths related to the HBs of the hydrogen-bonded complex ONA⋯CH2O, the black figures are for the monomers, the red, green blue and purple are for the complex in the S0, T1, S1 and S2 states, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Results and Discussion 3.1. Geometric Structures in the S0, S1, S2 and T1 States Fig. 1 shows the structure of the complex between orthonitroaniline (ONA) and formaldehyde (CH2O) in the ground state and the excited S1/S2 and T1 states. The optimized geometric structures of ONA in the S0, S1/S2 and T1 states are presented in Fig. S1 in supporting materials. We found that ONA in the monomer and in the complex is planer in the ground state where the nearby amino group and nitro group form an intramolecular hydrogen bond in ONA. However, in the excited S1/S2/T1 states, since the strong donor and the strong acceptor may alter their conformations, the complex forms the twisted nitro group [27]. Meanwhile, formaldehyde (CH2O) has a planar C2v geometry in the ground state but a bent Cs structure in the S1 and T1 states. The most important geometric parameters in the monomers ONA and CH2O in the excited states S1, S2 and T1 are shown in Table S1. The theoretical structure of the monomer CH2O shows good agreement with the experimental parameters, with differences within 0.001 Å for bond lengths and 0.1° for bond angles [48]. After optimization of ortho-nitroaniline (ONA) and formaldehyde (CH2O), their interactions with each other are investigated in this study. Since in particular of ONA the substituents are the proton donor amine group (\\NH2) and the acceptor nitro group (\\NO2), and the C_O and CH groups in the CH2O are the site groups that are responsible for the hydrogen bond formation in hydrogen accepting and donating environments. The hydrogen bonding complexes between ONA and CH2O are also optimized and molecular parameters are extracted. The general structures of three hydrogen-bonded complexes are presented in Fig. 1. It is obvious that there are three hydrogen bonds (HBs) in the S0 and T1 complexes, HB1: (ONA)N_O⋯HCHO, HB2:H2C_O⋯H\\N (ONA) and HB3: HN\\H⋯O_N\\O(ONA). The first two are intermolecular hydrogen bonds and the third one is an intramolecular hydrogen bond forming from the nitro group and amino group in ONA. However, in the S1 and S2 states there are only the two intermolecular hydrogen bonds, HB1:(ONA)N_O⋯HCHO and HB2:H2C_O⋯H−N(ONA). Inspection in the structures of the hydrogen bonded complexes displayed that a five-membered ring plus a six-membered ring or a nine-membered ring planar structure exists in the ground and excited states. The hydrogen bonds are considered as one of the important factors determining the structure of the compounds. With regard to the hydrogen bond interactions of the complex in the ground state, the hydrogen bond elongates the C=O CH2O by 0.005 Å and increases the NH bond in ONA involved in the HB2 by 0.002 Å, but shortens the CH bond in CH2O involved in the HB1 by 0.005 Å and the intramolecular hydrogen bond (HB3) has a increment by 0.081 Å, compared to the free monomers. In the excited states S1, S2 and T1 of the complex, the N_O bond in ONA involved in HB1: (ONA) N_O⋯HCHO elongates by 0.059 Å, 0.057 Å and 0.063 Å, respectively. And, the N\\H bond of ONA involved

in the HB2:H2C_O⋯H\\N(ONA) has a modest increment 0.028 Å, 0.009 Å and 0.035 Å, respectively, compared to the ground state complex. Meanwhile, another NH bond of ONA has a small increment in the S1 and T1 states. But the structure of CH2O in the complex is obviously not influenced by electron excitation, except for a small C_O bond increment by 0.009 Å, 0.004 Å and 0.002 Å, and a CH bond contraction in HB1: (ONA) N_O⋯HCHO by 0.005 Å, 0.003 Å and 0.001 Å, respectively, compared to the ground state complex, and a small shorting of the other CH bond. This is an obvious evidence of the locally excited (LE) character of the complex in the excited states S1, S2 and T1. The large elongation of the N_O bond in ONA is significantly due to the electron excitation, but the CH2O is not excited in the hydrogen bonds of the complex. Thus the formation of the HBs largely influences the CH2O moiety, but the electron excitations S0 → S1, S0 → S2, S0 → T1 mainly change the ONA moiety. 3.2. Electronic Spectra To gain insight into the transition nature of the absorption spectrum, based on the S0 optimized geometry of the hydrogen bond complex and the isolated ONA, TD DFT method is used to calculate the spectral properties of ONA and ONA⋯CH2O. The two calculated absorption spectra in the range of 200 nm–450 nm are depicted in Fig. 2, which exhibit three wide absorption peaks at near 210 nm, 270 nm and 380 nm, respectively. Meanwhile, the compilations of the excitation energies, oscillator strengths and dominant orbital contributions of the low-lying singlet

