Dipole Surface and Infrared Intensities for the - American Chemical

Article pubs.acs.org/JPCA

Dipole Surface and Infrared Intensities for the cis- and trans-HOCO and DOCO Radicals Xinchuan Huang,*,† Ryan C. Fortenberry,‡,⊥ Yimin Wang,¶ Joseph S. Francisco,§ T. Daniel Crawford,⊥ Joel M. Bowman,¶ and Timothy J. Lee‡ †

SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, California 94043, United States NASA Ames Research Center, Moffett Field, California 94035-1000, United States ¶ Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States § Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ⊥ Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States ‡

ABSTRACT: The vibrational spectra for the HOCO radical in both of its conformers and deuterated isotopologues are shown here for the first time. Building on previous work with coupled cluster quartic force fields (QFFs) in the computation of the fundamental vibrational frequencies for both cis- and trans-HOCO, coupled cluster dipole surfaces are now provided for both HOCO conformers and their corresponding deuterated isotopologues. These surfaces and subsequent vibrational configuration interaction (VCI) computations produce the intensities of transitions into vibrational states including the fundamentals, overtones, and first few combination bands of less than 4000 cm−1, slightly beyond the O−H stretch. Simulated spectra with an artificial full width at halfmaximum broadening of 10 cm−1 are also provided in order to aid in the characterization of HOCO’s vibrational frequencies and to assist detection in various laboratory or astronomical observations.

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INTRODUCTION Determination of the gas-phase vibrational spectrum of the HOCO radical has not been fully realized either experimentally or theoretically. The gas-phase O−H and CO stretches for the trans-HOCO conformer have been observed in the laboratory at 3635.702 (ref 1) and 1852.567 cm−1 (ref 2). Alternatively, all modes for both conformers except for the cis torsion have been assigned in the condensed phase in Ne, Ar, or CO matrixes,3−6 but actual gas-phase data have been difficult to determine conclusively. HOCO has been established as a necessary intermediate in the reaction of OH + CO,6−10 and recent computational studies have put tremendous effort into providing accurate descriptions of the OH + CO potential surface along with the fundamental vibrational frequencies for both HOCO conformers.11−15 Accuracy in the computation of the two experimentally known fundamental vibrational frequencies is often better than 4 cm−1 and occasionally coincident with experiment to around 1 cm−1 for the O−H stretch, especially. However, none of these previous studies have been able to provide the associated gas-phase anharmonic intensities for any of the modes. Whereas quartic force fields (QFFs) built around a relative few points close to the equilibrium geometry16 have given dependable results for the frequencies,17−19 the number of © 2012 American Chemical Society

points required for an accurate dipole surface has not been limited by similar means. Hence, larger and even global sufaces are often required for accurate reproductions of infrared intensities.20 The ability to compute anharmonic infrared intensities has been developed and applied to various problems over the past couple of decades.21−26 The MULTIMODE suite of computational programs27−30 is one tool that has been developed to make efficient use of the dipole surface in its programmatic functionality. This allows vibrational configuration interaction (VCI) to be extended within MULTIMODE to the computation of anharmonic intensities31−35 including other work on the HOCO systems.36 The study of the HOCO radical would greatly benefit from a listing of reliable anharmonic intensities corresponding to the fundamental vibrational frequencies, as well as some of the lower-frequency overtones and combination bands. HOCO’s role in the reaction of OH + CO and its significance in atmospheric studies of the carbon monoxide and carbon dioxide cycles both of the Earth and Mars37,38 would be better Special Issue: Joel M. Bowman Festschrift Received: October 16, 2012 Revised: November 27, 2012 Published: November 30, 2012 6932

dx.doi.org/10.1021/jp3102546 | J. Phys. Chem. A 2013, 117, 6932−6939

The Journal of Physical Chemistry A

Article

polynomial bases just as when fitting a potential. However, the polynomial bases used here must be covariant under the permuatation of two oxygen atoms, that is, effective charges of two oxygen atoms should interchange as their coordinates permute. The fitting basis is made up of permutationaly covariant polynomials of Morse-like functions, yij = exp(−rij/λ), where λ is fixed at 2.0 Bohr and rij is the internuclear distance of atoms i and j. By using such a fitting basis for the effective charges, the magnitude and orientation relative to the principal axis of the final dipole vector is invariant to any rotations or translations of the molecule. By solving the VCI equations28 and compiling the dipole surfaces, the Rαab dipole integrals can be solved

