Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (1): 7-15

#### The article information

Zhen-lin Zhang, Wen-lou Wang, Shi-lin Liu, Dong-ming Chen

Experimental and Density Functional Theory Calculation Studies on Raman and Infrared Spectra of 1, 1'-Binaphthyl-2, 2'-diamine
1, 1'联萘-2, 2'-二胺拉曼和红外光谱的实验和DFT计算研究
Chinese Journal of Chemical Physics, 2017, 30(1): 7-15

http://dx.doi.org/10.1063/1674-0068/30/cjcp1606118

### Article history

Accepted on: June 3, 2016
Experimental and Density Functional Theory Calculation Studies on Raman and Infrared Spectra of 1, 1'-Binaphthyl-2, 2'-diamine
Zhen-lin Zhanga, Wen-lou Wanga,b, Shi-lin Liua, Dong-ming Chena
Dated: Received on June 1, 2016; Accepted on June 3, 2016
a. Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China;
b. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
*Author to whom correspondence should be addressed. E-mail:dmchen@ustc.edu.cn
Abstract: The IR absorption, visible excited normal Raman, and UV-excited near-resonant Raman (UVRR) spectra of 1, 1'-binaphthyl-2, 2'-diamine (BINAM) were measured and analyzed. Density functional theory calculations were carried out to investigate its vibrational frequencies, infrared absorption, normal Raman, and near-resonance Raman intensities. The observed Raman and IR bands of BINAM were assigned with respect to the local vibrations of substituted 2-naphthylamine. Several Raman bands of BINAM were found selectively enhanced in the UVRR in comparison with the normal Raman spectrum. Possible excited state geometry distortion was discussed based on the resonance Raman intensity analysis.
Key words: 1, 1'-Binaphthyl-2, 2'-diamine    Resonance Raman    Infrared    Density functional theory    Molecular vibrations
I. INTRODUCTION

The axially chiral binaphthyl compounds play a crucial role in constructing new stable chiral structures as well as in developing effective asymmetric organocatalysts [1-18]. Various spectroscopic methods have been used to explore the structures and properties of the binaphthyl compounds, especially the 1, 1'-binaphthyl-2, 2'-diol, in the past several decades [19-31]. For example, Nogueira et al. measured the surface-enhanced Raman spectrum (SERS) of 1, 1'-binaphthyl-2, 2'-diol on silver colloids and proposed the empirical assignments for the observed SERS bands [19]. In Ref.[20], the vibrational circular dichroism (VCD) spectrum of 1, 1'-binaphthyl-2, 2'-diol was studied and density functional theory (DFT) calculations were carried out to assign the VCD bands. With ultralow-frequency Raman technique, Chang et al. studied the polymorphic transformation of crystalline binaphthyls [21]. Li et al. studied the normal and UV near-resonant Raman (UVRR) spectra of 1, 1'-binaphthyl-2, 2'-diol in basic solution and assigned the observed Raman bands on the basis of the DFT-calculations [22]. Vibrational spectroscopy and DFT calculations have also been extensively used to investigate the chiral conformational stability of the binaphthyl compounds [23-25]. The solutions of 1, 1'-binaphthyl-2, 2'-diol have also been studied by the sum-frequency generation (SFG) spectroscopies [26-30]. Recently, Liégeois have conducted a comprehensive theoretic study on the polarized Raman and vibrational Raman optical activities (VROA) of a series of 2, 2'-substituted binaphthyl compounds [31].

Among various binaphthyl compounds, the 1, 1'-binaphthyl-2, 2'-diamine (BINAM) derivatives have been found highly useful in developing new asymmetric catalysts [7-18]. For example, Guillena et al. have synthesized a series of BINAM-prolinamides and used them as catalysts in several organic reactions, such as the direct aldol condensation between aldehydes and aliphatic ketones [8-12]. Starting from BINAM, Duan et al. have obtained a novel axially chiral Rh-complex and applied it in the Rh-catalyzed enantioselective hydrosilylation of methyl ketone [13]. By using BINAM-derived phosphoric acids, Nelson et al. recently developed a chiral anion phase-transfer stretagy to achieve highly enantioselective α-amination of carbonyl compounds [15]. Barbas group has discovered a highly efficient organocatalytic domino approach for the direct construction of bispirooxindole derivatives with excellent stereocontrol by employing an organocatalyst containing a BINAM skeleton [17].

