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Structural and Vibrational Properties of Iodopentafluorobenzene: A Combined Raman and Infrared Spectral and Theoretical Study Michael Henry Palmer, Malgorzata Biczysko, Kirk A. Peterson, Christopher S Stapleton, and Simon P Wells J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08399 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Structural and Vibrational Properties of Iodopentafluorobenzene: A Combined Raman and Infrared Spectral and Theoretical Study Michael H. Palmer,∗,† Malgorzata Biczysko,∗,‡ Kirk A. Peterson,∗,¶ Christopher S. Stapleton,§ and Simon P. Wells§ †School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Scotland, UK ‡International Centre for Quantum and Molecular Structures, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, China ¶Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, USA §BrukerUKs, Banner Lane, Coventry CV4 9GH, UK E-mail: [email protected]; [email protected]; [email protected]

Abstract A combined study of the vibrational spectroscopy of iodopentafluorobenzene by new Raman and Fourier-Transform infrared (FTIR) spectroscopies, over the spectral range 300 to 3200 cm-1 (Raman) and 50 to 3400 cm-1 (FTIR), with a state-of-the-art theoretical investigation is reported. This has enabled reliable identification of numerous fundamental, overtone and combination band transitions in unprecedented detail. The theoretical analysis, beyond the double-harmonic approximation, is based on generalized second-order vibrational perturbation theory (GVPT2), with a hybrid coupled cluster/density functional theory (CC/DFT) approach. Anharmonic contributions to structural parameters, rotational constants, vibrational frequencies, and spectral intensities are incorporated. The procedures, of general applicability, enable rigorous comparison of theoretical methods with experimental results in vibrational spectroscopy.

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1. INTRODUCTION A recent reinvestigation of the neutral1 and ionic2 excited states of iodopentafluorobenzene (C6F5I), a compound widely used in synthetic chemistry, showed a paucity of structural and spectroscopic data. The structure, atomic labelling and inertial axes are shown in Fig.1. Structural features are limited to an rα structure derived from

13

C satellites in the

19

F NMR spectrum;3 no microwave

(MW) or electron diffraction (ED) studies have been reported. This is now rectified in this work by theoretical calculations of the equilibrium structure with an anticipated close equivalence to the experimental state. This same lack of detail also applies to the vibrational spectra of C6F5I. Although Long and Steele4,5 reported infrared (IR) and Raman (RA) spectra, their analysis was based on both a comparison with related compounds C6F5X (X = H, D, Cl, Br and I), and a force field based on C6F6, where analysis of in-plane frequencies and displacements was presented.4 Hyams et al6 offered tentative symmetry assignments for some vibrations from P-R wing separations observed in some band contours. In this paper, we present more precise Raman, far infrared (FIR) and mid-infrared (MID) spectra, with the combined FIR + MIR range from 50 to 3000 cm-1 and Raman spectra from 300 to 3000 cm-1. We have confirmed the presence of three bands in the FIR spectrum of C6F5I previously observed,7 but a fourth band was not found in either our spectroscopic or theoretical studies. Our detailed analysis of the experimental vibrational structure proceeds via a hybrid scheme, which combines equilibrium structure and harmonic vibrational frequencies from coupled cluster (CC) computations, with anharmonic corrections from density functional theory (DFT). This approach, denoted as CC/DFT,8,9 is combined with anharmonic generalized second-order vibrational perturbation theory (GVPT2) methods.10,11 This combination is believed to give the best theoretical predictions of IR and Raman spectra by comparison of its vibrationally averaged structures,

rotational

constants

and

fully

anharmonic

vibrational

frequencies

with 2


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experiment.9,11,12 Although these methods are only applied to C6F5I in this paper, the good agreement between our experimental and simulated IR and Raman spectra, further demonstrates the generality of CC/DFT predictions. We propose this provides a benchmark for future structural and spectroscopic studies from IR, Raman or MW experiments.

