D3CN HCCl3

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 3

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Blue-Shifted Hydrogen Bonding in the Gas Phase CH/DCN###HCCl Complexes Bedabyas Behera, and Puspendu Kumar Das J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12200 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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The Journal of Physical Chemistry

Blue-shifted Hydrogen Bonding in the Gas Phase CH/D3CN…HCCl3 Complexes B. Behera and Puspendu K. Das* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

*Corresponding author email: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Blue-shifting H-bonded complexes between CHCl3 and CH/D3CN have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. The change in FTIR peak intensity of the mixture of the two components as a function of temperature and composition provides the basis for identification of the H-bonded band in the infrared spectrum. On complex formation with CH3CN and CD3CN, the C-H stretching frequency of CHCl3 shifts to the blue by +8.7 and +8.6 cm-1, respectively. The molecular electrostatic potential calculation at the MP2/6-311++G** level has been used to arrive at the geometry of the complex. It has been reported in the literature that CHCl3 and CH/D3CN form red-shifting H-bonded complex in Ar matrix. The red shifting has been verified by doing ab initio calculations in the presence of Ar atoms, which has been attributed to the matrix effect at low temperature. The interaction of Ar with CH3CN makes the CH3CN more basic and as a result it becomes better hydrogen bond acceptor and causes red shift. The potential energy scans and NBO analysis of the Cl3CH⋅⋅⋅NCCH3 complex have been compared with those of F3CH⋅⋅⋅NCCH3 and Cl3CH⋅⋅⋅NH3 complexes. The change in electron density of the CHCl3 as a function of C-H⋅⋅⋅N distance shows that the approach of CH3CN to CHCl3 induces a shift in electron density from the H atom to the Cl atoms of CHCl3 which leads to C-H bond contraction and blue shifting of C-H stretching frequency. However, in the complex Cl3CH⋅⋅⋅NH3 where frequency shift to the red is reported, charge transfer and electrostatic interaction dominates.


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Introduction Blue shifting H-bond has received a significant experimental and theoretical interest due to its contrasting characteristics from the conventional red shifting H-bond1-15. The conventional Hbond (X-H⋅⋅⋅Y) is formed when a covalently bonded H atom of a proton donor (X-H) interacts with a lone pair or an electron rich center of a proton acceptor (Y)16. The interaction is primarily electrostatic in nature where the electronegative Y attracts H toward itself which lengthens the X-H bond, leading to a red shift of the X-H stretching frequency and an increase in intensity. These characteristics are regarded as the signature of the regular H-bonding. In 1989 the first observation of Budesinsky et al.1 indicated that the characteristics of H-bonding might be different in some systems when they reported a 7/4 cm-1 blue shift of the C-H/D stretching frequency of CH/DCl3 upon addition of triformylethane in solution. About 10 years later Hobza et al.2 reported blue shifting of C-H stretching frequency in benzene dimer and other carbon proton-donor-π-acceptor complexes and named this unusual H-bonding as anti-Hydrogen bonding. They also showed that apart from upward frequency shifting , this blue shifting-H bonding is accompanied with significant changes in intensity. Since then blue shifting of C-H stretching frequency where X is CF3 and CCl3 and Y is triformyl methane, benzene, ethylene oxide, dimethyl ether, etc. has been reported regularly in the literature1-2, 5, 10, 15, 17. The C-H⋅⋅⋅N type of H-bonding interaction has been investigated less compared to C-H⋅⋅⋅O type H-bonding. The calculation of C-H⋅⋅⋅N type blue shifting H-bonding was reported first by Fan et al.18 in CHF3-aniline complex. The CHF3-aniline complex is stable by 5-10 kJ mol-1 and the calculated blue shift of the C-H stretching frequency of CHF3 upon complexation with aniline is + 30 cm-1. Herrebout et al.19 observed a + 5 cm-1 blue shift of C-H stretch of CHF3 with either 3 ACS Paragon Plus Environment