informed by this reference data. Additionally, the cation form is known to exist in the interstellar medium,39−45 and it is reasonable to expect that either of the radical conformers may as well. With the growth of infrared telescopic capabilities as evidenced by missions such as the Stratospheric Observatory for Infrared Astronomy (SOFIA), the Herschel Space Observatory, and the upcoming James Webb Space Telescope (JWST), highly accurate reference data for known or potential interstellar molecules or those that may be key to understanding the atmospheres of extrasolar planets is imperative. This problem goes beyond simply knowing the vibrational transition frequencies but also knowing their intensities. Laboratory-based reference data are not readily available due to the complexities of the HOCO system itself, but theoretical computations can give comparable data such that either laboratory or astronomical observations may be better informed.

R αab =

THEORY The quartic potential is defined from force constants Fij... and displacements Δi to be 1 2

∑ FijΔiΔj +

+

1 24

ij

1 6

∑ FikjΔiΔjΔk ijk

A(νa → b) =

∑ FikjlΔiΔjΔk Δl (1)

ijkl

∑ Vi(1)(Q i) + ∑ Vij(2)(Q i , Q j) + ij

∑

... +

(N ) V ijk ... N (Q i , Q j , Q k , ..., Q N )

ijk ... N

■

(2)

COMPUTATIONAL DETAILS As a review of previous associated work, QFFs have been used in the prediction of the fundamental vibrational frequencies of the HOCO radical, anion, and cation.11,12,46,47 To briefly summarize, 743 symmetry-unique points describe the QFFs for each system where bond lengths were displaced by 0.005 Å and bond angles by 0.005 radians. A set of displacements up to a maximum of four, regardless of the combination, constitutes the QFF. At each of the displaced points on the surface, spinrestricted open-shell48−50 referenced unrestricted coupled cluster51,52 singles, doubles, and perturbative triples [CCSD(T)]53 aug-cc-pVXZ (where X = T, D, and 5)54,55 energies were computed and extrapolated to the complete one-particle basis set limit.56 Further corrections were made for corecorrelation,57 scalar relativity58 and higher-order electron correlation effects. Full use of these corrections gave the abbreviated CcCRE QFF. The geometries for the trans- and cisHOCO radicals were obtained from fitting of the CcCR QFF (refs 11 and 12) because the method for higher-order electron correlation effects that we used in the CcCRE QFF yielded relatively no additional correction to the energies. The dipole surface utilized in this work is defined from a set of seven nested QFFs for each conformer with increasingly larger displacements for the bond lengths, bond angles, and the torsional angle. This gives 5201 total points per conformer on

N is the number of modes for each level of the expansion. Truncations at the N = 4 and 5 levels, for instance, are the fourand five-mode representation (4MR and 5MR) computations where the full expansion for any HOCO system is 6MR. For a given symmetry, the V(1) i term is the one-mode potential for the ith mode, V(2) is the intrinsic two-mode potential for the ij corresponding ith and jth modes, and so on for larger mode terms up to V(N) ijk...N. The coordinates of a given normal mode i are represented by Qi. In the dipole moment surface (DMS), a three-dimensional vector, μ⃗ , is represented as a vector sum of effective charges 4

μ⃗ (X) =

∑ qi(X) ri ⃗ i=1

(5)

from ref 23. The transition energy in eq 5, E(νa→b), is in cm−1, Rαab is in Debye, and NAv is Avogadro’s Number.23 Because the transition from state a to state b may be composed of various combinations of normal modes in the mode representation coupling in the MULTIMODE definition of the potential, eq 2, it is necessary to distinguish anharmonic vibrational states (a) from normal modes (i) here. Various normal modes may contribute to a given transition whether for its frequencies or for its intensities.

