Duncan, Timothy V., \u2700

Miller, Charles E.

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Haverford College: Haverford Scholarship

Haverford College Haverford Scholarship Faculty Publications Chemistry 2000 The HCO2 potential energy surface: Stationary point energetics and the HOCO heat of formation Timothy V. Duncan '00 Class of 2000, Haverford College Charles E. Miller Haverford College Follow this and additional works at: https://scholarship.haverford.edu/chemistry_facpubs Repository Citation Duncan, Timothy V., and Charles E. Miller. "The HCO potential energy surface: Stationary point energetics and the HOCO heat of formation." The Journal of Chemical Physics 113 (2000): 5138. This Journal Article is brought to you for free and open access by the Chemistry at Haverford Scholarship. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Haverford Scholarship. For more information, please contact nmedeiro@haverford.edu. The HCO2 potential energy surface: Stationary point energetics and theHOCO heat of formationTimothy V. Duncan and Charles E. Miller  Citation: J. Chem. Phys. 113, 5138 (2000); doi: 10.1063/1.1312824 View online: http://dx.doi.org/10.1063/1.1312824 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v113/i13 Published by the American Institute of Physics.  Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 29 Mar 2013 to 165.82.168.47. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsThe HCO2 potential energy surface: Stationary point energeticsand the HOCO heat of formationTimothy V. Duncan and Charles E. Millera)Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041-1392~Received 1 June 2000; accepted 4 August 2000!The energies of six stationary points on the OH1CO→HOCO→H1CO2 potential energy surfacehave been calculated using the G3 and CBS-QB3 methods. An analysis combining ab initio andexperimental enthalpies yielded DH f298 K (trans-HOCO)5242.961.5 kcal mol21 (243.861.4 kcal mol21) at the G3~CBS-QB3! level of theory. These results confirm the revised HOCOheat of formation derived from photoionization spectroscopy and suggest that the HOCO potentialwell is 8.8 kcal mol21 shallower than previously thought. We discuss the implications of theseresults for accurate Rice–Ramsperger–Kassel–Marcus modeling or quantum mechanical scatteringcalculations of the OH1CO reaction. © 2000 American Institute of Physics.@S0021-9606~00!32437-0#The OH1CO→HOCO→H1CO2 reaction has been thesubject of extensive experimental1–6 and theoretical7–14 in-vestigation due to its importance in atmospheric chemistry,in combustion, and as the prototypical complex-forming bi-molecular reaction. The crucial role played by the HOCOintermediate in determining the reaction dynamics has beenemphasized by crossed molecular beam scatteringmeasurements,1 femtosecond laser detection of the productevolution,5,6 measurements of the differential cross sectionsfor the H1CO2 reaction,15–17 and the observation of scatter-ing resonances in calculations of the quantum mechanicalreaction probability.9–14 The wealth of data available for thissystem has made it a benchmark for the study of four-atomreaction dynamics.Despite widespread interest in the OH1CO reaction, theenergetics of the HCO2 potential energy surface remainpoorly defined. Figure 1 illustrates that the reaction proceedsfrom OH1CO over a negligibly small barrier at TS1, intothe potential well containing both cis- and trans-HOCO con-formers, then over the TS2 barrier to H1CO2. The experi-mental enthalpies18 of the OH1CO entrance channel and theH1CO2 exit channel provide absolute reference points forthe characterization of the entire potential surface. Analysisof reaction rate data has produced general agreement thatTS2, the @H{{{OCO# transition state, lies 1 – 2 kcal mol21above the zero point energy of the OH1CO entrancechannel,2–4 but the exact energy of TS2 has not been defini-tively established. The HOCO well depth had long been ac-cepted as 35.4 kcal mol21 based on the DH f298 K(trans-HOCO)5252.5 kcal mol21 derived from photoion-ization measurements19 and corroborated by ab initiocalculations.8 However, reanalysis of the threshold region ofthe photoionization spectra20,21 recently led to the conclusionthat DH f298 K (trans-HOCO)>246.5 kcal mol21, or that theHOCO well depth extends no more than 29.4 kcal mol21 