ammonia or pyridine at low temperature (188 K). From the analysis of bending and stretching frequencies of H bonded CHF3, they concluded that the observed blue shift is not due to the strengthening of the C-H bond due to complexation, rather it is due to the change in the Fermi resonance interaction caused by H-bond formation. Recently a small blue shift of about +3 cm-1 was found by Rutkowski et al.20 when CHCl3 was added to CD3CN in liquefied Kr at ~120 K. It was shown by ab initio calculation at the MP2/6-311++(2d,2p) level that the C-H bond length indeed decreases by 0.0008 Å and the C-H harmonic stretching frequency is blue-shifted by +1 cm-1. From calculations they concluded that the Cl3CH⋅⋅⋅NCCH3 complex assumes a linear Hbonded structure under their experimental condition. Ito et al.10 studied the same complex in argon matrix (20 K) and calculated a red shift of -25 cm-1in the C-H stretching frequency of chloroform. The C-D stretching frequency in Cl3CD⋅⋅⋅NCCH3 complex was found to shift to the red by -18 cm-1. These red shifts instead of the blue shifts seen earlier were attributed to environmental effect due to the surrounding Ar atoms. At low temperature, the matrix molecules often interact strongly with the probe molecules which affect the spectral properties of the probe molecules and alter their vibrational frequencies 21-23. Sometimes the matrix stabilizes one type of complex over another. For example, CH3OH and C2H2 form two complexes: one weak O-H··π (CH3OH··C2H2) complex in N2 matrix below 20 K and another strong O··HC (n-σ) complex in both Ar and N2 matrices24-25. Jovan Jose et al.25calculated the molecular electrostatic potential (MESP) minimum of C2H2 surrounded by N2 molecules and Ar atoms. The MESP minimum of C2H2 becomes more negative when surrounded by N2 compared to that by Ar. This implies greater basic character of C2H2 in N2 matrix, favoring the formation of O-H··π (CH3OH⋅⋅⋅C2H2) complex more compared to that in Ar matrix. The effect of Ar environment on vibrational frequency of CO2 was studied computationally by 4 ACS Paragon Plus Environment

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Jovan Jose et al.23 In CO2(Ar)m clusters the asymmetric stretch of CO2 is red shifted and the red shift increases from 0.73 to 5.9 cm-1 on increasing the number of Ar atoms in the cluster from m =1 to 8. In the CH3CN⋅⋅⋅HCCl3 complex, the Ar matrix can only change the nature of the Hbonding, thereby shifting the X-H stretching frequency to the red or to the blue. The Ar and CO2 van der Waals complexes were probed by slit supersonic expansion26 at very high spectral resolution (0.05 cm-1 or 0.01 cm-1). About 250 individual rovibrational transitions were assigned which belong to the asymmetric stretch of the CO2 moiety in Ar–CO2 and (CO2)2 complexes. Moudens et al.27 investigated the (H2O)mArn clusters with n varying from 0 to 20 and m from 1 to 5 in a supersonic jet expansion. From both experiment and calculation they were able to speculate on the average size of the argon solvation shell around the water cluster and quantify the effect of the attached argon atoms on vibrational characteristics of the small-sized clusters. This paper focuses on the gas phase FTIR study of CH/D3CN⋅⋅⋅HCCl3 complexes followed by ab initio calculation to understand the nature of the frequency shift. The gas phase data have been compared to those of the low temperature matrix data. The matrix effect is evaluated computationally by reoptimizing both monomers as well as the complex in Ar environment. The potential energy scan and NBO analysis of Cl3CH⋅⋅⋅NCCH/D3 complexes have been done and compared with those of F3CH⋅⋅⋅NCCH3 and Cl3CH⋅⋅⋅NH3 complexes. Experiment The chemicals used in the experiments, CH3CN, CD3CN, and CHCl3 were all obtained from Sigma Aldrich and CDCl3 from Euriso-Top and used without further purification. The details of the experimental set-up and procedures have been described elsewhere15,

28

and only a brief

description will be given here. All infrared spectra were recorded in the gas phase in a FTIR 5 ACS Paragon Plus Environment


spectrometer (Bruker Optics Vertex 70) equipped with a liquid nitrogen cooled photovoltaic mercury-cadmium-telluride (PV-LN-MCT) detector, a KBr broadband (0- 20000 cm-1) beam splitter in a multi-pass gas cell of 8.0 meter path length. The gas cell is coupled to a home built vacuum line for sample introduction. The volume of the gas cell is 2.8 l and has two KCl windows and three gold-coated mirrors on it. Each spectrum was recorded with a spectral resolution of 2 cm-1 and averaged over 512 scans. For each measurement, first, we loaded the Hbond acceptor (CH/D3CN) inside the sample cell and then CH/DCl3 in the desired proportion and waited for10 min for the H-bonded complex formation process to be completed. The temperature variation studies have been performed by heating the gas cell in a controlled way. To convert the interferogram to a spectrum we have used Blackman Harris 3 term apodization function. For spectral subtraction, band integration and further processing of the spectra, we have used OPUS 7.0 software. Gas phase FTIR spectra The gas phase FTIR absorption spectra of CD3CN⋅⋅⋅HCCl3 complexes and their respective monomers in the C-H stretching region are presented by stack plots in Figure 1. Monomer spectra are subtracted with proper weight factor from the mixture spectra to obtain the spectra of the complex. The FTIR spectra of 20 Torr CD3CN (Figure 1A) has a sharp peak centered at 3106 cm-1 flanked by two broad humps on the two sides. The central sharp peak matches well with the band at 3105 cm-1 reported by Pace et al.29 who assigned it as the combination band of the C-C stretch (831.5 cm-1) and C≡N stretch (2278 cm-1). The C-H stretching fundamental mode of CHCl3 is located around 3033 cm-1.30 The FTIR spectra of CHCl3 (Figure 1B) has three bands at 3045, 3033 and 3023 cm-1. The sharp peak at 3033 cm-1 and other two broad bands on both the