V (Q 1 , Q 2 , ..., Q N ) i

8π 3NAv E(νa → b) ∑ |R αab|2 3hc(4πε0) α=x ,y,z

(in km/mol)

However, this is expanded once more in the MULTIMODE formulation of VCI. The potential is decoupled into hierarchical order of mode coupling terms28

=

(4)

where α denotes a cardinal direction x, y, or z. Ψa(Q) represents the VCI wave function associated with vibrational state a, and Q are the six mass-scaled, rectilinear normal modes of the trans or cis configuration in the principle axis frame. Therefore, the band infrared intensity of a given transition, νa→b, is

■

V=

∫ Ψa(Q)μα (Q)Ψb(Q) dQ

(3)

where X denotes all of the internuclear distances in the molecule; qi(X), which depends on the molecular configuration (X), is the effective charge on the location of ith atom; and

⎛ xi ⎞ ⎜ ⎟ ri ⃗ = ⎜ yi ⎟ ⎜ ⎟ ⎝ zi ⎠ contains the X,Y,Z coordinates of the ith atom. Each of the four effective charges are represented as a linear combination of 6933



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Table 1. Theoretical Predictions of Fundamental Vibrational Frequencies (in cm−1) for cis- and trans-HOCO ν1 ν2 ν3 ν4 ν5 ν6 ν1 ν2 ν3 ν4 ν5 ν6

trans-HOCO

cis-HOCO

mode

this worka

5MR-lb

DVR(6)c

VPTd

exp.e

O1−H stretch CO2 stretch H−O1−C bend C−O1 stretch O1−C−O2 bend torsion O1−H stretch CO2 stretch H−O1−C bend C−O1 stretch O1−C−O2 bend torsion

3641.0 1862.0 1212.7 1052.0 616.0 475.4 3452.3 1824.1 1280.2 1042.4 601.2 540.2

3641.0 1862.0 1212.7 1052.0 616.0 475.4 3452.3 1824.1 1280.2 1042.4 601.2 540.2

3642.2 1861.0 1217.2 1052.7 617.2 499.9 3450.4 1824.3 1282.1 1045.6 601.3 562.9

3641 1854 1217 1057 614 507 3458 1815 1282 1042 596 545

3635.702 1852.567

a

CcCR 6MR results. bCorrected (see text) 5MR-l CcCR QFF results reported here, which differ from those in ref 11 for trans, but the cis results are purely from ref 12. cCcCR full-dimensional DVR(6) vibrational energies from ref 15. dVPT2 results from CCSD(T)/ANO1 and ANO2 force fields in ref 13. eThe trans-HOCO ν1 gas-phase frequency was observed by Petty and Moore (ref 1), while ν2 was observed by Sears, Fawzy, and Johnson (ref 2).

Table 2. MULTIMODE 4MR Intensities (in km/mol) with 6MR Frequencies (in cm−1) for the Fundamental Frequencies and First Few Overtones and Combination Bands for trans- and cis-HOCO and DOCO trans-HOCO mode 2ν2 ν1 ν3 + ν4 + ν5 + 2ν3 ν4 + 2ν4 ν2 ν5 + ν5 + 2ν5 ν3 ν4 2ν6 ν5 ν6

ν2 ν2 ν2 ν3

ν3 ν4

cm

−1

3700.6 3641.0 3068.4 2894.8 2472.7 2402.8 2255.2 2089.0 1862.0 1826.2 1659.7 1232.1 1212.7 1052.0 943.6 616.0 475.4

intensity 3.87 80.37 0.10 1.01 0.37 1.74 3.61 15.86 203.56 1.52 2.41 7.94 221.62 79.88 8.17 5.78 84.49

trans-DOCO cm

−1

3695.8 2685.1 2931.1 2757.8 2445.2 2152.3 1977.6 1795.2 1859.8 1668.6 1489.6 1179.6 1086.4 902.6 710.2 590.1 368.0

cis-HOCO

intensity 4.12 47.97 0.77 0.16 0.35 13.11 12.58 8.74 214.61 1.57 1.03 0.40 229.91 10.68 2.10 7.73 34.16