On the other hand, despite of its significance in asymmetric catalysis, the spectroscopic properties of BINAM have not been well studied. To the best of our knowledge, the experimental and theoretical study of Raman and IR spectra of BINAM has never been reported. Therefore, the present study was conducted with the intention to provide a detailed vibrational analysis for BINAM by conjunct investigation of IR/Raman experiments and DFT calculations. The assignments of the observed IR and Raman bands have been proposed based on the local vibrations of substituted 2-naphthylamine. The UVRR spectrum of BINAM was also measured and its resonant enhancement pattern was discussed and used to analyze the possible distortion of the excited state structure.

II. Experimental and Computational Methods

1, 1'-binaphthyl-2, 2'-diamine (>99%) was purchased from Alfa Aesar and was used without further purification. UV-Vis absorption spectra were measured with a 5 mm light-path quartz cell at room temperature using Shimadzu UV-2401PC spectrometer. Raman spectra were recorded on a LABRAM-HR 800 micro-Raman spectrometer equipped with an air-cooled CCD detector, a notch filter, and a 600 grooves/mm grating for the visible excitation and a 2400 grooves/mm grating for UV. The 325 nm line of a He-Cd laser and the 488.0 nm line of an Ar ion laser were used as the excitation sources with the power 3.5 and 5 mW, respectively, on the sample. The collection time of the CCD detector was 40 s for Raman measurements. In order to minimize any damage of the sample due to the prolonged exposure to laser radiation, the BINAM solid sample was mixed with KBr powder in a weight ratio of 1:10 and pressed into a disc of 1 cm diameter, which was placed on a rotating holder during Raman measurements. IR spectrum of BINAM was measured on a Nicolet 6700 FT-IR spectrometer as KBr pallet.

DFT calculations were carried out using Becke's three-parameter hybrid functional (referred as B3LYP) [32, 33], which has been proven to be suitable for studying the structures and properties of binaphthyl systems [22, 25, 29-31]. In order to reduce computational cost, initial searching of steady structure of the studied molecule was carried out by geometry optimization with relative small basis sets, 6-31G, without any symmetry constraint. The obtained structures were then used for the final optimization using 6-311++G(d, p) basis sets with suitable symmetry (C2 point group) constraint. Analytic frequency calculations (using B3LYP/6-311++G(d, p)) at the optimized structure were done to confirm the optimized structures to be an energy minimum and to obtain the theoretical vibrational spectra. Due to the neglect of the anharmonicity and the incomplete basis sets, the DFT calculations tend to slightly overestimate the vibrational frequencies. These systematic discrepancies between the computed and experimental frequencies can be corrected by scaling the calculated frequencies with a single factor of 0.98, which has been found to be appropriate for the planar conjugate systems [34, 35]. Assignment of individual vibrational frequency was carried out by inspecting the calculated Cartesian displacements of the corresponding normal mode. Electronic absorption spectrum of BINAM was theoretically studied with TDDFT calculations [36-38]. The simulation of UV-resonance Raman spectrum was carried out with frequency-dependent polarizability using coupled-perturbation theory (CPHF). All DFT/TDDFT calculations were performed with the Gaussian 09 program suite [39].

III. Results and discussion A. Ground-state geometries and UV-visible absorption spectrum

Table I lists the bond lengths, bond angles and dihedral angles of BINAM optimized with B3LYP/6-311++G(d, p). Figure 1 shows the structural sketch and atomic labels of BINAM used in this work. B3LYP/6-311++G(d, p) calculations indicate that the two naphthyl rings of BINAM molecule are almost perpendicular with each other. The dihedral angle between the two naphthyl groups is about 91.4°. The C1-C2, C2-C3, and C9-C10 distances of BINAM are calculated to be 1.394, 1.425, and 1.432 Å, respectively, while the C1-C9 distance is found to be 1.431 Å. The C-N bond length of BINAM is 1.398 Å.

Table I Selected structural parameters of BINAM calculated with B3LYP/6-311++G(d, p). Distance in Å and angles or dihedral angles in (°).
 FIG. 1 Structural sketch of BINAM with the atomic labels.