Figure 1: Molecular structure of C6F5I: showing atom labelling and rotational axes

2. EXPERIMENTAL METHODS AND COMPUTATIONAL APPROACHES 2.1 Mid-infrared (MIR) and far-infrared (FIR) Spectroscopy Both series of measurements were performed on a Bruker Vertex 70 spectrometer. The C6F5I sample (CAS Registry Number 827-15-6) obtained from Sigma-Aldrich, was used without further purification. The sample was introduced into a variable path length liquid transmission cell (Specac Omnicell); the path length was set to 50 µm by a PTFE spacer between polyethylene windows. For the FIR measurements, the instrument was configured with a dedicated FIR silicon beam splitter, a FIR DLaTGS detector, polyethylene windows and ceramic source. Background measurements were taken after overnight purging with dry air. Spectrum collection was again preceded by purging of the instrument prior after injection of the sample into the transmission cell. Because of the cut-off points in the detector and optics, we define the FIR region to lie 3 ACS Paragon Plus Environment


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below 400 cm-1. It was measured with a dedicated FIR detector, windows and at a scanner velocity of 2.5 kHz. The measurement parameters were 1000 scans each containing 4563 data points over range. With 4 points being measured for each wavenumber, this corresponds to a −1

resolution of better than 0.5 cm . Six of the strongest FIR bands for 1H216O (in cm-1 with intensities in km/mol given in parentheses),13,14 are: 202.8(330), 150.6(290), 170.4(270), 208.5(260), 227.9(250) and 100.6(180); their absence in our FIR region confirms the absence of water. The MIR measurement used a wide range silicon beam splitter, room temperature DLaTGS MIR-FIR detector and a ceramic source. The sample was introduced into a Specac Omnicell®, with a pathlength of 50 µm and potasium bromide windows. . The MIR measurement contained 30 scans of 1883 data points over this range, with 4 points per wavenumber, this corresponds to −1

a resolution of 6 cm . No extra spectral changes were observed when the measurements were increased to 500 scans. The MIR and FIR spectra shown in Table 3 and Figs. 2, 3 and 4 are all in absorbance units (log of incident to transmitted spectral power). 2.2 Raman Spectroscopy The Bruker Bravo handheld Raman spectrometer is a dispersive Raman instrument that uses sequentially shifted excitation (SSE) as a means of fluorescence rejection. The Bravo utilizes two lasers, covering between 700 and 1100 nm. This gives the Bravo a spectral range of 3200 cm-1 – 300 cm-1. The Bravo uses a CCD detector and achieves a spectral resolution of 2 cm-1.. The laser power output was < 100 mW for both lasers. The Raman spectrum was run in ‘bench-top’ mode, where the integration time (5000 ms) and total scans (20) were manually set. The Raman spectrum is linear in Raman intensity. In order to extract the Raman signals from the acquired spectrum, the Bravo fluorescence software recognizes and separates the Raman signal from fluorescence and other spurious signals 4 ACS Paragon Plus Environment

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automatically, by SSE.15,16 The wavelength of the excitation lasers is sequentially varied during the measurements, with consequential change in the position of Raman signals on the detector. Spurious signals such as fluorescence, remain at constant position (and implicit wavelength) during this process and are electronically eliminated. In short, the SSE algorithm extracts only the “moving” signals which relate directly to the Raman signature. 2.3 Computational details The equilibrium structure and harmonic force field calculations employed the explicitly correlated coupled cluster singles and doubles with perturbative triples approach, CCSD(T*)F12b,17,18,19 where (T*) indicates the perturbative triples contribution was scaled by the ratio MP2-F12/MP2. In regards to the basis set, the C and F atoms utilized the cc-pVDZ-F12 sets20 while the cc-pVDZ-PP-F12 set was used for I.21 The -PP indicates a small-core (28 electron) relativistic pseudopotential (PP) was employed on the I atom.22 Auxiliary basis sets required by the explicitly correlated approach corresponded to DZ-quality OptRI sets23,21 for the resolution of the identity of the F12 multi-electron integrals, together with ccpVTZ/JKFIT (C, F)24 and def2-QZVPP/JKFIT (I)25 for density fitting of the Fock and exchange matrices and aug-cc-pVTZ/MP2FIT26 and cc-pVDZ-PP-F12/MP2FIT21 for density fitting of the remaining integral quantities. The geminal exponent was set to 1.0 ao-1. All coupled cluster computations were performed with the MOLPRO package,27 although the harmonic frequencies were obtained with the CFOUR program28 using the CCSD(T*)-F12b energies from MOLPRO. The equilibrium structure and vibrational force fields were also evaluated by density functional theory (DFT) using the B3LYP functional,29 together with the cc-pVTZ(-PP) basis set (cc-pVTZ for C and F with cc-pVTZ-PP for I).30,22 A refined hybrid force-field, denoted by CC/DFT,8 was developed by addition of the cubic and semi-diagonal quartic B3LYP force constants to the 5 ACS Paragon Plus Environment