the dipole surface (743 × 7), where 868 of these are nonplanar. The QFFs with the smallest displacements are the standard 0.005 Å and 0.005 radians displacements. From these, six more QFFs are created where the smaller displacements are 61.8% smaller than their next-largest counterparts; there do exist some rounding errors in the actual displacement values. This scale is so chosen to balance the dipole surface between focusing on points at lower energies while containing enough higher-energy points. The exceptions to this rule are the QFFs with the firstand second-smallest displacements. The difference between the displacements in these QFFs and the next-largest is often larger than the standard ∼61.8%. These nonuniform increases in the displacements are adopted to produce a more continuous surface between the original QFF (the QFF used to compute energies for the fundamental frequencies, which is the first QFF in the succession) and the other, larger ones. The maximum bond lengths used in the largest QFF are nearly 50% longer and shorter than the CcCR equilibrium bond length (because displacements are both positive and negative). The maximum bond angles used in the largest QFF allow the angles to just

cm

−1

3623.9 3452.3 3097.6 2854.1 2418.3 2549.4 2307.6 2073.9 1824.1 1881.9 1640.5 1203.5 1280.2 1042.4 1059.7 601.2 540.2

cis-DOCO

intensity 3.10 14.96 0.38 2.84 0.39 1.17 1.31 15.47 276.83 12.45 2.28 0.62 0.89 151.94 37.68 28.40 102.41

cm

−1

3629.0 2551.6 2939.1 2778.8 2364.3 2231.7 2073.4 1910.0 1827.5 1655.4 1496.1 1081.1 1123.1 960.9 865.0 539.8 446.9

intensity 3.43 14.75 1.41 1.22 0.47 4.44 9.37 6.83 177.70 4.36 0.71 0.87 84.40 75.65 0.68 30.38 50.52

approach π. The maximum torsion used in the largest QFF is nearly π/2. At each point on the total dipole surface, CCSD/ aug-cc-pVTZ dipole moments are computed from analytic derivatives of the electronic wave functions within each of the Cartesian cardinal directions and done so with the PSI3 suite of programs.59 In trans-HOCO, for instance, the C−O1 individual displacements are 0.005, 0.013, 0.023, 0.038, 0.063, 0.103, and 0.167 Å for each QFF. For this coordinate in the largest QFF, the discplacement step of 0.167 gives a maximum bond length variation (when i = j = k = l to give 4Δi from eq 1) of ±0.668 Å. This is slightly less than half of the 1.348 50 Å C−O1 bond length given in ref 11. The O1−H bond displacements are 0.005, 0.010, 0.017, 0.028, 0.046, 0.074, and 0.120 Å, where 0.480 Å (four times the largest displacment) is slightly more than half of the 0.957 73 Å O−H bond. The CO2 displacements are 0.005, 0.011, 0.021, 0.035, 0.056, 0.091, and 0.147 Å. The O1−C−O2 bond angle is displaced by 0.005, 0.011, 0.032, 0.052, 0.085, 0.137, and 0.222 radians. The angle at 0.888 radians, the largest possible angle variation, is 50.9°. 6934



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Figure 1. Simulated spectrum for trans-HOCO.

When combined with the 126.949° reference O1−C−O2 bond angle, the maximum angle utilized is 177.8°. The displacements for the H−O1−C bond angle are 0.005, 0.025, 0.045, 0.073, 0.119, 0.193, and 0.313 radians and behave similarly. The dihedral angle displacements are 0.005, 0.035, 0.050, 0.090, 0.147, 0.238, and 0.358 radians, where the largest displacement is 22.1°, giving a maximum torsional angle of 88.4°. In cisHOCO the C−O1 displacements for the seven QFFs are 0.005, 0.013, 0.023, 0.038, 0.063, 0.102, and 0.165 Å. The O1−H displacements are 0.005, 0.010, 0.017, 0.028, 0.046, 0.075, and 0.121 Å. The CO2 displacements are 0.005, 0.011, 0.021, 0.034, 0.056, 0.091, and 0.147 Å. The O1−C−O2 bond angle is displaced by 0.005, 0.011, 0.030, 0.050, 0.082, 0.133, and 0.216 radians, while H−O1−C has displacements of 0.005, 0.025, 0.045, 0.073, 0.119, 0.192, and 0.311 radians. The torsional displacements with respect to the minimum values for cisHOCO are the same as those for the trans conformer. In the present dipole moment fit, 6181 coefficients of covariant polynomials of a maximum degree 8 are determined in a least-squares fashion. This is under the constraint that the total charge is zero, and the fitting reproduces the ROHFCCSD/aug-cc-pVTZ dipole moments computed for 10 402 configurations at both the trans and cis minima. More details on the covariant fitting of the DMS are given elsewhere.33,60 The RMS fitting error of the dipole moment is 1.3 × 10−4 au for the 6276 configurations whose potential energies are less than 2195 cm−1 (0.01 au) above the trans global minimum. The RMS error is 2.7 × 10−4 au for the 1198 configurations in the 2195−4390 cm−1 energy range above the trans minimum and is 2.1 × 10−3 au for the 1485 configurations in the 4390− 10 973 cm−1 range. The RMS fitting error of the dipole moment surface is 0.03 au when considering the entire data set. The dipole moment surface is available to the community upon request to the authors.