Figure 2(a) displays the UV-Vis absorption spectrum of R-BINAM dissolved in CH2Cl2, which shows a quite broad peak with the absorption maximum at 348 nm. Figure 2(b) gives the theoretical spectrum of BINAM calculated with TD-B3LYP/6-311++G(d, p), where a band-width of 2600 cm-1 (0.322 eV) was utilized for each electronic transition in the spectral simulation. Table II tabulates the calculated orbital energies and symmetries of frontier molecular orbitals of BINAM, and Table III lists the calculated excitation energies and oscillator strengths of BINAM calculated with TD-B3LYP/6-311++G(d, p). Table III also gives the weights of major configurations for each excited state. According to the TD-B3LYP calculations, the near-UV absorption of BINAM are contributed from the transitions from the ground state to 11B, 11A, 21B, and 21A states, with 11B, 21B, and 21A being of relatively large oscillator strengths. As shown in Table III, the transition energies of these four states are quite close with each other, while they are well separated from higher excited states (31B, etc.). Thus the observed broad 348 nm peak in Fig. 2(a) can be attributed to the overlap of the 11B, 21B, and 21A states.

Table II Frontier orbital energies of BINAM by B3LYP/6-311++G(d, p) calculations.
 FIG. 2 (a) UV-Vis absorption spectra of BINAM measured in CH2Cl2 solution. The arrows mark the wavelength positions of excitation lines for Raman experiments. (b) UV-Vis absorption spectra of BINAM calculated by TD-B3LYP/6-311++G(d, p) calculation.
Table III TD-B3LYP calculated excitation energies ($\Delta E$), oscillate strengths (f), and main configurations for the low-lying excited states of BINAMa.
B. Vibrational spectra 1. Infrared absoption spectrum

BINAM has 38 atoms and 108 degrees of internal freedom, which, according to the C2 symmetry, can be classified as Γ=55A+53B. According to the vibrational selection rules, all of these modes are active for both IR and Raman transitions. As the two naphthyl rings of BINAM are nearly perpendicular to each other, most vibrational modes of BINAM can be considered as the in-phase and out-of-phase combinations of the corresponding local vibrations of two isolated naphthylamine groups. The molecular vibrations of naphthalene have been studied by Scherer [40], but it is not very useful in helping our vibrational assignments since the substituents in BINAM significantly alter the mode compositions. Therefore, we conducted a supplementary frequency calculation on 1-chloro-2-naphthylamine molecule and used the corresponding modes to describe and classify the normal vibrations of BINAM. In this way, the in-phase or out-of-phase coupling (so called vibrational exciton coupling) of the corresponding local vibrations of two naphthylamine groups generates A and B symmetry blocks of normal modes of BINAM. The calculated atomic Cartesian displacements of 1-chloro-2-naphthylamine are shown in Figs. S1 and S2 in the supplementary materials. Detailed vibrational assignments of BINAM will be discussed in the following parts of the article.

Figure 3 compares the experimental and theoretical IR spectra of BINAM, where the theoretical IR spectrum has been simulated from B3LYP-calculated harmonic frequencies (scaled with 0.98) and intensities. By comparing with the DFT-calculated band positions and intensities, the observed IR bands of BINAM are assigned as listed in Tables IV and Table V. In the Tables, the mode numbers are according to those of isolated 1-chloro-2-naphthylamine molecule, and the single quotation marks are added after the mode numbers for the B symmetry modes so as to distinct them from the A modes. It is noticed that, while both A and B modes are IR active, B3LYP calculations indicate that most of the strong IR absorptions of BINAM in the high frequency region (950-1600 cm-1) are due to the in-plane naphthyl modes belong to the B symmetry block.

 FIG. 3 IR absorption spectra of BINAM (a) experimental (in KBr pellet), (b) calculated with B3LYP/6-311++G(d, p).
Table IV Observed and calculated vibrational frequencies of BINAM in the range of 1000-1650 cm-1.
Table V Observed and calculated vibrational frequencies (in cm-1) of BINAM in the range of 400-1000 cm-1.

The vibrations in the region 1200-1650 cm-1 are mainly due to the C=C stretches of naphthyl rings, and some of which are coupled with the NH2 scissoring vibrations. The strongest absorption was measured at 1616 cm-1, which is rather broad and can be decomposed into a main band at 1616 cm-1 and a shoulder band at 1605 cm-1. DFT-calculations show a very strong IR band at 1624 cm-1 and a shoulder band at 1615 cm-1. However, according to calculation the 1624 cm-1 band itself is an overlapped band by two normal modes which can be assigned to v7 and v7 (the C=C stretches of the naphthyl rings), both of which are severely coupled with the NH2 scissoring vibrations. The shoulder band at 1605 cm-1 can be assigned to v8, which is calculated at 1615 cm-1 and is also severely coupled with the NH2 scissoring. Other predominant IR absorptions in 1200-1650 cm-1 region were measured at 1506, 1468, 1426, 1383, 1348, 1289, 1249, and 1208 cm-1. They are thought to correspond to the calculated bands at 1516/1513 (v10/v10'), 1468(v11'), 1431(v12'), 1377(v13'), 1346(v15'), 1283 (overlapped by v16 and v16'), 1247(v17'), and 1211 cm-1(v18'), respectively.