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(harmonic quadratic) CCSD(T)-F12b set results. After treatment to remove unwanted resonances, the vibrational frequencies were obtained via a generalized second-order vibrational perturbation theory approach (GVPT2).10 IR intensities and Raman activities were evaluated at the DFT level using deperturbed VPT2 (DVPT2) computations.10,11 Vibrationally averaged structures were also obtained from VPT2 computations.31 All these DFT and VPT2 computations were performed using the GAUSSIAN-09 suite of programs32 following the procedures recommended in Ref 33. 3. RESULTS 3.1 Equilibrium and vibrationally averaged structural parameters C6F5I is planar with C2v symmetry in its ground electronic state (X1A1). The measured dipole moment of 1.06 D (extrapolated to infinite dilution in C6H6 solution),34 is very similar to that of piodofluorobenzene (0.90 D),35 which demonstrates that the moments from the 2,3,5,6-tetrafluoro moiety largely cancel. The present calculated dipole moment for C6F5I, at the B3LYP/cc-pVTZ (PP) level) is 0.88 D, oriented along the rotational axis A (Figure 1). Two sets of computed structural parameters for C6F5I, B3LYP/cc-pVTZ(-PP) and CCSD(T*)F12b/cc-pVDZ-F12(-PP), are compared in Table 1. The mean absolute differences (MAD) for CC/CF bond lengths are 0.006 Å for the current results, but the C1-I bond discrepancy is 0.021 Å. The MAD for the angles is only 0.15º, but the largest discrepancy again lies near the I-atom, with C2C1C6 differing by 0.4º. Two of the NMR bond lengths3 were assigned in the nematic phase NMR spectra, but the maximum difference between the present theory and the NMR data are small, 0.0155 Å for the C3-F3 / C5-F5 bonds and 0.0104 Å for the C5-C4 / C3-C4 bonds. The angle differences are small (< 0.4º). Overall the level of agreement is very good. Table 1: Equilibrium (req) and vibrationally averaged (r0) structural parameters of C6F5I obtained at the B3LYPa and CCSD(T*)-F12bb levels of theory, along with vibrational corrections (∆vib), experimental rα parameters and rotational constants (A,B,C). Distances in Å , angles in degrees, rotational constants in MHz. 6 ACS Paragon Plus Environment

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B3LYPa

C1-I C2-F2 C3-F3 C4-F4 C1-C2 C2-C3 C3-C4 C2C1C6 C1C2C3 C2C3C4 IC1C2 F2C2C3 F3C3C4 F4C4C3 A B C

CCSD(T*)-F12bb

req

ro

∆Vib

req

ro

2.0974 1.3334 1.3327 1.3302 1.3893 1.3880 1.3879 118.56 121.01 119.76 120.72 118.18 119.88 120.04 1026.444 365.742 269.658

2.1009 1.3362 1.3352 1.3326 1.3932 1.3919 1.3917 118.53 121.03 119.75 120.74 118.16 119.89 120.05 1021.416 364.196 268.466

0.0035 0.0029 0.0025 0.0024 0.0038 0.0039 0.0038 0.03 0.03 0.01 0.02 0.01 0.01 0.00 −5.028 −1.546 −1.192