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sides of the sharp peak at 3045 and 3023 cm-1 have been assigned. The sharp peak at 3033 cm-1 is due to the Q branch of the C-H stretch which is strong. The peak position and features are in good agreement with the reported values of the Q-branch.30 The broad band at 3045 cm-1 is due to the R branch and the other band at 3023 cm-1 is due to the P branch of the C-H stretch. We have checked for the dimerization of CHCl3 by gradually increasing the CHCl3 pressure from 10 to 80 Torr and looking for any new band in the 3000-3100 cm-1 region. We did not find any new band and concluded that in our working pressure range of CHCl3 there is no dimer formation at room temperature. Figure 1C shows the spectra of CHCl3 and CD3CN mixture, where the CD3CN pressure was kept constant at 20 Torr and the pressure of CHCl3 was increased successively in steps of 10 Torr up to 40 Torr. Upon addition of CHCl3 to CD3CN no new peak was observed and only the intensity of the PQR bands of CHCl3 increased albeit unevenly over the range. The intensity in the R band region increased more rapidly than in the P band region which might be due to the appearance of a new band in this region whose intensity gets enhanced with increasing CHCl3 pressure. To investigate further, the pressure of the mixture was kept fixed and the temperature was slowly increased from 292 to 348K (Figure 1D). A new band at 3041.6 cm-1 appears when the monomer spectra of CHCl3 and CD3CN are subtracted from the spectrum of the mixture (Figure 1E). With increase in temperature, the spectral intensity in the R-band region goes down but the rest of the spectrum remains unchanged which confirms that the increase in intensity in the R band region at room temperature is due to a band which disappears as the temperature of the complex is raised. We ascribe this intensity loss to the disappearance of C-H··N H-bonding between CHCl3 and CD3CN. The new 3041.6 cm-1 band is due to the H-bonded C-H stretch which is +8.6 cm-1 blue-shifted from the free C-H stretch of CHCl3 at 3033 cm-1.



Figure 1. Room temperature gas phase FTIR spectra of H-bonded complexes of chloroform and acetonitrile-d3 in the C-H stretching region. A: spectrum of acetonitrile-d3 at 20 Torr, B: spectrum of CHCl3 at 60 Torr, C: spectra of mixture of monomers at different compositions, D: spectra of mixture of monomers at a fixed composition at different temperatures, E: spectrum of the complex where monomers’ spectra have been subtracted from the spectrum of the mixture. The partial pressures and temperatures are shown in the inset. In a similar fashion, we have the studied the H-bonded complex between CH3CN and CHCl3. The spectra of the complex and their respective monomers are shown by stack plots in Figure 2. The gas phase FTIR spectra of CH3CN at 20 Torr (Figure 2A) shows major peaks at 2970, 2954, 2940, 2902, 2878, 2868 and 2854 cm-1 along with some rotational fine structure at lower resolution. The spectra of CH3CN31-32 and its assignment33 in the C-H stretching region have been reported in the literature. The rotational fine structure centered at 3009.6 cm-1 is assigned as the degenerate C-H stretching frequency of the CH3 group33. The three peaks at 2940, 2954 and 8 ACS Paragon Plus Environment

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2970 cm-1 are assigned as P, Q, R branches of symmetric C-H stretching frequency33. The peak at 2878, 2868 and 2854 cm-1 are assigned to P, Q, R branches of the first overtone of CH3 deformation frequency. The band at 2902 cm-1 is ascribed as a combination band of CH3 deformation and CH3 rocking motion33. Like in CD3CN….HCCl3 complexes, with addition of CHCl3 at a fixed pressure of CH3CN (20 Torr), the intensity in the PQR region of the C-H stretch of CHCl3 increases (Figure 2C). With increase in temperature (373 K) at a fixed ratio, the intensity in the R band region decreases which confirms the presence of a new band in this region. After subtraction, the isolated peak (Figure 2E) is obtained at 3041.7 cm-1 which is blue shifted by +8.7 cm-1 with respect to the free C-H stretch. From the experiments we found that CHCl3 forms blue shifted H-bonded complex with both CH3CN and CD3CN in the gas phase at room temperature. We could not detect the CDCl3 and CH/D3CN complex in our experiment because of the spectral overlap of both C-D and C-N stretching bands. Ito et al.10 reported that in Ar matrix the CH/DCl3 form red shifting H bonding with CH/D3CN. The band positions and spectral shifts of H-bonded complexes of CHCl3⋅⋅⋅CH/D3CN in the gas phase and in Ar matrix are collected in Table 1. In order to get further insights about the structure of the complex, the red shifting of C-H stretch in CHCl3⋅⋅⋅CH/D3CN complexes in Ar marix, etc. we turn into quantum chemical calculation.