Fig. 2. Calculated absorption of isolated ONA and the hydrogen-bonded ONA⋯CH2O complex.

J. Yang, A.Y. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 199 (2018) 194–201

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Table 1 Vertical excitation energies E (eV) and wavelengths λ (nm) and the corresponding oscillator strengths f for the singlet and triplet excited states of the hydrogen-bonded ONA⋯CH2O complex as well as the free o-nitroaniline at the geometry of the ground state at the B3LYP/aug-cc-pVDZ level. States

S1 S2 S3 S4 S5 S6 States

T1 T2 T3 T4 T5 T6

ONA⋯CH2O

ONA E(λ)

f

E(λ)

f

3.29(377) H → L 97.6% 3.81(325) 4.40(282) 4.62(268) 5.08(244) 5.09(244)

0.0879

3.21(386) H → L 97.7% 3.79(327) 3.85(322) 3.97(312) 4.38(283) 4.59(270)

0.0830

0 0.0005 0.1414 0.0292 0.0034

0 0.0033 0.0001 0.0003 0.1436

ONA

ONA⋯CH2O

E(λ)

E(λ)

2.46(504) H → L 97.4% 3.08(402) 3.31(375) 3.39(366) 3.93(316) 3.97(312)

2.36(526) H → L 97.7% 3.12(398) 3.27(380) 3.30(375) 3.38(367) 3.83(323)

and triplet excited states of ONA and ONA-CH2O are presented in Table 1. An inspection of Table 1 clearly suggests that the singlet transition which dominates the absorption spectrum is S0 → S4/S6 with oscillator strength f = 0.14, respectively for the free ONA and the complex. Their excitation energies are calculated to be 4.63/4.59 eV, corresponding to the maximum adsorption peak wavelength of 268/270 nm for the free ONA and the hydrogen-bonded complex. The maximum adsorption wavelength of 268 nm of the monomer ONA is very close to the wavelength at 266 nm used in the transient absorption spectrum [37]. Therefore, it is effective to prove that the monomer ONA molecule most probably be excited to the electronically excited S4 state after quick excitation at 266 nm. By contrast, the electronic excitation energies of the hydrogen bonded complexes in the S1/T1 excited states correspondingly decrease compared to the isolated ONA, due to the intermolecular hydrogen bonding interactions. This means that the hydrogen bond in both S1 and T1 states are stronger than the ground state. Both for the free ONA and the complex the oscillator strength of the lowest singlet excited state S1 is very small, which probably due to the typical π → π* transition character of S0 → S1 transition. Their relatively weak absorption peak is calculated to be 377 nm and 386 nm, corresponding to transition energies of 3.29 eV and 3.21 eV for the free ONA and ONA⋯CH2O, respectively. It evidently confirms that the hydrogen bond complex ONA⋯CH2O is more easily excited to the low excited state than the monomer ONA. 3.3. Frontier Molecular Orbitals The frontier molecular orbitals (MOs) of the hydrogen-bonded ONA⋯CH2O complexes are shown in Fig. 3. According to the calculation results, the S1/T1 state of the hydrogen-bonded complex corresponds to the orbital transition from HOMO to LUMO (with 98% and 97% orbital contribution), thus only the HOMO and LUMO orbitals are described here. The π character for the HOMO and the π* character for the LUMO suggest that the S1 and T1 states are of distinct ππ* feature. The excited state involves a transition from the HOMO to the LUMO orbitals, which presents an electron density transfer to the NO2 group of the π conjugated system from the NH group and aromatic cycle, which suggests ONA in the excited states forms the hydrogen bonds more readily than in the ground state. And, the change of electron density in the N_O and N\\H groups can directly affect the forming of the hydrogen