be-low the OH1CO asymptote. Such a large change in theHOCO well depth has significant consequences for the exist-ing literature on OH1CO rate calculations since the mostsophisticated Rice–Ramsperger–Kassel–Marcus3,4 andquantum mechanical scattering calculations8–10,13,14,22 em-ployed the 35 kcal mol21 well depth. The possibility of sig-nificant errors in the energetics of the key stable points of theempirical potential surface8,9,12,22 suggested that there mightbe more subtle energetic discrepancies for other regions ofthe potential. Therefore, we began an ab initio characteriza-tion of the full six-dimensional HCO2 potential energy sur-face with the goal of determining a chemically accurate po-tential. Here we report accurate energies for the keystationary points.The HCO2 stationary point energies have been calcu-lated using the CBS-QB323 and G324 model chemistries.These methods produce results that deviate from experimen-tal heats of formation by mean absolute values of 0.87 and1.02 kcal mol21, respectively. Stationary point structureswere optimized at the B3LYP/6-31111G(3d f ,3pd) levelof theory. The optimized structures agreed with publishedvalues8 and were used as the starting points for CBS-QB3and G3 calculations. Thermochemical quantities for theHOCO stationary points have been calculated both at 298.15and 0 K; Table I lists the results. To facilitate comparisonwith existing thermochemical information, Table II presentsenthalpies of each stationary point relative to the enthalpy ofOH1CO. Note that the ab initio reaction enthalpies forOH1CO→H1CO2, DHr3n298 K (G3)5224.9 kcal mol21 andDHr3n298 K ~CBS-QB3!5225.4 kcal mol21, reproduce the ex-perimental value18 DHr3n298 K5224.85 kcal mol21 within0.6 kcal mol21.We combined the ab initio results from Tables I and IIwith the experimental enthalpies for OH1CO and H1CO2to generate two determinations of DH f298 K ~trans-HOCO! foreach model chemistry. These results are collected in TableIII. The experimental uncertainties in the OH, CO, H, andCO2 thermochemistry18 are negligible compared to the esti-mated uncertainties in the HOCO energetics. The uncertain-a!Electronic mail: cmiller@haverford.eduJOURNAL OF CHEMICAL PHYSICS VOLUME 113, NUMBER 13 1 OCTOBER 200051380021-9606/2000/113(13)/5138/3/$17.00 © 2000 American Institute of PhysicsDownloaded 29 Mar 2013 to 165.82.168.47. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsties listed for each determination of DH f298 K ~trans-HOCO!,1.5 kcal mol21 for CBS-QB3, and 1.8 kcal mol21 for G3, aredominated by the uncertainties from the three separate abinitio calculations required to evaluate the HOCO heat offormation. Averaging all four determinations yields our bestestimate of 243.3 kcal mol21 to which we assign a conser-vative uncertainty of 2 kcal mol21. Similar calculations pro-duce DH f298 K (cis-HOCO)5241.4 kcal mol21, DH f298 K(TS2)5217.9 kcal mol21, and DH f298 K (TS3)5235.3 kcal mol21. Our DH f298 K ~trans-HOCO! agrees wellwith the recently revised lower limit of 246.5 kcal mol21reported by Ruscic and Litorja21 and suggests a further up-ward revision of 3 kcal mol21. The present result also im-plies that trans-HOCO lies 26.2 kcal mol21 below OH1CO,1.4 kcal mol21 below H1CO2, and 25.4 kcal mol21 belowTS2. Thus, unimolecular decomposition of trans-HOCO re-quires less than 27 kcal mol21 to surmount both the OH1COand H1CO2 dissociation barriers.Several immediate ramifications for the OH1CO reac-tion may be deduced from the revised energetics of theHCO2 potential. Most important, the conclusions drawn fromprevious theoretical treatments of the reaction should be con-sidered suspect until the effect of decreasing the HOCO welldepth by 9 kcal mol21 can be thoroughly investigated. Weestimate the effect that the shallower potential well has onthe microcanonical rate constant for HOCO dissociation bynoting that k(E) depends inversely on the density of states,r(E),k~E !5N~E !hr~E ! . ~1!Evaluating the density of vibrational states r(Ev)25 at the oldand new HOCO well depthsk~26.2 kcal mol21!k~35.4 kcal mol21! >r~35.4 kcal mol21!r~26.2 kcal mol21! ~2!we anticipate that the calculated rate constant for the revisedthermochemistry is approximately a factor of 3 larger thanthe rate constant calculated from the old thermochemistry.19The sparser density of states near the