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Figure 2. Room temperature gas phase FTIR spectra of H-bonded complexes of chloroform and acetonitrile in the C-H stretching region. A: spectrum of acetonitrile at 20 Torr, B: spectrum of CHCl3 at 60 Torr, C: spectra of both 1:1 complex and monomers mixture at different compositions, D: spectra of both complex and monomers mixture at different temperatures E: spectrum of the complex where monomer spectra have been subtracted. The partial pressures (Torr) are shown in the inset. Table 1. Observed C-H stretching frequency (νComplex), frequency-shift (Δνshift)a, andreported frequency-shift ofCl3CH⋅⋅⋅NCCH/D3 complexes.

Complex

νComplex (cm-1)

∆νShift (cm-1)

∆νShift (cm-1) (Ar matrix) 10

Cl3CH⋅⋅⋅NCCD3

3041.6

+ 8.6

-25

Cl3CH⋅⋅⋅NCCH3

3041.7

+ 8.7

-25


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a

The C-H stretching frequency in pure CHCl3was observed at 3033 cm-1. Thus, Δνshift = νC-H in

complex

- νC-H in CHCl3.

Computational details Ab initio calculations were performed by Gaussian 09 D 01 version 26 set of programs34 at the MP2 level of theory and 6-311++G** basis set with added polarization(**) and diffuse function(++). The geometries of the complexes and its constituent monomers were optimized without any constraints on molecular geometry. Harmonic vibrational frequencies were calculated by the analytical second derivative method. All geometries converged to their global minima without the presence of any negative frequency which is indicative of transition states. At global minima, anharmonic force field is computed in the Gaussian package by using the linear relationship between normal (Q) and cartesian displacement (X) coordinates as Q=L+M1/2X, where L is the mass-weighted cartesian force constant matrix and M is the diagonal matrix of atomic mass35. Anharmonic frequencies were calculated at MP2/aug-cc-pvdz, CCSD/cc-pVDZ and QCISD/aug-cc-pVDZ levels of theory. This type of calculation had been performed earlier and the frequency shifts (∆νC-H = νC-Hcomplex - νC-Hmonomer) calculated were in good agreement with the experimental values36. All binding energies were corrected for zeropoint energy and basis set superposition error by using the counterpoise method. The counterpoise method was applied as a part of the geometry optimization of the complex. The molecular electrostatic potential (ESP) was obtained to predict the electrophilic and nucleophilic sites on the van der Waal surface of the molecule37. We also carried out potential energy scans of the complexes where the X⋅⋅⋅Y distance in small increments was varied while all the other geometrical parameters were optimized6. We did NBO calculations at the MP2/6-311++G** 11 ACS Paragon Plus Environment


level and the second-order perturbation energy was computed at M06-2X/6-311++G* level of theory.38-39 All analysis were done with only the Cl3CH⋅⋅⋅NCCH3 complex and we expect that the Cl3CH⋅⋅⋅NCCD3 complex will follow the trend. When two monomers interact they always tend to approach each other in a manner so that their molecular electrostatic potentials (MESP) are complementary at the point of interaction. The position of the ESP maximum (Vmax) or minimum (Vmin) on the molecular surface (topographical distribution), determines the binding site and the geometry of the complex. Usually, the magnitude Vmax indicates the electron-deficient points (+ve) and the magnitude of Vmin the electron-rich points (-ve) on the molecular surface. The MESP maximum and minimum of both CHCl3 and CH3CN were calculated for looking at the geometry of the stable 1:1 complex and for assessing the effect of the inert matrix on the ground state vibrational spectrum of the complex. The MESP was used in a different way by Gadre et al.40-41 to model the interaction between different DNA base pairs in a “lock and key” arrangement. Instead of using Vmax/Vmin directly they used bond critical points of MESP which are points where all the first order partial derivatives of the given MESP vanishes, to locate and identify the points of interaction between the base pairs. We have used the complementarity of the MESP at the point of interaction. MESP surfaces of CHCl3 and CH3CN are shown in Figure 3. The highest Vmax of CHCl3 is located on the top of the H atom while three degenerate potential maxima of lower potential are found on the three chlorine atoms. An even smaller Vmax is present at the center of the degenerate ring which we have not shown. The Vmin appears as a degenerate ring below the plane of the three chlorine atoms. The CH3CN has three degenerate potential maxima located at the top of each H atoms and one potential maximum below the carbon atom of the CH3 group. A potential well (Vmin) is present at the top of the nitrogen atom of CH3CN. The MESP calculation of Ar atom 12 ACS Paragon Plus Environment