2.0765 1.3302 1.3290 1.3265 1.3944 1.3913 1.3916 118.19 121.25 119.70 120.91 118.13 119.87 120.04 1026.131 367.523 270.603

2.0800 1.3331 1.3315 1.3289 1.3983 1.3952 1.3954 118.15 121.27 119.69 120.92 118.11 119.87 120.04 1021.103 365.977 269.411

rαc

1.334 1.347 1.330 1.389 1.395 1.385 118.7 120.1 121.0

120.2

a

Computations at B3LYP/cc-pVTZ(-PP) level.

b

Equilibrium structures from CCSD(T*)-F12b/cc-pVDZ-F12(-PP) computations along with vibrational corrections at B3LYP/cc-pVTZ(-PP) level.

c

Experimental rα structure derived from 13C satellites in the 19F NMR spectrum from Ref. 3.

3.2 Vibrational spectra overview We present a detailed analysis of the FIR, MIR and Raman spectra. This analysis, summarized in Table 2, makes major use of the new GVPT2 and CC/DFT calculations; these include predicted intensities for all fundamentals, overtones and combination bands where up to 2 quanta are involved. It will be seen that this limitation does not account for all the low-frequency bands observed. Further, although we can readily derive difference bands from the fundamentals, we are not able to determine their intensities. The vibrations reported in Table 2 7 ACS Paragon Plus Environment


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have high intensity in either IR and/or Raman spectra. A further important aspect which assists the assignments, is the ratio of the IR and Raman intensities (IIR and IRA); most bands have contributions from both types of spectra. The wide range of calculated intensities makes it essential to expand some regions of the spectra, as shown in Figures 2 to 4. The absence of Hatoms in C6F5I leads to all fundamentals lying in a narrow range below 1700 cm

−1

with 14 below

−1

500 cm . A comparison of the calculated harmonic frequencies obtained by the two procedures, CCSD(T*)-F12b and B3LYP, is shown in Table 3 alongside the principal atomic motion in the vibrations. The two sequences by symmetry are very similar, and the CCSD(T*)-F12b and B3LYP values are generally in good agreement, with the mean absolute difference 10 cm-1; a single difference of 51 cm-1 occurs for mode 15 (b1).

Overall the statistical adjacent R2

correlation is 0.999, and this similarity justifies use of the B3LYP approach for the evaluation of anharmonic corrections. The new FIR, MIR and Raman experimental spectra are compared with theoretical spectra line-shapes next. An additional list of calculated and assigned transitions from this work is reported in the Supplemental Material.

Table 2: Comparison of the experimental vibrational frequencies with computed −1

anharmonic frequencies and intensities for the fundamentals, (ν in cm ). A selected list of overtones and combination bands is also shown where selection is based on a high calculated intensity for either IR or RA or both. The units for infrared intensities (IIR) are km/mol and 4 −1

Raman intensities (IRA) are Å u . A further selection of weak transitions is shown in the Supplemental Material. Experimental spectroscopy FIR MIR Raman Freq. Intensity Freq. Intensity Freq. Intensity

73 132 173

0.544 0.432 0.445

----

----

----

----

Theoretical spectroscopy Anharmonic Intensity frequency IR RA Assignment v20 98 0.018 0.069 v30 119 0.237 0.807 v19 170 0.050 0.001

Symmetry b1 b2 b1

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204 214 264 282 309 350 356 386 -492 -592 --------------------------

1.227 1.140 -0.497 0.735 0.778 0.610 0.899 -0.808 -1.370 --------------------------