reported previously for the CcCR QFF. Comparison with previous predictions of the fundamental vibrational frequencies is also given in Table 1 along with the two experimentally known gas-phase frequencies of trans-HOCO. VCI computations require transformation of the coordinates into MorseCosine coordinates in order to have proper limiting behavior.16 The VCI results reported here for the trans conformer differ from those reported in ref 11. A slightly perturbed minimum geometry was used in the previous VCI computations, resulting in some impurities of the bases. The ν1 and ν6 modes are those most affected. This has been corrected, and the proper 5MR-l VCI results are listed in Table 1. The VPT frequencies reported in ref 11 are unaffected by this. As a result of this correction, not only do the proper CcCR 5MR-l results for trans-HOCO agree with the 6MR computations reported here to better than 0.1 cm−1, but the CcCR 5MR results for cis-HOCO from ref 12 do as well. The DVR(6) method is coded to robustly treat any arrangement of four atoms and, concurrently, six degrees-offreedom. Theoretically, it should give more trustworthy results for the systems that it can treat as compared to 4MR/5MR results with MULTIMODE. As listed in ref 15 and in Table 1, the CcCR VCI ν6 mode differs from that reported by DVR(6) by more than 20 cm−1, while the VPT results (which are not affected by the minor defect mentioned previously for VCI) reported in in ref 11 agree well with DVR(6). The discrepancy between MULTIMODE/VCI and DVR(6)/VPT ν6 fundamentals could result from the fact that the version of MULTIMODE adopted in this study is single-referencebased, not reaction/torsional-path-based. The normal coordinate defined at the trans and cis minima is rectlinear and not suitable to accurately describe the full torsional path connecting the trans and cis configurations. However, the purpose of this study is to provide reliable, semiquantitative anharmonic intensities. The VCI intensities reported here should not be greatly affected by the inexact torsional treatment present in the MULTIMODE VCI computations. Infrared Intensities and Simulated Spectra. In Table 2, 6MR fundamentals, overtones, and first combination bands for cis- and trans-HOCO of less than 4000 cm−1 are reported along with the affiliated 4MR intensities. The intensities exhibit better than 95% convergence in the mode representation space by

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RESULTS AND DISCUSSION Vibrational Frequencies. The fundamental vibrational frequencies, including new 6MR results, for both cis- and transHOCO are given in Table 1. Furthermore, Table 2 contains the first few combination bands and overtones for both forms of HOCO. These overtones and combination bands as well as all of the deuterated isotopologue frequencies have not been 6935



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Figure 2. Simulated spectrum for trans-DOCO.