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show that it has 228 surface minima (Vmin) each of value -0.084 kcal/mol and 116 surface maxima each of value + 0.084 kcal/mol. So overall Ar molecular surface is electron rich and it will interact with surface maxima of other atoms. The complementary combinations of MESP of the monomers were used for making initial guesses for the starting geometry of the complex. Four possible combinations between Vmax and Vmin such as (a) Vmax(H) of CHCl3 and Vmin(N) of CH3CN, (b) Vmax(H, Cl) of CHCl3 and Vmin(N) of CH3CN, (c) Vmax(Cl) of CHCl3 and Vmin(N) of CH3CN, (d) Vmax(H) of CH3CN and Vmin(degenerate ring) of CHCl3 were taken as initial guess. On optimization, the initial geometry gets modified and is shown in Figure 4. Binding energies and frequencies corresponding to the optimized

structures

are

shown

in

Table

2.

Figure 3. The molecular electrostatic potential (eV) topographical analysis of monomers calculated on at MP2/6-311++G** level of theory. The ESP maxima and minima were calculated on the isodensity surface of 0.001 au: a. potential maxima of CHCl3 b. potential minima of CHCl3, c. potential maxima of CH3CN, and d. potential minima of CH3CN. 13 ACS Paragon Plus Environment


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Without counterpoise correction, the structure b has a bent geometry as shown in Figure 4, but after imposing counterpoise correction the geometry became the same as that of a. The binding energy of structure c is only -1 kJ/mol which indicates that it is not very stable at room temperature. Structure d shows positive binding energy which makes it unstable. Thus from calculation, it can be safely concluded that the structure a is, in all likelihood, the stable optimized universal structure of the CHCl3 and CH3CN H-bonded complex at room temperature.

Figure 4. Possible geometries for the CHCl3 and CH3CN complex at room temperature. All geometries were optimized at the MP2/6-311++G** level of theory. Table 2. Calculated binding energies and frequencies of the four possible structures. Complex

ΔE/ kJ.mol-1

ΔE(CP)/ kJ.mol-1

νanharm/ cm-1

Δνanharm/ cm-1

A

-18.9

-13.6

3102.4

25.7

B

-22.3

-

3085.1

8.4

C

-10.5

-1

3073.9

-2.8

D

-9.3

0.1

3075.3

-1.4


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The C-H stretching frequency of CHCl3 obtained from calculation is 3076.7 cm-1 which is about 43 cm-1 lower than the experimental frequency of 3033 cm-1. On complexation with CH3CN and CD3CN, our calculation shows that the C-H stretching frequency of CHCl3 is blue shifted by 25.7 and 21.5 cm-1, respectively, to 3102.4 and 3098.2 cm-1. We then calculated the blue shift and the extent of contraction of the C-H bond, at different levels of theories which are frequently used in the literature in dealing with the H-bonding interaction. The changes in bond-lengths and frequencies are listed in Table 3. The blue shift and the shortening of C-H bond-length obtained from calculations are consistent with our observation of blue-shift but the extent of the shift is much lower than what is predicted by calculations. Calculations show contraction of the C-H bond length upon complexation, which is consistent with the blue-shift of the C-H stretching frequency. However, infrared experiments10 on the same complexes in Ar matrix reported redshifts of about 25 cm-1 of the C-H stretch. Table 3.Frequency shifts and changes in C-H bond-length in Cl3CH. . .NCCH/D3 complexes at different levels of theory. Anharmonic (a) and harmonic (h) calculation results are shown. Level of theory

Cl3CH⋅⋅⋅NCCH3 -1

Cl3CH⋅⋅⋅NCCD3

ΔR/Å -0.0004

ΔνShift/cm-1 + 21.5

ΔR/ Å -0.0004

MP2/6-311++G** (a)

ΔνShift /cm + 25.7

MP2/aug-cc-pvdz (a)

+ 10.5

-0.0002

+ 12.6

-0.0002

CCSD/cc-pVDZ (h)

+ 32

-0.0009

+ 32.0

-0.0009

QCISD/aug-cc-pVDZ (h)

+ 35.8

-0.0012

+ 35.8

-0.0012



Now the question is why these complexes exhibit red-shifting H-bonds in Ar matrix. It is well documented that the matrix environment alters the vibrational frequency of a molecule with respect to the free molecular vibrational frequency21-22. Therefore, we decided to do the calculations in a matrix environment at low temperature. Table 3 lists the C-H stretching frequency of CHCl3 in five different environments reported in the literature. Interestingly in the Ar and N2 matrices10 the C-H stretching frequencies observed at 3053 and 3063 cm-1, respectively, are higher than the gas phase molecular C-H stretching frequency of 3033 cm-115. In other words in Ar and N2 environments the C-H stretching frequency gets blue shifted by 20 and 30 cm-1, respectively, which indicates that there is interaction between CHCl3 and Ar/N2. The CH stretching frequencies in pure chloroform liquid (300K)31 and in liquid Kr (120 K)20 are almost the same (Table 4) which points to the fact that Kr has no interaction with CHCl3 up to 120 K. It could be due to the fact that the temperature (120 K) is not low enough for the formation of a stable complex. The difference C-H stretching frequency in the pure liquid state and the gas phase could be due to the formation of intermolecular H bonded complexes in the liquid state. Table 4. C-H stretching frequencies of CHCl3 in different environments.