In the region 950-1200 cm-1, IR bands were observed at 1147, 1129, 1112, 1022, and 964 cm-1 and the corresponding absorptions are calculated at 1152 (v20'), 1110 (v21), 1099 (NH2 rocking), 1026 (v22'), and 965 cm-1 (v23'+NH2 rocking), respectively, with middle or weak intensities.

In 700-950 cm-1 region, IR absorptions were observed at 921, 869, 815, 782, and 761 cm-1 and calculated at 920 (v24'), 856 (γ4)/855 (γ4), 810 (γ_5/γ5), 775 (γ6), and 745/743 cm-1 (γ7/γ7), respectively. According to the calculation, most of them are due to the out-of-plane wagging of the hydrogen atoms on the naphthyl rings. In the 400-700 cm-1 region, our B3LYP-calculation predicted three strong IR bands at 545 (γ10+NH2 wagging), 511 (NH2 wagging+γ8), and 490 cm-1 (v29+NH2 wagging) which can be assigned to the in-plane or out-of-plane skeletal deformation of the naphthyl rings together with NH2 wagging). Experimentally, only weak absorptions were observed probably because of the interference of broad background in the low-frequency region.

2. Normal Raman spectrum excited at 488.0 nm

Figure 4(a) displays the normal Raman spectrum of BINAM in solid discs with 488.0 nm excitation, while the calculated non-resonant Raman is shown in Fig. 4(b). The experimentally observed and DFT-calculated frequencies and assignments of Raman bands of BINAM are listed in Tables IV and Table V.

 FIG. 4 Normal Raman scattering spectra of BINAM (a) experimental (solid) with the excitation wavelength λex=488 nm, (b) calculated with B3LYP/6-311++G(d, p).

In the non-resonance Raman spectrum of BINAM (Fig. 4(a)), strong Raman bands were observed at 1618, 1573, 1476, 1439, 1381, 1358, 1024, 852, 668, 577, and 528 cm-1. As shown in Tables IV and Table V, most of the vibrations of BINAM in the 900-1650 cm-1 region are due to the naphthyl in-plane CC stretching and CH bending. In the 1550-1650 cm-1 region, three bands were observed at 1618, 1594, and 1573 cm-1, respectively, in the normal Raman spectrum of BINAM. The 1573 and 1618 cm-1 bands of BINAM are much stronger than the 1594 cm-1 band. Theoretically, B3LYP/6-311++G(d, p) calculations give rise to three bands at 1573, 1602, and 1624 cm-1, respectively, with similar relative intensities to the experiment. According to our DFT calculations, the strong 1573 cm-1 band can be attributed to v9, the stretching of C2C3/C6C7/C9C10 bonds of the naphthyl rings. On the other hand, the B3LYP/6-311++G(d, p) calculation manifests that the bands at 1602 and 1624 cm-1 are associated mainly with the naphthyl C3C4/C1C2/C5C6 stretching but they also contain significant contributions from NH2 scissoring. Similar to that observed in IR spectrum, the 1624 cm-1 Raman band in Fig. 4(b) also comes from two overlapped normal modes, v7 and v7. Two strong Raman bands were observed at 1476 and 1439 cm-1 (Fig. 4(a)), which are thought corresponding to the calculated bands at 1474 (v11) and 1434 cm-1 (v12). Based on our DFT calculations, these two bands are assigned to the CC stretching of naphthyl ring, and both of them also involves significant contributions from in-plane CCH bending.