---

---

---

---

------491 --611 -714 748

------0.143 --0.343 -0.237

---354 386 442 492 520 584 611 699 ---

---3379 2594 2209 8435 494 6621 494 164 ---

806 976 1001 -1080 1148 --

3.005 3.099 3.261 -3.237 0.641 --

-1364 1407 -1470 1482 1508 1513 1573 1591 1606 1635 --

-0.827 0.389 -2.141 3.356 3.112 -0.441 0.440 0.386 1.120 --

806 --1085 -1149 1220 1268 1301 -1408 ----1513 ---1635 2539

416 --278 -297 106 117 91 -890 ----159 ---1526 139

211 211 271 279 306 344 357 381 437 490 517 582 614 690 737 747 764 810 988 993 1086 1090 1151 1229 1278 1290 1355 1410 1410 1478 1493 1507 1510 1581 1589 1608 1629 2546

1.063 2.139 0.044 0.100 0.991 1.386 0.450 0.000 0.021 0.593 0.142 0.000 2.204 0.069 0.282 0.038 0.368 80.481 158.790 20.026 28.412 59.522 2.595 0.227 0.139 0.007 4.275 1.111 4.473 16.444 323.626 14.024 219.485 5.386 2.089 1.948 4.368 0.386

3.069 0.179 0.044 0.400 0.015 1.674 2.344 2.987 3.788 9.463 2.063 13.558 0.749 0.845 0.025 0.136 0.918 2.068 0.430 0.032 0.350 0.685 1.059 1.448 2.335 0.388 0.009 10.062 0.099 0.009 0.105 0.000 0.844 0.079 0.027 3.082 23.982 8.852

v11 v18 v29 v10 v28 v17 v9 v13 v27 v8 v17v19 v7 v16 2v17 v15

a1 b1 b2 a1 b2 b1 a1 a2 b2 a1 b1+b1 a1 b1 2b1 b1

ν26

b2

2v13 v6 v25 v16+v13 v6+v10 v5 v24 2v16 v23 v4 v15+v16 v3 v15+v12 v25+v8 v22 v24+v9 v2 v5+v8 v24+v27 v21 v1 2v23

2a2 a1 b2 b1+a2 a1+a1 a1 b2 2b1 b2 a1 b1+b1 a1 b1+a2 b2+a1 b2 b2+a1 a1 a1+a1 b2+b2 b2 a1 2b2

3.3 The Raman spectrum Although this spectrum has a more limited overall spectral range, more bands are present in the range 350 to 600 cm-1 and the absence of some strong IR absorption is a simplification. There is clearly a close comparison between theory and experiment in Figure 2, within the whole range from 300 to 2800 cm-1. This provides immediate support for our spectral assignment derived from 9 ACS Paragon Plus Environment


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the calculated states. Eight fundamentals missing in the present Raman spectrum are due to the lower limit of 300 cm-1, but these are observed in the FIR spectrum as discussed below. In the low frequency range, the 354 cm-1 peak is found to be a doublet from the calculations. The present assignment of this band consists of contributions from two fundmental transitions υ9 (a1) and υ17 (b1). The former is more intense in the Raman spectrum with the reverse in the infrared; this is discussed further below. This is an accidental near degeneracy, since the two vibrations have completely differing atomic motions (Table 3); υ9 is a C-F in-plane bending motion, while υ17 is a C-I out-of-plane deformation mode. The two isolated bands at 386 (a2) and 442 (b2) cm-1 are identified as υ13 and υ27 respectively, with the former absent from the IR spectrum. The two bands at 492 and 584 cm-1 are the most intense in the Raman spectrum, but the calculated intensities are reversed relative to experiment. These are both a1 modes, where the higher wavenumber one υ7 (computed at 582 cm-1) has the larger calculated Raman cross-section when compared with υ8 (computed at 490 cm-1); the reverse is observed. Both have very weak bands on their high wavenumber sides, both in the calculations and spectra. The υ7 is accompanied by the fundamental υ16 (observed at 611 cm-1, computed at 614 cm-1) which is weaker than the υ8 sattelite near 520 cm1

which is a combination band ν17 + ν19. These are followed to higher wavenumber by further weak

combination bands up to 806 cm-1, and a further fundamental (υ6). Above 1000 cm-1, the calculated and observed Raman spectra show a very close correspondence. The isolated relatively strong peak at 2539 cm-1, is the overtone 2ν23.