HOMO

LUMO

Fig. 3. HOMO and LUMO of the hydrogen-bonded complex ONA⋯CH2O in the ground state.

bonding. It indicates that the intermolecular hydrogen bonds would be strengthened in the S1 and T1 states of the complex, compared with the ground state. By comparison, electron densities of HOMO and LUMO are only localized on the ONA moiety, which again suggests that the excited states S1 and T1 are LE states where only the ONA moiety is excited, but the CH2O moiety remains in its ground state. It makes us easy to discuss the hydrogen binding energy under study. 3.4. Natural Bond Orbital and AIM Analyses AIM and NBO are the simple and fast methods for studying the influence of intermolecular interactions and electron excitation on the bonds of or between ONA and CH2O. Three important values obtained from AIM calculations are electron density ρ, Laplacian ∇2ρ and local electron energy density H at the bond critical points (BCPs) for the HBs in the complex ONA⋯CH2O. Wiberg bond order b is obtained from NBO calculations. These four parameters for the interacting system between ONA and CH2O in four electronic states are listed in Table 2. Table S2 in supporting materials lists the corresponding data for the isolated ONA and CH2O. It is notable that in the free monomers ONA and CH2O the electron excitations S0 → S1/S2/T1 mostly reduce BCP electron density ρ and the Laplacian ∇2ρ of the N_O ONA bond and C_O CH2O bond. This means that the π electrons have been excited. In the complex, the hydrogen bonding interaction reduces the bond order b and electron density ρ of the C_O CH2O bond in the ground state, which interprets the CO bond elongation and frequency red shift in the ground state in the complexes compared to the free monomer. It is understood that ∇2ρ N 0 and H N 0 mean a noncovalent closedshell interaction, such as hydrogen bonds, ionic bonds and so on. In the complex, we note that the three hydrogen bonds has only a small electron density ρ b 0.03 a.u., a positive Laplacian 0 b ∇2ρ b 0.1 a.u., and a very small local electron energy density H, which imply three noncovalent closed-shell interactions in the ground state. Fig. 4 shows that the molecular graphs for the hydrogen-bonded complex ONA⋯CH2O and the free ONA in the different electronic states. As the hydrogen bonding complex is excited to the electronically excited states, the bond order b and ρ(BCP) of HB1(O⋯H) increase, so the HB1 should be strengthened, which interprets the N_O bond elongation and frequency red in S1/S2/T1 states. The electron excitation of the hydrogen-bonded complex leads to increase of the HB2(O⋯H) bond order and electron density ρ in the S1/S2 states and decrease in the T1 states. Meanwhile, for the intramolecular hydrogen bond (HB3) formed in the complex, the bond order and BCP electron density also increase in the T1 state, which means that the HB3 is strengthened compared to the ground state. In the excited states S1/S2/T1 of the complex, the bond order b, electron density ρ and the Laplacian ∇2ρ of the ONA N_O bond is significantly reduced, which means the ONA N_O has been electronically

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Table 2 Wiberg bond order b, electron density properties ρ, ∇2ρ and H at the BCPs for HB1: (ONA)N_O⋯HCHO, HB2:H2C_O⋯H\ \N(ONA) and HB3: HN\ \H⋯O_N\ \O(ONA) in the hydrogen bonding complexes, the local electron energy density H at the O⋯H BCP is almost zero. HB1

C\ \H

N_O

O⋯H

b

ρ

∇2ρ

H

b

ρ

∇2ρ

H

b

ρ

∇2ρ

H

S0(C) S1(C) S2(C) T1(C)

1.414 1.460 1.457 1.327

0.477 0.407 0.410 0.409

−0.984 −0.642 −0.661 −0.680

−0.593 −0.449 −0.455 −0.452

0.911 0.909 0.911 0.910

0.278 0.282 0.281 0.279

−1.069 −1.114 −1.093 −1.076

−0.297 −0.307 −0.302 −0.298

0.005 0.014 0.010 0.007

0.009 0.016 0.012 0.012

0.034 0.047 0.036 0.046

0.001 0.000 0.000 0.001

HB2

C_O

S0(C) S1(C) S2(C) T1(C) HB3

S0(C) T1(C)