With 488 nm excitation, the strongest Raman band of BINAM was observed at 1381 cm-1, and also a strong Raman band at 1358 cm-1 was observed at the low-frequency side of the 1381 cm-1band. B3LYP/6-311++G(d, p) calculation of BINAM predicts a very strong Raman at 1368 cm-1 (v14) and two shoulder bands at 1361 (v14') and 1351 cm-1 (v15), respectively, at low-frequency side of the 1368 cm-1 band. As the v15 mode (1351 cm-1) is calculated much weak in Raman intensity, we assign the observed 1381 and 1358 cm-1 bands to v14 and v14', both of which involves the in-plane stretching of C9C10/C5C6/C8C9 bonds. The weak or shoulder bands at 1289, 1276, 1215, 1208, 1158, and 1146 cm-1 are assigned to the v16, v17, v18, v23, v19'/v19, and v20'/v20 modes. Most of these modes are due to the in-plane CCH bending of the naphthyl groups, with exception of v23 that is dominated with C1-C1 stretching between two naphthyl groups. The strong Raman band at 1024 cm-1 is calculated at 1026 cm-1 (v22). This mode also involves in-plane CCH bending, but it contains a significant contribution of in-plane deformation of the naphthyl rings.

Our DFT calculations manifest that the non-resonance Raman band in the 400-950 cm-1 region is mainly due to the in-plane deformations of naphthyl ring. The observed weak Raman band at 960 cm-1 was calculated at 945 cm-1 and is assigned to in-plane deformation v24 mixed slightly with out-of-plane deformation γ3. The observed 852 cm-1 band of BINAM is assigned to v25, a naphthyl in-plane deformation mode involving largely the changes of the C3C4C10/C6C7C8/C5C6C7 bond-angles. DFT calculations predict this band to appear at 844 cm-1, with a pretty strong Raman intensity. Two weak bands at 794 and 774 cm-1 were detected in the normal Raman spectrum. Based on matching of calculated and observed frequencies and intensities, we assign them to the calculated bands at 787 and 768 cm-1. According to the DFT-calculations, these two modes belong to B symmetry and correspond to the in-plane skeleton deformations mixed with the C-H out-of-plane wagging of the naphthyl rings. DFT calculation for BINAM predicts a rather strong Raman band at 665 cm-1. It can be readily assigned to the observed middle-strong band at 668 cm-1. The calculated Cartesian atomic displacements of this mode suggest it to be v27, the in-pane deformation of the naphthyl ring mainly involving the bending of C7C8C9/C10C5C6 bond angles.

In the low-frequency region, two strong Raman band was observed at 577 and 528 cm-1, respectively. We assign 528 cm-1 band to v28, the in-plane naphthyl deformation mainly involving the translational separation of two benzo rings of naphthyl, which is calculated at 530 cm-1. In the calculated Raman spectrum, a moderately strong band was predicted at 577 cm-1, which, according to the calculated atomic Cartesian displaces, can be assigned to the out-of-plane deformation of the naphthyl ring γ9 mixed mildly with in-plane vibration like v29. Another low-frequency Raman band observed at 436 cm-1 is thought corresponding to the calculated band at 433 cm-1, which can be assigned to the in-plane deformation of the naphthyl ring v30' combined with a γ9-like out-of-plane deformation.

According to our calculation, the most of the strong bands observed in non-resonance Raman belong to A symmetry, with the exceptions of the 1358 cm-1 band that belongs to B symmetry and the 1618 cm-1 band that is composed of two modes belong to A and B symmetries respectively.

 FIG. 5 UV near-resonance Raman scattering spectra of BINAM (a) experimental with the excitation wavelength γex=325 nm, (b) calculated with CPHF-B3LYP/6-311++G(d, p).
3. UV resonant Raman (UVRR) spectrum excited at 325 nm

Figure 5(a) displays the resonance Raman spectrum (RRS) of BINAM excited with UV light at 325 nm, where the wavelength of the incident light is nearly in resonance with the electronic absorptions of BINAM at 348 nm. As shown in Fig. 5(a), evident RR bands were observed at 1618, 1595, 1567, 1475, 1438, 1380, 1358, 1289, 1277, 1213, 1158, 1147, 1024, 851, 668, 579, and 528 cm-1, most of which also appear in the 488 nm excited normal Raman with considerably intensities. However, in comparison with the normal Raman (Fig. 4(a)), the RR bands at 1618, 1475, 1358, 1289, 1158 cm-1 show clear increase in relative intensities. Especially, the RR band 1618 cm-1 displays greatly enhancement, which is even stronger than the 1380 cm-1 band, the strongest band in normal Raman.