HOCO dissociationlimit of the shallower potential should increase the impor-tance of reactive scattering resonances to k(E) in this energyregion. This conclusion may be tested experimentally by ob-taining rovibrational spectra of HOCO in the neighborhoodof the dissociation threshold. The HOCO potential well de-termined by the present study requires only three quanta ofthe OH stretching vibration (Ev;30 kcal mol21) to exceedthe dissociation barriers into both the OH1CO and theH1CO2 channels. Interesting tunneling dynamics may al-ready exist for molecules possessing two quanta of OHstretching excitation. In fact, all HOCO excited vibrationalstates have energies above the H1CO2 thermodynamicFIG. 1. A schematic diagram of the HCO2 potential energy surface. The abinitio enthalpies calculated in the present work are shown as the connectedlines. The experimental values for DH f298 K ~trans-HOCO! from Refs. 19and 21, labeled 1 and 2, respectively, are included for comparison.TABLE I. Ab initio energies ~hartrees! for the HCO2 potential surface.OH CO OH1CO t-HOCO TS3 c-HOCO TS2 H1CO2 H CO20 KCBS-QB3275.649 693 2113.182 257 2188.831 950 2188.872 480 2188.859 555 2188.869 239 2188.830 252 2188.871 831 20.499 818 2188.372 013G3 275.694 906 2113.267 368 2188.962 274 2189.000 974 2188.988 103 2188.997 986 2188.962 205 2189.001 301 20.501 003 2188.500 298Expt~kJ/mol!a38.39 2113.81 275.42 2191.6 2177.116 216.035 2393.151298 KCBS-QB3275.646 388 2113.178 952 2188.825 340 2188.868 314 2188.855 569 2188.865 101 2188.826 073 2188.865 897 20.497 457 2188.368 440G3 275.691 601 2113.264 063 2188.955 664 2188.996 766 2188.984 124 2188.993 829 2188.958 007 2188.995 372 20.498 642 2188.496 730Expt~kJ/mol!a38.987 2110.53 271.543 2194.6 2175.523 217.999 2393.522aExperimental enthalpies taken from Refs. 18 and 21.TABLE II. Enthalpies ~kcal mol21) on the HCO2 potential relative toOH1CO.OH1CO t-HOCO TS3 c-HOCO TS2 H1CO20 KCBS-QB3 0.0 225.4 217.3 223.4 1.1 225.0G3 0.0 224.3 216.2 222.4 0.0 224.5Expta 0.0 227.8 224.3298 KCBS-QB3 0.0 227.0 219.0 224.9 20.5 225.4G3 0.0 225.8 217.9 223.9 21.5 224.9Expta 0.0 229.4 224.9aExperimental enthalpies taken from Refs. 18 and 21.5139J. Chem. Phys., Vol. 113, No. 13, 1 October 2000 HCO2 energyDownloaded 29 Mar 2013 to 165.82.168.47. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsthreshold and may possibly dissociate via quantum tunnel-ing. Clearly, a great deal remains to be learned about thiswell-studied system.This work has been supported by Haverford College, theCamille and Henry Dreyfus Foundation Faculty Start-upGrant Program for Undergraduate Institutions, and the Pitts-burgh Supercomputer Center ~CHE-980011P!.1 M. Alagia, N. Balucani, P. Casavecchia, D. 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Calculated heats of formation for trans-HOCO, cis-HOCO,TS2, and TS3.aDH1 DH f 1 DH2 DH f 2trans-HOCO0 KCBS-QB3 225.4 243.5 20.4 242.7 245.8 ExptbG3 224.3 242.3 0.2 242.1 242.7 Average298 KCBS-QB3 227.0 244.1 21.5 243.5 246.5 ExptbG3 225.8 242.9 20.9 242.8 243.3 Averagecis-HOCO0 KCBS-QB3 223.4 241.4 1.6 240.7G3 222.4 240.4 2.1 240.3 240.7 Average298 KCBS-QB3 224.9 242.0 0.5 241.5G3 223.9 241.0 1.0 241.0 241.4 AverageTS20 KCBS-QB3 1.1 217.0 25.0 217.3G3 0.0 218.0 24.5 217.8 217.5 Average298 KCBS-QB3 20.5 217.6 25.0 217.0G3 21.5 218.6 23.4 218.5 217.9 AverageTS30 KCBS-QB3 217.3 235.3 7.7 234.6G3 216.2 234.2 8.3 234.0 234.6 Average298 KCBS-QB3 219.0 236.1 6.5 235.5G3 217.9 235.0 7.1 234.9 235.3 AverageaExperimental enthalpies for OH, CO, H and CO2 taken from Ref. 18.DH15DH fab initio(X)2DH fab initio(OH1CO); DH f 15DH11DH fexpt(OH1CO) where DH fexpt,0 K(OH1CO)5218.03 kcal mol21, DH fexpt,298 K(OH1CO)5217.10 kcal mol21 DH25DH fab initio(X)2DH fab initio(H1CO2);DH f 25DH21DH fexpt(H1CO2) where DH fexpt,0 K(H1CO2)5242.33kcal mol21, DH fexpt,298 K(H1CO2)5241.95 kcal mol21.bReference 21.5140 J. Chem. Phys., Vol. 113, No. 13, 1 October 2000 T. V. Duncan and C. E. MillerDownloaded 29 Mar 2013 to 165.82.168.47. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

The HCO2 potential energy surface: Stationary point energetics and the HOCO heat of formation

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The HCO2 potential energy surface: Stationary point energetics and the HOCO heat of formation

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