Figure 3. Simulated spectrum for cis-HOCO.

comparing 3MR and 4MR. Furthermore, the reliability of the computed intensities is confirmed in the test of the basis set convergence and in the DMS fitting tests done with different fitting orders and expanded geometry sets. Additionally, 4MR is the highest level currently coded in MULTIMODE for the computation of infrared intensities. The intensities are computed for the J = 0 energy level. The frequencies and intensities for the deuterated isotopologues are also reported. The frequencies in Table 2 are ordered based on the descending values of trans-HOCO, which is why some are out of order for the other conformers/isotopologues. Our independent MP2/aug-cc-pVTZ double-harmonic intensities share similarities with the anharmonic ones that we are reporting in Table 2. The O−H stretch is the major exception where the harmonic frequencies are 60% larger for the trans isomer and more than 100% larger for the cis, showcasing a viable need for anharmonic corrections to the intensities as well as the frequencies. The dipole moment for the equilibrium geometry of trans-HOCO is 2.886 D, and it is 1.855 D for cisHOCO. Because the electronic structure computations are made within the Born−Oppenheimer approximation, the

dipole moments and reference geometries of the deuterated species are the same as their HOCO counterparts. For all of the systems studied here, trans-HOCO and DOCO as well as cis-HOCO and DOCO, the same rudimentary structure is present in the vibrational spectra. Figures 1−4 are visual simulations of the vibrational spectra for these species. The spectra are plotted with an artificial full width at halfmaximum (fwhm) broadening of 10 cm−1 to complement current high-resolution infrared experimental spectroscopic techniques. Additionally, the idealized stick spectra are also provided as lines underneath each of the spectral curves in order to give a better understanding as to the shape of the curves and for comparison with ultrahigh resolution spectra. For each of the spectra, there are four major peaks. The ν1 O1− H stretch is set far to the blue of the other three peaks. The middle two peaks have the largest amplitudes, and the higherfrequency one corresponds to the ν2 CO2 stretch (see below). The mode that causes the other peak varies between isomers but, interestingly, still gives the same qualitative structure. The fourth major peak observed in these HOCO radical vibrational spectra results from the torsional motion in 6936



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Figure 4. Simulated spectrum for cis-DOCO.

ν6, with some broadening resulting from the ν5 O1−C−O2 bend. For trans-HOCO and DOCO, the fundamental peaks are, in fact, the most intense. From Table 2, the O1−C−O2 ν5 bend has the smallest absorption of the six fundamentals at 5.78 km/ mol for trans-HOCO and 7.73 km/mol for trans-DOCO. The other bend (ν3) is conversely the most intense at 221.62 and 229.91 km/mol. In fact, the spectra for the trans isotopologoues, shown in Figures 1 and 2, respectively, for HOCO and DOCO, are qualitatively similar. Granted, the ν1 O1−H stretch decreases by 955.9 cm−1 and, conjointly, 32.4 km/mol in intensity, but the shapes of the spectra are still roughly similar. The shoulder to the red of ν3 caused by ν4 is also noticeably smaller for DOCO as compared to that for HOCO, but it should still be observed in both spectra. The 2ν4 frequency at 2089.0 cm−1 creates a shoulder in the trans-HOCO spectrum as well, but it is to the blue of ν2 at 1862.0 cm−1. A shoulder also exists to the blue of ν2 at 1859.8 cm−1 for trans-DOCO, but it is caused by the ν4 + ν3 combination band at 1977.6 cm−1. Comparison of the cis- and trans-HOCO vibrational spectra clearly yields identifiers to distinguish the two conformers. The change in geometry has already been shown to decrease the dipole moment in the equilibrium structure of the cis conformer. The cis-HOCO simulated spectrum is shown in Figure 3. The ν2 CO2 stretch is now the most intense transition for cis-HOCO, but it is already large in the trans. However, the ν3 H−O1−C bend has almost no transition strength in the cis conformer, even though it is the most intense band for the other conformer. The second-most intense band in cis-HOCO falls to the ν4 C−O1 stretch at 1042.4 cm−1 and 151.94 km/mol. Because this mode increases the effective size of the molecule in its motion, the dipole moment shifts to a greater degree in this more compact conformer than it does in the trans. Whereas ν4 created a shoulder to the red of ν3 in the trans conformer, only ν4 remains, and this peak does not have a reddened shoulder. The cis ν6 torsional mode at 540.2 cm−1 is qualitatively comparable in both position and intensity to the ν6 mode in the other conformer, but the close proximity of the ν5 O1−C−O2 bend at 601.2 cm−1 broadens and raises this peak slightly in our simulated spectra. The ν1 O1−H stretch is also lower in frequency and much less intense in the cis conformer than that in its trans-HOCO counterpart. The two conformers