Theoretical simulation of UV-resonance Raman spectrum (Fig. 5(b)) was carried out with frequency-dependent polarizability using coupled-perturbation theory. Experimentally, the resonance Raman spectrum was recorded by using the 325 nm line of a He-Cd laser as the excitation source, which is blue-shifted by 2034 cm-1 (0.252 eV) from the UV absorption band (348 nm). This energy difference must be considered in calculation model since it is known that the resonance Raman intensities sensitively depend on the wavelength difference between the incident laser line and the electronic absorption maximum. Therefore, the energy of the incident light was set as 4.01 eV, blue shifted by 0.252 eV with respect to the TDDFT calculated UV-absorption peak, for theoretical simulation of UVRR. As shown in Fig. 5, the calculations well reproduce the enhancement pattern of UV near-resonance Raman spectrum. It is noticed that the calculations predicted increased relative intensities for the 1624, 1474, 1351, 1282, 1158 cm-1 bands of the UVRR spectrum (Fig. 5(b)) as compared with theoretical normal Raman (Fig. 4(b). This is consistent with the experiment observations that the corresponding bands (1618, 1475, 1358, 1289, 1158 cm-1) are enhanced in 325 nm excited spectrum (Fig. 5(a)) as compared with 488 nm excited one (Fig. 4(a)). Especially, the 1624 cm-1 band was predicted to be significantly enhanced by UV excitation. Meanwhile, the 1381 cm-1 band remains as one of the strongest bands, which is also consistent with the experiment. According to the calculations, the 1624 cm-1 band in the Raman spectrum is composed of two modes, i.e., 1625 cm-1 (v7+NH2 scissoring) and 1624 cm-1 (v7+NH2 scissoring), and their intensity ratio is about 1:2.1. Thus one can reasonably considered that the intensity of the 1618 cm-1 band in Fig. 5(a) comes mainly from the B symmetric γ7 mode (with mixing of NH2 scissoring). The enhancement pattern of UV resonant Raman of BINAM may result from both the non-zero Franck-Condon overlap due to excited state pseudo-Jahn-Teller distortion and the vibronic coupling between the excited states, which are discussed in the next section.

IV. Discussion

Resonance Raman is much different from normal non-resonance Raman in physical mechanisms. For resonance Raman in which the incident irradiation is resonant with an electronic transition, Raman signals can be enhanced either due to the non-zero Frank-Condon overlap between the ground and the resonant excited state (A-term mechanism) or due to the vibronic coupling between the electronic states (B-term mechanism) [41-45]. Unless the vibrational wave functions are non-orthogonal, the vibrational overlap integrals are zero and the A-term mechanism dose not contribute Raman intensities. For any vibrational mode of a molecule, non-orthogonality of these wave functions will subsist if, between the ground state and excited states, there is either a displacement of the potential energy minimum along the normal coordinate, or a difference of vibrational frequency, i.e. a change in shape of the potential energy surface. Symmetry arguments indicate that such a displacement may only occur for totally symmetric modes unless the molecular symmetry is altered in the excited state. However, if a change of molecular symmetry accompanies the electronic transition, this restriction is relieved. In this case, Raman bands attributed to non-totally symmetric fundamentals may acquire intensity under resonance conditions from either the A- or B-terms of the Raman polarizability. Nevertheless, it is generally the case that the A-term contribution is much more important for totally symmetric fundamentals when the incident radiation is in resonance with a strong electric-dipole-allowed electronic transition. The domination of the totally symmetric modes (A modes) in the UVRR of BINAM, as manifested by the strong intensities of 1380, 1475, 1024, 851, 668, and 528 cm-1 bands in Fig. 5(a), may be considered as the consequence of A-term enhancement.

In Fig. 5(a), several non-totally symmetric modes (B modes), such as the 1618, 1358, 1289, and 1158 cm-1 bands, were also measured with appreciable intensities. Clark and Dines [42] have pointed out that RR bands attributed to non-totally symmetric fundamental vibrations may acquire intensity via three mechanisms: (i) A-term activity due to a change of molecular symmetry in the resonant electronic state; (ii) A-term activity due to excited-state Jahn-Teller distortion; (iii) B-term scattering, involving the vibronic coupling of the resonant state to a nearby electronic excited state. The mechanism (ii) can be considered as a specific case of mechanism (i), and it has been used to explain to the Q-band excited resonant Raman of the porphyrin compounds, in which the non-totally symmetric modes (B1g and B2g) have been found dramatically enhanced due to the excited state Jahn-Teller distortion [46, 47]. We consider that both the mechanisms (i) and (iii) are responsible for the enhancement of the non-totally symmetric modes in the UVRR of BINAM for the following reasons.