Environment

νCHCl3(cm-1)

Gas (300 K)

303312

Matrix (Ar, 20 K)

305310

Matrix (N2, 20 K)

306310

Liquid (300 K)

302331

Liquid (Kr, 120 K)

302420


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What is the arrangement of Ar atoms surrounding CHCl3? In order to arrive at the most stable optimized geometry of CHCl3(Ar)m complex, we use the molecular electrostatic potential to make initial guesses on the arrangement. The point of maximum electrostatic potential (Vmax) of CHCl3 is situated at the top of the H atom where we placed an Ar atom first and optimized the geometry of the complex. For the second Ar atom, we calculated the MESP topographical distribution of the CHCl3(Ar) complex and repeated the same strategy of identifying the point of maximum potential to place another Ar atom. Successively, in each step, we calculated the MESP of the complex and added one more Ar atom at the new Vmax point and optimized the geometry (Figure 5) as was suggested by Jose et al.23 The positions of Vmax after addition of each Ar atom are given in the supporting information. After optimization, the position of the Ar atom is not the same as where it is initially placed. The anharmonic frequency of C-H stretch was calculated for every structure (Table 5) up to m = 7 (structure 5g). We could not proceed beyond 7 Ar atoms because the complex size became too large to handle within our resources. At a very low temperature the Ar matrix is known to stabilize in a face-centered cubic lattice25 by occupying the lattice sites. The guest molecules are normally trapped at the lattice sites except for small molecules which could be trapped in interstitial sites. In the structure 5g, we have sequentially added four more Ar atoms at the Vmax and optimized the geometry and surprisingly it stabilized at a face centered cubic lattice (Figure 5i), where the three Cl atoms and 11 Ar atoms occupy the lattice sites and the smaller H and C atoms are confined to the interstitial sites. The changes in bond lengths (m=1-11) and anharmonic frequencies of CHCl3(Ar)m (m=1-7) are given in Table 5. After addition of four Ar atoms to CHCl3(Ar)7, there is no significant change in the C-H bond length of CHCl3. Therefore, the anharmonic frequency (3105.1 cm-1) calculated for structure 5.5g is taken as the C-H stretching frequency of CHCl3 in Ar matrix. The C-H stretch 17 ACS Paragon Plus Environment


shows a 28.3 cm-1 blue shift with respect to free CHCl3 which is in agreement with the reported experimental shift of 20 cm-1 10.

Figure 5. The optimized structures of CHCl3(Ar)m (m=1-11) obtained by successively adding one Ar atom at a time at the Vmax of CHCl3(Ar)m-1 complexes.


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Figure 6. Molecular electrostatic potential (eV) surface maxima of the Cl3CH⋅⋅⋅NCCH3 complex calculated at the MP2/6-311++G** level. The ESP maxima and minima were calculated on the isodensity surface of 0.001 au Table 5. Anharmonic frequency, C-H bond-length, frequency shift and bond length change of CHCl3(Ar)m complex at the MP2/6-311++G** level. Bond length changes and frequency shifts are shown with respect to gas phase CHCl3. Due to large size of CHCl3(Ar)8 and CHCl3(Ar)11 complexes, frequencies couldn’t be computed.

Complex Number

No. of Ar νanahrm(com) atoms /cm-1

R(com) /Å

Δνanahrm /cm-1

ΔR /Å

A

1

3097.49

1.0833

20.75

-0.00155

B

2

3099.65

1.08334

22.91

-0.00151

C

3

3092.62

1.08411

15.88

-0.00074

D

4

3104.92

1.08372

28.18

-0.00113

E

5

3101.63

1.0837

24.89

-0.00115

F

6

3104.61

1.08361

27.87

-0.00124

G

7

3105.07

1.08369

28.33

-0.00116

H

8

-

1.08363

-

-0.00122

I

11

-

1.08373

-

-0.00112

The molecular electrostatic potential of the CH3CN⋅⋅⋅HCCl3 complex was calculated (Figure 6). Optimization of the geometry of the complex with Ar atoms placed around it was difficult due to its large size. We placed electron rich Ar atoms on the points where the MESP maxima (Vmax) of the complex were found. We added up to 7 Ar atoms around the complex and optimized the geometry in each case. For a given number of Ar atoms different possible arrangements of Ar atoms around the complex are possible but some of the geometries did not converge. Only 9 geometries converged (Fig. 7). The anharmonic frequencies of each structure were computed. 19 ACS Paragon Plus Environment