Table 3: Comparison of the calculated harmonic frequencies (in cm−1) from the CCSD(T)F12b and B3LYP methods with the principal atomic motion Mode

Symmetry

CCSD(T*)F12b

1 2

a1 a1

a

1661 1544

b

B3LYP 1650 1529

Assignmentc ring def. C-F m/p sym str. 10


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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

a1 a1 a1 a1 a1 a1 a1 a1 a1 a2 a2 a2 b1 b1 b1 b1 b1 b1 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2

1440 1311 1104 818 586 494 357 279 208 647 378 132 668 604 342 206 165 73 1654 1525 1293 1171 1001 751 439 303 268 112

1426 1298 1098 808 587 497 357 282 204 666 388 132 719 632 352 211 166 80 1649 1515 1304 1163 998 756 447 311 277 130

C-F o sym str. Ring-ip def. ring def. + C-X str. C-X str. + ring def. Ring-only ip def. Ring-only ip breathing CF- ip bend C—F(o/m) ip bend C6F5-X str. Ring-only oop def. Ring-only oop def. C—F(o/m) oop bend I-ring oop def. I-ring oop def. I-ring oop def. C6F5-X oop bend C—F(m/p) oop bend ring-I oop bend Co-Cm sym str. C-F o/m asym str. Ring-ip def. ring in-plane def. C-F str C-F- ring C-F bend Ring-only ip def. C—F(all) ip bend C—F(m/p) ip bend ring-I ip bend

a

Computations at CCSD(T*)-F12b/cc-pVDZ-F12(-PP) level.

b

Computations at B3LYP/cc-pVTZ(-PP) level.

c

Abbreviations: out-of-plane: oop; in-plane: ip; deformation: def.



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Figure 2. The Raman spectrum separated into two wavenumber ranges (in black), compared with the present calculated values (in red). The experimental spectrum in blue is a 10 times expansion, which allows additional weak structure to be observed.


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3.4 The FIR and MIR spectra. 3.4.1 The overall correlation of theoretical band positions with the experimental spectra. The assignments, shown in Figs. 3 and 4, are the results of fully anharmonic GVPT2 computations at the CCSD(T*)-F12b/B3LYP level. These include all fundamentals, overtones and combination bands arising from 1 or 2 quanta. The intensities shown in blue have all been increased by a factor of nine to show the positions of several low-intensity bands, which include several fundamentals. The stick diagram theoretical spectral line-shapes (red) have been convoluted by means of Lorentzian distribution functions with a half-width at half-maximum (HWHM) of 2 or 5 cm-1 depending on the range, giving the continuous curves shown, also in red. The close correspondence of the most intense bands to the calculated anharmonic frequencies, shown in Table 2, is indicated in these Figures. The large number of overtones (O) and combination (C) bands lead to a rich and complex spectrum. A survey of the FIR + MIR regions of the spectrum in Figs. 3 and 4, shows remarkable levels of agreement both in line positions and their relative intensities over the whole range from around 100 to 3000 cm-1. The lowest frequency is computed poorly, and it appears that there are probably insufficient other vibrational states for the FIR region to account for all the observed peaks. Although difference frequencies can be derived from the present fundamentals, the intensities for these bands are not yet implemented; we neglect further discussion of difference bands in this light. However, using the available data, most peaks in the FIR + MIR spectra can be interpreted in terms of fundamentals, overtones and combination bands, even with the limitation of a total of two quanta excited (due to the version of software available to us). A further selection of the O + C bands are shown in the Supplemental Material. The most intense bands in the IR spectrum lie near 1500 cm-1, and are assigned to υ2 and υ22, with the latter the more intense. The wide range of the IR absorption intensity makes several of the fundamentals barely visible; weak bands, v19, ν15 and ν4 are shown in the supplementary material. 13 ACS Paragon Plus Environment


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A multiplet near 170 cm-1 is attributed to v19; similarly, ν15 is assigned to the lower value of a very weak doublet at 714 and 719 cm-1. The position of ν4, is found in the Raman spectrum at 1301 cm1

.