N\ \H 2

O⋯H 2

b

ρ

∇ ρ

H

b

ρ

∇ ρ

H

b

ρ

∇2ρ

H

1.880 1.842 1.859 1.883

0.402 0.394 0.398 0.401

0.477 0.372 0.426 0.445

−0.690 −0.676 −0.682 −0.688

0.733 0.726 0.754 0.683

0.333 0.311 0.326 0.304

−1.600 −1.407 −1.504 −1.394

−0.444 −0.397 −0.422 −0.393

0.018 0.063 0.040 0.011

0.015 0.038 0.026 0.011

0.044 0.132 0.086 0.037

0.000 0.002 0.001 0.000

O⋯H b

ρ

∇2ρ

H

0.022 0.078

0.027 0.053

0.096 0.183

0.001 −0.002

excited. The more ρ decreases, the more ∇2ρ is reduced. However the bond order b and electron density properties ρ, ∇2ρ and H of the C_O bond in the CH2O moiety have almost no changes as the complex is excited from the ground state to the excited states, thus the CH2O moiety still remains in the ground state in the excited states of the complex. Thus the LE character of the excited states is confirmed once again by the electron density properties. In the ground state and the three excited states of the hydrogen bonding complex, the electron density ρ of the CH CH2O bond involved in the HB1 increases but the electron density ρ of the NH ONA bond involved in the HB2 decreases, which explains the CH bond contraction and frequency blue shift but the NH bond elongation and frequency red shift.

3.5. Vibrational Absorption Spectra Our present works on the ONA⋯CH2O complexes highlight the effects of the hydrogen bonds and the different electronic excited states on the vibrational frequencies of the molecules. The characteristic frequencies of the hydrogen-bonded groups are sketched. The calculated infrared absorption spectra and the vibrational frequencies of the hydrogen-bonded groups in different states at the spectral range from 1000 to 3750 cm−1 are shown in Fig. 5 and listed in Table S3 of the supporting materials. Fig. 5 points out the different shifts of the corresponding vibrational frequencies relative to the free monomers due to the formation of the hydrogen bonding interactions and electron excitation. Comparison with the calculated frequencies in monomers in

Fig. 4. The molecular graphs for the hydrogen-bonded complex ONA⋯CH2O and the free ONA in the different electronic states. The purple, orange, and green points stand for the nucleus, BCPs and CCPs, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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199

Fig. 5. The calculated vibrational absorption spectra of free CH2O and the complex.

the ground state shows that the excited states weaken the N_O and NH stretching frequencies by the different degrees. As shown in Table S3, the electronic excitation S0 → S1/S2/T1 shifts the N_O stretching frequency from 1556 cm−1 to 1271 cm−1, 1271 cm−1 and 1375 cm−1, respectively. This means a large frequency red shift. Meanwhile, in the S1/S2 excited states, the two NH stretch modes are red shifted by the excited states. The symmetry stretching mode is red shifted from 3561 cm−1 to 3531 cm−1 and 3521 cm−1, respectively. And, the asymmetry stretching mode also has a modest red shift from 3717 cm−1 to 3679 cm−1. Also the C_O stretch frequency of the free CH2O has a large red shift by electronic excitation S0 → S1/T1 from 1802 cm−1 to 1357/1322 cm−1 with a large red shift of 445/480 cm−1. It is notable that the S0 → S1 excitation of the free CH2O causes a large frequency blue shift of the CH symmetry and asymmetry stretching modes from 2889 cm−1 and 2958 cm−1 to 2993 cm−1 and 3099 cm−1 respectively, but the S0 → S2 excitation leads to a large frequency red shift of the CH modes to 2560 cm−1 and 2874 cm−1, and the S0 → T1 excitation has only a small effect on these two modes. The large red shifts of the N_O modes in ONA and the C_O group in CH2O suggest the π electron-excited characteristics. An important observation is that in the hydrogen-bonded complex the infrared spectral features are significantly different from the free monomers. As expected, the most important shifts of the vibrational frequencies concern the characteristic groups that are involved in the hydrogen-bonding interactions in the ground state. In the case of the complex, the N_O stretch frequency in the ONA moiety is shifted by the HBs from 1556 cm−1 in the free monomer to 1554 cm−1 in the complex with a small red shift 2 cm−1. The NH symmetry stretch mode is from 3561 cm−1 to 3547 cm−1 featuring a small red shift for the stretching vibration, in comparison with the isolated ONA. The C_O frequency in the CH2O moiety is also red shifted from 1802 cm−1 to 1787 cm−1. Instead, blue shifts have been observed for the CH stretch modes in the CH2O moiety upon formation of the HBs, the symmetry and asymmetry stretching modes have blue shifts 29 cm−1 and 58 cm−1, respectively. The CH stretch modes in the CH2O moiety are different from the other stretching vibrations, their frequency blue shifts as participated in a hydrogen bond can be well interpreted by the theory of the coupling between intramolecular and intermolecular