Due to limitation in available computational time, the larger structures (> 4 Ar atoms) and their anharmonic frequencies could not be computed. The corresponding C-H bond lengths and anharmonic frequencies are shown in Table 6. The frequencies and bond lengths of the complexes were compared with those of CHCl3 in Ar matrix. Irrespective of the number of Ar atoms in all the complexes the C-H bond length increases and the C-H stretching frequency shifts to the red, which is in good agreement with experiments. The highest red shift calculated was 9.9 and 9.52 cm-1 for structures b and e, respectively, which are both lower than the experimental value10 of 25 cm-1. It could be due to the number of Ar atoms in the complex is not sufficient to reproduce the matrix environment. The above calculations show that the Ar matrix can shift the C-H stretching frequency in CHCl3 toward a higher wave number (blue) region but C-H stretching frequency in the complex toward a lower wave number (red) region. The CH3CN has a sigma hole opposite to the CH3 group where the Ar atom interacts42-43. The interaction of Ar with CH3CN makes the CH3CN more basic and as a result it becomes better hydrogen bond acceptor25 forming red shifted H-bond. In the presence of a single Ar atom on the back side of the CH3 group, a red shift of -9.9 cm-1 to the C-H stretch has been reported by calculation.


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Figure 7 Cl3CH⋅⋅⋅NCCH3 and Ar complex i.e., Cl3CH⋅⋅⋅NCCH3(Ar)m (m=1-4), these structures are obtained by successively adding Ar atom at Vmax of Cl3CH⋅⋅⋅NCCH3.



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Table 6. Anharmonic frequency, C-H bond length, frequency shift and bond length change of Cl3CH⋅⋅⋅NCCH3 in Ar matrix calculated at the 6-311++G** level of theory. Bond length change and frequency shift are cited with respect to CHCl3(Ar)m(m=11). Complex structure

No. of Ar νanahrm(com.) atoms /cm-1

R(com.) /Å

Δνanahrm /cm-1

ΔR /Å

a

1

3102.52

1.08451

-2.55

0.00078

b

1

3095.17

1.08445

-9.9

0.00072

c

2

3100.04

1.08449

-5.03

0.00076

d

3

3100.35

1.08449

-4.72

0.00076

e

3

3095.55

1.0845

-9.52

0.00077

f

3

3098.68

1.08471

-6.39

0.00098

g

4

3098.68

1.08452

-6.39

0.00079

i

5

-

1.08467

-

0.00093

j

7

-

1.08466

-

0.00092

To get insight into the origin of blue-shifting of the C-H frequency in complexation with CH3CN, we examined two other similar complexes, F3CH⋅⋅⋅NCCH3 and Cl3CH⋅⋅⋅NH3 computationally where in the former we have replaced the H bond donor and in the latter the H bond acceptor. From calculation the F3CH⋅⋅⋅NCCH3 complex shows greater blue shift (+33.9 cm1

) than Cl3CH⋅⋅⋅NCCH3 (+25.7 cm-1) while the Cl3CH⋅⋅⋅NH3 complex shows large red shift (-36.9

cm-1). To probe further, these complexes were subjected to potential energy scans (PES), where the distance (C⋅⋅⋅N) between two monomers were changed from as far as 7 Å to as close as 2.3 Å in small steps of 0.1 Å while other geometric parameters were optimized. A comparison between PESs of Cl3CH⋅⋅⋅NCCH3 with Cl3CH⋅⋅⋅NH3 and F3CH⋅⋅⋅NCCH3 is presented in the Figure 8. As the monomers approach each other, the binding energy (∆Erel) gradually decreases and this downward trend continue up to the equilibrium distance when the energy is zero. 22 ACS Paragon Plus Environment

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Beyond the equilibrium distance, energy goes up again. During the entire scan, the C-H bond length first increases, reaches a maximum and then decreases. The solid and dashed line parallel to the x-axis shows the reference C-H bond length of free CHCl3 and CHF3, respectively. The arrow indicates the C-H bond length in the complex at the equilibrium geometry. The arrow above the solid or dashed line implies that at the equilibrium geometry the C-H-bond is elongated and that below indicates that the C-H bond is contracted. Consequently the C-H stretching frequency will shift to the red or blue with respect to free CHCl/F3. The potential energy scans validate that Cl3CH⋅⋅⋅NH3 forms red shifting and Cl3CH⋅⋅⋅NCCH3, and F3CH⋅⋅⋅NCCH make blue shifting H-bonds.