Although the present band origin measurements are in excellent agreement with the previously recorded fundamentals,4,5,6 significant differences do occur for ν20 (reduced by (-)41), ν26 (34), υ23(12), υ8 (11) and υ16 (10 cm-1). Some bands have been re-assigned with ν20 < ν30 < ν14 < ν19.


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Figure 3: Experimental low frequency FIR spectral region of C6F5I. Assignments are superimposed. A linear ramp has been subtracted from the experimental data, to produce a nearly horizontal base-line. The theoretical positions of υ18 and υ11 are nearly degenerate, with intensities close to 1: 2 respectively. The ‘stick’ diagrams (red) of the theoretical spectrum have intensities increased by a factor of nine, to show where some very weak bands, including combination bands are located. The theoretical spectral line-shapes have been convoluted by means of Lorentzian distribution functions with half-width at half-maximum (HWHM) of 5 cm−1.



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Figure 4: Experimental MID-IR spectral region of C6F5I. Assignments are super-imposed. The experimental spectrum intensity has been increased by 30 times (blue), to show extra weak structure. The ‘stick’ diagrams (red) of the theoretical spectrum show some very weak combination bands are located. The theoretical spectral line-shapes have been convoluted by means of Lorentzian distribution functions with half-width at half-maximum (HWHM) of 5 cm−1.


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3.4.2. The detailed assignment of the FIR, MIR and Raman spectra of C6F5I. Starting from the lowest energy transitions, ν20 has been assigned to the new peak observed −1

−1

below 100 cm . We assign the band at 132 cm to ν30 (computed at 119 cm

−1

) rather than to ν

14

as suggested previously, because of the negligible calculated IR intensity of the latter; ν14 is calculated at 132 cm−1 and was not detected previously. The previously unassigned band at 173 cm−1 is ν19. The band at 214 cm−1 has previously been attributed to both ν18 and ν29; we assign it to the former in view of its much larger calculated IR intensity and closer position. The weak transition observed at 264 cm−1 is assigned to ν29. The peak at 611 cm−1 previously assigned as ν15 has been reassigned to ν16, which shows a larger calculated IR intensity and closer match −1

with the calculated band position. The peak at 696 cm , (Table S2) previously attributed to an overtone, can now be assigned as the ν8 + ν11 combination band, which shows significant calculated IR intensity. The band at 714 cm−1 has been reassigned to ν15, which is predicted to be more −1

intense than the ν26 fundamental; the latter is attributed to shoulder at 748 cm . The very strong peak near 1000 cm−1, shows two maxima. We assign the 976 cm

−1

band as ν25, while the

broadened base may arise from either of the predicted combination bands, υ16+υ13 and/or υ6+υ10, both of which have high IR intensity. The 1005 cm-1 peak (Table S2) is reassigned to the ν12 + ν17 transition instead of the ν10 + ν26 combination. Peak broadening is observed for several intense bands, and this can be attributed to the nearby non-fundamental transitions. An example is the band at 1080 cm-1 assigned to the ν5; it is broadened by intensity from the strong ν6 + ν10 combination band (computed at 1086 cm-1). Similarly, the broad band at 1450-1550 cm-1, dominated by ν2 and ν22 fundamentals, is accompanied by several non-fundamental transitions; intensity from these is necessary to match well the experimental spectrum line shape. Finally, we have reassigned the bands at 1606 and 1635


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cm-1 to the ν21 and ν1 fundamentals. The band at 1730 cm-1 (Table S2) is assigned to the υ7 + υ24 combination.