Firstly, as mentioned above, the broad 348 nm band in the UV absorption spectrum of BINAM can be attributed to the overlap of three electronic transitions, i.e., 11B, 21B, and 21A. These transitions correspond to the electronic excitation from HOMO, HOMO-1 to LUMO, LUMO+1. The orbital energies of HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 are -6.354, -5.540, -5.492, -1.253, -1.179, -0.428 eV, respectively, indicating that the four frontier molecular orbitals, i.e., HOMO, HOMO-1 to LUMO, LUMO+1, are well separated from other orbitals. Moreover, the HOMO/HOMO-1 and LUMO/LUMO+1 are nearly degenerate, which hints a pseudo-Jahn-Teller distortion may occur for the electronic configurations (37b)1(39a)1 and (38a)1(38b)1. Under this circumstance, the two configurations, (37b)1(39a)1 and (38a)1(38b)1, can be effectively coupled by certain non-totally symmetric modes. If a molecule undergoes a change of symmetry upon excitation then the A-term active modes are those that are totally symmetric in the subgroup formed by the symmetry operations common to the ground- and excited-state point groups (common group). The B modes of BINAM, while they are non-totally symmetric under the ground state point group (C2 group), are totally symmetric under the distorted C1 group at the excited states. Accordingly, these modes can be enhanced via the A-term mechanism.

Secondly, non-totally symmetric modes (B modes of BINAM) can be enhanced via the B-term mechanism. According to Ref.[42], the B-term of Raman polarizability is proportional to the vibronic coupling integral between two excited states ( and ), , and inversely proportional to the energy difference of these states (ve-vs). In our UVRR experiment, the excitation line (325 nm) locates at high energy side of the main absorption band (348 nm). From TDDFT calculated excitation energies and oscillator strengths (see ﻿Table III), one may reasonably assumed that the 21A and 21B states are favorably in resonance with incident light. Symmetry argument leads to non-zero vibronic coupling between 21A and 21B states via B-modes. Moreover, the energy difference between this two states is quite small (0.0208 eV according to TD-B3LYP/6-311++G(d, p)). Therefore, both requirements for the B-term contributions to Raman intensities are satisfied for the B-modes of BINAM.

Furthermore, the dramatic enhancement for the 1618 cm-1 (experimental) band may be intuitively understood by the general empirical rule that the vibrational coordinates responsible for converting a molecule from its ground state equilibrium conformation to the excited-state geometry will give rise to resonance-enhanced Raman bands (Tsuboi's rule) [48]. For BINAM, the lone pair of NH2 group can interact with the naphthyl π-system through hyper-conjugation. The electronic excitations from the ground to the 11B and 21B states induce an obvious redistribution of electron density for the NH2 groups and the naphthyl rings, which can result in large changes for the C-N bond distances and the NH2 bong-angles. Our DFT calculation manifests that the 1624 cm-1 (calculated) mode contains a significant contribution from the NH2 bending and a moderate one from C-N stretching, thus according to Tsuboi's rule, 1624 cm-1 band is resonantly enhanced with the UV excitation.

V. Conclusion

We have studied the IR absorption, visible excited normal Raman, and UV-excited near-resonance Raman spectra of 1, 1'-binaphthyl-2, 2'-diamine (BINAM). Density functional theory (DFT) calculations were carried out to study the vibrational frequencies and the ground-state structure of BINAM. The measured IR and Raman spectra of BINAM were found in good accordance with the calculations. The assignments of observed IR and Raman bands were proposed on the basis of the calculated and measured frequencies and intensities. In comparison with the visible excited normal Raman, several bands of BINAM were found dramatically enhanced in the UV resonance Raman spectrum. Possible excited state structure distortion was discussed based on the RR intensity analyses.

Supplementary materials: Atomic cartesian displacements for the normal modes of 1-chloro-2-naphthylamine are shown.

VI. Acknowledgments

This work was supported by the National Natural Science Foundation of China (No.21273211, No.21573208), USTC-NSRL Association Foundation (No.NSRLLHJJ(14-15-012), and the Supercomputation Center of USTC.

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1, 1'联萘-2, 2'-二胺拉曼和红外光谱的实验和DFT计算研究

a. 中国科学技术大学化学物理系, 合肥 230026;
b. 中国科学技术大学国家同步辐射实验室, 合肥 230026