hyperconjugations [38]. In the CH2O moiety there is a strong intramolecular hyperconjugation between one lone pair of the oxygen atom and the CH antibonding orbital, which transfers electron density from LP(O) to σ*(CH) in the free CH2O, but in the hydrogen bonded complex transfers more electron density in the opposite direction from σ*(CH) to LP(O), leading to a net decrease of electron density on the σ*(CH). The decrease of electron density on the σ*(CH) in the complex compared to the monomer causes the CH bond involved in the HBs strengthening such that the bond is shortened and its stretching frequency is blue shifted. As the hydrogen bonding complex is excited to the states S1 and T1, there is a big change on ONA moiety. The N_O stretching mode has a large frequency red shift from 1554 cm−1 in the ground state to 1284/1159/1372 cm−1 in the S1/S2/T1 state. Meanwhile, the symmetry NH stretching frequency red shifts from 3547 cm−1 to 3128/3412/ 3025 cm−1 in the three excited states. The change of C_O and CH stretching frequencies in the CH2O moiety is small compared to the ONA moiety. The C_O stretching frequency in the CH2O has a small red shift from 1787 cm−1 to 1750/1771/1776 cm−1. The asymmetry CH stretching mode has a modest blue shift in the three excited states from 3016 cm−1 to 3079/3054/3031 cm−1 and the symmetry CH stretching mode has a smaller blue shift by 15 cm−1/6 cm−1 in the S1/S2 excited states. So the ONA moiety in the complex is sensitive to the electron excitation, but the CH2O moiety is mainly influenced by the formation of hydrogen bonds. In order to understand better the vibrational stretch frequencies, the absorption spectra and vibrational frequencies for the isotope-replacing system are presented in Fig. S2 in supporting materials. The isotope-replacing system provides information similar to the system without isotope replacing. 3.6. Hydrogen Bond Strengthening In the four different electronic states of this complex, the three hydrogen bonds, HB1: (ONA)N_O⋯HCHO, HB2: H2C_O⋯H\\N(ONA) and HB3: HN\\H⋯O_N\\O (ONA) have been taken into consideration in the present work. The hydrogen bonding energy ΔE can be determined by the energy of hydrogen bonded complex minus the energies of the free monomers.

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Table 3 The O⋯H distance (Å), the O⋯HC/N_O⋯H, O⋯HN/C_O⋯H and O⋯HN/H⋯ON angles (°) and the interactional energies ΔE (kJ/mol) of the HBs calculated by (TD) DFT method at the B3LYP/aug-cc-pVDZ level. HB1: (ONA)N_O⋯HCHO, HB2:H2C_O⋯H\ \N(ONA) and HB3: HN-H⋯O_N\ \O(ONA). HB1