Figure 8. Potential energy scans of C-H bond distance and relative binding energy as a function of C⋅⋅⋅N distance in Cl3CH⋅⋅⋅ NCCH3, F3CH⋅⋅⋅ NCCH3and Cl3CH⋅⋅⋅ NH3complexes. Next the electron density on each atom was calculated by NBO population analysis at the MP2/6-311++G** level of theory for all the three complexes. At each step of the potential energy scan, the electron densities were calculated which are displayed in Fig. 9.

Figure 9. Change in the electron density on atoms of (a) Cl3CH⋅⋅⋅NCCH3, (b) F3CH⋅⋅⋅NCCH3, and (c) Cl3CH⋅⋅⋅NH3 complexes after subtracting the electron density of respective atoms in unbound form as a function of donor-acceptor distance. Figure 9 shows that at large C···N separation (> 6 A) , the change in the electron density on all the atoms is insignificant. As the donor and the acceptor come closer, the electron density on the H atom forming the H-bond decreases and simultaneously that on the Cl/F atom of the donor increases. A similar observation was made for the H-bond acceptor. The electron density on the N atom of CH3CN/NH3 continues to increase as the distance becomes shorter and shorter and the 24 ACS Paragon Plus Environment

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electron density on the atom next to it (C/H) keeps getting smaller and smaller. This separation of electron density between C and H of CCl/F3H makes C more electronegative and H more electropositive and leads to C-H bond contraction. This can be seen by comparing the electron density of C and H atoms of Cl3CH and F3CH of Cl3CH···NCCH3 and F3CH···NCCH3 complexes (Figure 9 (a) and (b)). The electron density difference between C and H is more in the case of F3CH···NCCH3 complex than in Cl3CH···NCCH3 which causes larger C-H bond contraction in the former than in the latter.

Figure 10. Nitrogen to C-H charge transfer energy (E2) in Cl3CH⋅⋅⋅NCCH3, Cl3CH⋅⋅⋅NH3, and F3CH⋅⋅⋅NCCH3 complexes as a function of distance between the monomers. These energies are obtained by NBO analysis at the M06-2X/6-311++G** level of theory. Another factor which contributes significantly to H bonding is the charge transfer interaction which is best assessed by NBO second-order perturbation analysis. In the three complexes considered above, we found a significant amount of charge transfer from the lone pair orbital of N (H-bond acceptor) to the C-H (H-bond donor) antibonding orbital. Due to this charge transfer the C-H bond became weak and elongated. The N to C-H charge transfer energies as a function 25 ACS Paragon Plus Environment


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of C···N distance between the two monomers are presented in Figure 10. In Figure 10 the Cl3CH···NCCH3 and F3CH···NCCH3 complexes have about the same charge transfer energy as a function of C···N distance. When the NCCH3 acceptor is replaced by NH3, a significant increase in the charge transfer energy is observed. In Figure 10 the charge transfer energy between Cl3CH···NCCH3 and Cl3CH···NH3 starts around the same value at long distances and become different from 4.4 Å downward. In Figure 8, the C-H bond distance between Cl3CH···NCCH3 and Cl3CH···NH3 gets separated at 4.4 Å which shows that the charge transfer is the reason for the C-H bond elongation in the Cl3CH···NH3 complex, which dominates the H-bonding interaction and leads to red shift. 4.6 Conclusion Blue shifting H-bond between CHCl3 and CH/D3CN has been observed in the gas phase FTIR spectra of the complexes and confirmed by ab initio calculations. However, the environment and experimental condition around the H bonded complex can alter the nature of the shift. For example, both the complexes show red shift in Ar matrix. By doing the calculations in Ar environment we have shown that CHCl3 and CH/D3CN form red shifting H-bond as reported in Ar matrix due to the change in the nature of the interaction in Ar environment. In H bond formation the X-H of H-bond donor can undergo either elongation or contraction.

The

elongation is generally aided by electrostatic attraction between H and Y, and charge transfer from Y to the antibonding orbital of the X-H. The contraction is caused by charge rearrangement in X-H due to induction interaction of Y. The contribution from all of these interactions varies with the nature of H bond donor and acceptor. The dominance of some of these effects at equilibrium decides the X-H bond length and nature of the frequency shift.


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Supporting information The Supporting information is available free of charge on the ACS publication website at DOI: Position and magnitude of electrostatic Potential maxima (Vmax) of different CHCl3(Ar)n Clusters (n=0-7) are shown in the supporting information (Figure S1). AUTHOR INFORMATION *Corresponding author Email: [email protected] Phone number: +918022932662

fax: +918023600416

Acknowledgement The FTIR spectrometer used in this work was obtained from a FIST grant of the Department of Science and Technology, Govt. of India.

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Graphical Abstract 99x57mm (600 x 600 DPI)

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D3CN HCCl3

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