4. DISCUSSION The availability of the relative intensities, and the generally excellent fit with the experimental profile, is the most critical aspect of the current work. Correct positioning of the vibrational states in the IR and Raman spectra, whilst important, is merely the first stage. It must be accompanied by acceptable relative intensity ratios for reliable science. It seems true that acceptable frequencies are obtained first as the methodology improves, whereas intensities are more demanding. Long and Steele4 predicted the in-plane frequencies for C6F5I from a force field analysis based studies of C6H6 and C6F6, but the C-I stretching and C-C-I bending force constants were assumed. Their framework coordinate choice interchanges b1 with b2 vibrations, and are converted to our system in the current work. Subsequently,5 their experimental IR spectra, which used the conventional intensity classes: weak (w), medium (m) and strong (s) with further variations ‘very (v), and very-very (vv)’ led to 10 assignments. After allowing for our more precise measurements, we consider that their vibrations ν3, ν5, ν6, ν10, ν24 and ν25 were correctly identified. A contemporary study by Hyams et al,6 led to 22 proposed assignments for fundamentals. After allowances of up to 37 cm-1 (with a mean difference of 19 cm-1) in the positions, many are basically correct. .However, we do not believe that one of the claimed a2 modes (14) has been identified; both studies do not find ν12 either. Two overtones at 696 and 764 cm-1 were assigned by Hyams et al. to 2ν26 (b2) and 2ν10 (a1) respectively. We believe that a more acceptable assignment is 2ν17 (2b1 calculated at 690) and 2ν13 (2a2 at 764 cm-1); 2ν10 (a1) lies near 559 cm-1. None of the set of 13 combination bands assigned by Hyams et al,6 has sufficient intensity to be 21 ACS Paragon Plus Environment


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observed in the spectra, and alternative assignments with bands showing stronger intensities are shown in Table 2. One example is the absorption reported6 at 750 cm-1, attributed to 438+311 (2 x a1); the only combination bands within +/- 10 cm-1 from this wavenumber value are modes 27+28 (2b2, at 742 cm-1, 19+7 (b1 + a1 at 753 cm-1) and 16+14 (b1+a2 at 757cm-1) all showing IR intensity below 0.01 km/mol.

5. CONCLUSIONS The previous studies4,5,6 of the IR and Raman spectra of C6F5I, are typical of their period (1960s); several fundamentals were correctly identified, but not all by any criterion. The weakness of assignments based on correlations between molecules where none are confidently assigned, is that false correlations will occur. A feature of the current study, is determination of the intensities of all contributing vibrational states under 2 quanta conditions. The present detailed analysis of over 100 fundamentals, overtones and combinations (nonfundamental), where both intensity and wavenumber position are considered, has allowed us to test the agreement between theory and experiment.

The visual comparison of success of our

simulation, even correlating with the weak portions of the spectral envelopes, is spectacular. These allow a detailed analysis of the Raman spectrum with the corresponding similarly anharmonic calculations. In the infrared energy window up to 3200 cm-1 over 100 transitions have been detected spectroscopically. Assuming the correlation is real, the mean absolute error between these procedures is 7 cm-1. We believe that this methodology is rigorous, within the 2 quanta caveat. The close correlation obtained between our CCSD(T*)-F12b and B3LYP hybrid approach (CC/DFT), clearly validates the determination of the anharmonic corrections at the B3LYP level, as already demonstrated also by the comparison with those computed previously at the CCSD(T) level.12,36 22 ACS Paragon Plus Environment

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We believe these methods are of general applicability certainly for semi-rigid molecules up to approximately 20 atoms and perhaps with some symmetry such as C2V.

We believe that

reinvestigation of many of the IR and Raman spectra from the second half of the 20th Century, would be well repaid, and more importantly would allow generalizations to be made with confidence.

SUPPORTING INFORMATION DESCRIPTION Tables S1 and S2, which include a more detailed comparison between theory and experiment for many anharmonic frequencies and intensities. Also included is Figures S1, which depict the environments of some of the very weak bands in the FIR and MIR spectra.

ACKNOWLEDGMENTS We thank the following for their support: National Natural Science Foundation of China (Grant No. 91641128). The support of the COST CMTS-Action CM1405 (MOLIM: MOLecules In Motion). The Edinburgh Parallel Computing Centre for provision of super-computing facilities. M.B. thanks Dr. Julien Bloino for fruitful discussions.



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