S0 S1 S2 T1

HB2

HB3

O⋯H

O⋯HC/N_O⋯H

O⋯H

O⋯HN/C_O⋯H

O⋯H

O⋯HN/H⋯ON

E

2.180 2.222 2.360 2.270

120.9/164.8 144.3/81.2 145.7/81.0 126.7/168.3

2.387 1.773 1.934 2.380

141.9/116.1 163.7/121.1 166.8/122.0 119.5/103.2

1.988

120.1/111.9

−18.66 −55.99 −28.55

1.667

137.6/106.1

The calculated hydrogen bonding energies, the corresponding hydrogen bond lengths and angles both in the ground and excited states are listed in Table 3. The hydrogen bonding distances in the four states S0, S1, S2 and T1, in the range of 2.1–2.4 Å, are typical HB lengths. In the complex, it can be evidently confirmed that the hydrogen bond is significantly strengthened since the binding energy increases from − 18.66 kJ/mol in the ground state to −55.99/−28.55 kJ/mol in the S1 and S2 excited states. Furthermore, the HB1 length between oxygen (O) and hydrogen (H) atom increases 0.004/0.180 Å and the HB2 length has a large contraction 0.614/0.453 Å respectively, compared to the ground state. In the S1 state, the HBs are stronger and their interacting distances are shorter than the S2 state. In the T1 state, the HB1 has a increment 0.009 Å, but the HB2 and HB3 lengths are shortened by 0.007 Å and 0.321 Å, respectively, compared to the ground state. However, the hydrogen bond energy in the T1 excited state cannot be calculated. From Table S1 in the supporting materials, we found that the geometric structure of the monomer ONA has a very difference from the ONA moiety in the hydrogen complex in the T1 excited state, which means that the calculated equation for the interaction energy in the LE states cannot be used to calculate the hydrogen bond energy in the T1 excited state. As a result, the calculated bond lengths are in agreement with the trend of the binding energies of the complexes. So, they can predict the stability for the three different configurations of this complex. 4. Conclusion In this work, we have been motivated to theoretically study the hydrogen bonded ONA⋯CH2O complexes using the DFT and TD DFT methods. The geometric structures and the hydrogen binding energies of the complex as well as the free ONA and CH2O are investigated. To study the systematic features of the hydrogen bond and the electronic excitations, we calculate the electronic spectra and IR spectra. All the calculated spectral features are in good agreement with the experimental result. As a result, it is evidently concluded that the hydrogen bonding and electron excitation have different influence on the complex as well as the monomers ONA and CH2O. The ONA moiety is strongly influenced by electron excitation but not sensitive to the HBs. From the red shift influence on the N_O bond stretching frequencies in the different electron states and the binding energies of the complexes, we concludes that the hydrogen bonds are strengthened in the excited states compared to the ground state. The CH2O moiety is almost unaffected by the excited states in the complex, except that the CH stretching frequencies have a modest blue shift in the S1/S2 excited states. Instead, the CH2O moiety is sensitive to the HBs. The formation of hydrogen bonds leads to a large blue shift of the symmetry CH2 vibration and the asymmetry CH2 vibration. The blue shift of the latter is larger than the former. When the complex is excited to the electronic states, the ONA moiety is excited and the CH2O moiety remains in the ground state. Appendix A. Supplementary Data Fig. S1 shows the optimized geometric structures of the monomer ONA in the S0, S1/S2 and T1 states, respectively. Table S1 lists the lengths (Å) of the N_O and N\\H bonds of in the free ONA and the C_O and

C\\H bonds in the CH2O monomer bonds in the excited states S1, S2 and T1. Table S2 lists Wiberg bond order b, electron density properties ρ, ∇2ρ and H at the BCPs for the N_O and N\\H bonds in the free ONA and the C_O and C-H bonds in the CH2O monomer calculated at the B3LYP/aug-cc-pVDZ level. Fig. S2 shows the calculated vibrational absorption spectra of the ground state free CH2O and the complex with isotope replacing in the states S0, S1, S2 and T1. Table S3 lists vibrational frequencies of the N_O and NH stretch modes in ONA and the C_O and CH stretch modes in H2C_O for the no-isotope-replacing system calculated by DFT and TD DFT at the B3LYP/aug-cc-pVDZ level. Table S4 lists vibrational frequencies of the N_O and ND stretch modes in ONA and the C_O and CD stretch modes in HDC_O for the isotope-replacing system. Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2018.03.062.

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