are not without their similarities, however. A shoulder to the blue of the large ν2 amplitude is also caused by the 2ν4 overtone in the cis-HOCO spectrum at 2073.9 cm−1 and 15.43 km/mol, very close to the values for this transition in the trans-HOCO spectrum. Deuterating the cis-HOCO radical reveals a vibrational spectrum with smaller absorption amplitudes than its nondeuterated equivalent. The increased mass restricts the motion of the modes and, hence, the shift in dipole moment. The exception to this is ν3 at 1123.1 cm−1. Even though cis-HOCO has almost no intensity associated with this mode, it shifts up to 84.40 km/mol in DOCO. As a result, the cis-DOCO spectrum, given in Figure 4, has a double peak after the artificial broadening is considered. Even though vibrational excitation into the ν3 state is not the most intense transition, it still qualitatively changes the appearance of the spectrum compared to cis-DOCO with this splitting of the third peak from the blue. Like trans-HOCO, cis-DOCO also has a shoulder to the blue of the ν2 at 1827.5 cm−1 caused by the ν4 + ν3 combination band at 2073.4 cm−1. However and unlike trans-DOCO, the 2ν4 overtone is not lower in frequency than ν2, but it contributes to the overall ν2 peak height considering the 10 cm−1 fwhm broadening. The ν1 O1−H stretch decreases upon deuteration of the cis conformer by 900.7 cm−1, but the intensity is little changed because it stays at slightly less than 15 km/mol.

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CONCLUSIONS For cis- and trans-HOCO and DOCO, the vibrational spectra give roughly the same qualitative picture with four major peaks. The intense ν2 CO2 bond stretch in the 1820 to 1865 cm−1 and 175 to 275 km/mol ranges is a common signpost for all of the related systems present in this study because it is the mode least affected by deuteration or conformation. From this clear position, one higher-frequency peak is observed to the blue, and two lower-frequency peaks are also present to the red in each of the spectra. Depending on the conformer or the isotopologue chosen, unique markers are still present for each of the four systems. These markers are retained after the application of artificial broadening, which present less idealized spectra that are necessary for comparison to interstellar and extraterrestrial spectra, especially. The ν1 O1−H stretch varies, naturally, between isotopologues, but it also changes with 6937



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conformation as a result of the small structural interference present in the cis-HOCO geometry. Additionally, the peak splittings, broadenings, and shoulders present uniquely identify each system, in addition to the exact positions of each vibrational transition. Although the computations executed in this study do not precisely match previous experimental results for the known ν1 and ν2 gas-phase fundamental frequencies of trans-HOCO, the values given here are very close to experiment and in the same accuracy range or better than many ν1 and ν2 frequencies previously computed.11,13,15 We are not proffering extreme accuracy for the intensities and amplitudes for the transitions into these vibrational states, but we are providing a semiquantitative, reliable picture as to what these previously muddled spectra should be in their pure forms. We hope that the simulated spectra provided here will assist in further analysis of the OH + CO potential surface and the HOCO system(s) in various atmospheric and astrochemical environments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.H.); Timothy.J. [email protected] (T.J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.H. and T.J.L. are delighted to dedicate this work to Joel M. Bowman Festschrift, on occasion of his 65th birthday, and gratefully acknowledge the support, advice and collegiality that Professor Joel Bowman has shown over the many years in which we have collaborated. The work undertaken by X.H. and T.J.L. was made possible through NASA Grant 10-APRA100096 and 10-APRA10-0167. X.H. also acknowledges funding from the NASA/SETI Institute Cooperative Agreement NNX12AG96A. The U.S. National Science Foundation supported the work by R.C.F. and T.D.C. through a MultiUser Chemistry Research Instrumentation and Facility (CRIF:MU) Award CHE-0741927 and through Award CHE1058420. R.C.F. was also funded, in part, through the NASA Postdoctoral Program administered by Oak Ridge Associated Universities. J.M.B. thanks the Department of Energy (DE DFG02-97ER14782) for financial support. The artificial broadening program used was provided by Dr. Micah Abrams of Abrams Scientific Consulting.

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