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Reply to Comment on "Thermochemical and Kinetics of the CHOH + (S)N Reactional System": The Role of Duplet Atomic Nitrogen 4

Rhayla Mendes Ferreira, Orlando Roberto-Neto, Francisco Bolivar Correto Machado, and Rene Felipe Keidel Spada J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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The Journal of Physical Chemistry

Reply to Comment on “Thermochemical and Kinetics of the CH3OH + (4S)N Reactional System”: The Role of the Atomic Duplet Nitrogen. Rhayla Mendes Ferreira,† Orlando Roberto-Neto,‡ Francisco B. C. Machado,¶ and Rene F. K. Spada∗,§ †Departamente de Física, Universidade Federal do Espírito Santo, Vítória, 29.075-910, Espírito Santo, Brazil ‡Divisão de Aerotermodinâmica e Hipersônica, Instituto de Estudos Avançados, São José dos Campos, 12.228-001 São Paulo, Brazil ¶Departamento de Química, Instituto Tecnológico de Aeronáutica, São José dos Campos, 12.228-900 São Paulo, Brazil §Departamento de Física, Instituto Tecnológico de Aeronáutica, São José dos Campos, 12.228-900 São Paulo, Brazil E-mail: [email protected]

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Recently we reported thermal rate constants for the hydrogen abstraction of methanol (CH3 OH) by atomic nitrogen in the ground state (( 4S)N). 1 After the publication, it was brought to our attention two previous works that we have missed 2–4 and the fact that our calculated rate constants do not agree with the measured ones. Also, it was pointed out that we considered different reaction paths from the one identified as the dominant channel by the experimental work. 3 We are thankful to Prof. J. M. Roscoe for bringing us this query. In this context, we tested the proposed path and compared the results with the previously reported ones. 1,3 It is worth noticing that the scope of our work was to study elementary reactions considering only ground state atomic nitrogen, while the experimental work 3 carried out the measurements with active nitrogen. Roscoe and Roscoe 3 states that the active nitrogen was created via microwave and high voltage discharges, obtaining the same results for both methods. Also with high voltage discharges, Foner and Hudson 5 measured metastable atomic nitrogen concentrations in active nitrogen, obtaining the ratios of 0.0068 ([( 2D)N]) and 0.0025 ([( 2P)N]) in relation to [( 4S)N]. Roscoe and Roscoe 3 also states that the importance of the excited species formed during the discharge should decrease rapidly with increasing reaction time. However, the formation of these metastable species during the measurement was discarded. An example mechanism of formation for metastable atomic nitrogen in active nitrogen starts with the reaction of 6 ground state atomic nitrogen and (A3 Σ+ u )N2 , presented in reaction 1,

2 1 + ( 4S)N + (A3 Σ+ u )N2 −−→ ( P)N + (X Σg )N2

(1)

3 −3 The concentration of (A3 Σ+ u )N2 was estimated by Roscoe and Roscoe to be about 10

times the nitrogen atom concentration. As a possible next step, Taghipour and Brennen 7 reported the reaction with atomic nitrogen in the ground state (reaction 2) as the main

2


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removal route of ( 2P)N in active nitrogen,

( 4S)N + ( 2P)N −−→ ( 4S)N + ( 2D)N

(2)

Motivated by this data and the fact that we could only find equilibrium structures similar to those proposed by Roscoe 3 on the duplet surface, i.e. the reaction of methanol with the metastable duplet atomic nitrogen, we studied a reaction path that occurs in two steps, presented below as R4 and R5.

SP

4 CH3 OH + N −−→ CH3 OH···N −−→ CH3 ONH

SP

5 CH3 ONH −−→ CH3 + HNO

(R4) (R5)

First, at R4, the reactants are arranged in a reactant well (RW4 ), and after proceeds to a stable geometry CH3 ONH via a saddle point (SP4 ). At R5, the system is dissociated to CH3 + HNO, being the last on the singlet state. R4 and R5 are also presented in Figure 1. These stationary structures, obtained with BB1K/aVTZ, are in great concordance to the ones suggested in the experimental work. 2,3

Figure 1: Representation of elementary reactions R4 and R5 with selected bond lengths calculated with the BB1K/aVTZ methodology.

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The energy profile, calculated with CCSD(T)/CBS//BB1K/aVTZ for R4 and R5 is presented in Figure 2. The connectivity of these points were checked by IRC calculations or a relaxed scan for the initial reactants to RW4 . For completeness, the energy profile for R1 , R2 , and R3 is also reproduced. 1 CH3OH+2N (61.0) SP2 (56.6)

60

SP4 (38.4)

40

RW4 (34.1) Energy (kcal/mol)

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SP1 (28.1) SP3 (22.4)

CH3O+NH (25.4) CH3+NOH (20.4) CH2OH+NH (15.7)

SP5 (6.4) 0

CH3OH+4N

CH3+HNO (-4.0)

-20

-40

CH3ONH (-33.6)

Figure 2: Adiabatic energy profile for reactions R1 to R5 calculated with CCSD(T)/CBS// BB1K/aVTZ Our calculations present the final products at R5 (CH3 + HNO) -4.0 kcal/mol relative to the reactants (CH3 OH + ( 4S)N). Based on values of tabulated enthalpies at 0 K from Active Thermochemical Tables (ATcT 8–10 ), the relative energy of the final product is -5.0 kcal/mol, and in the NIST Chemistry Webbook 11 it is equal to -3.1 kcal/mol (as stated in references 2 and 3). Thus, our value differ by no more than 1 kcal/mol from standard results already in the literature. Considering the energy gap between the quartet and duplet surfaces at the reactants, our result is equal to 61.0 kcal/mol, and according to NIST Atomic Spectra Database 12 this value is 55.0 kcal/mol (for the energy difference of ( 4S)N and ( 2D)N). Despite 4


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this difference, the saddle point on R4 is below the reactants by more than 22 kcal/mol, while on the quartet surface, the barriers height are at least 22.4 kcal/mol (R3). Therefore, the rate constants for R4 are expected to be much larger than for the reactions on the quartet surface (R1 to R3). It can be seen from Figure 2 that the two saddle points (SP4 and SP5 ) present lower energy than the initial reactants. It is worth noticing that R4 is not a bimolecular reaction, but R4 and R5 form an addition-elimination reaction mechanism with the CH3 ONH as a metastable intermediate. As such, the intermediate upkeep is dependent of the pressure of the bath to dissipate the excess energy via an inert third body. Once the measurements were not made under the high-pressure limit, 3 the formed CH3 ONH dissociates due to the excess energy in vibrational modes. However, the following elimination reaction (R5) presents a barrier of 40.0 kcal/mol, while the reverse barrier for R4 is equal to 72.0 kcal/mol. Thus, the elimination of HNO is the dominant path rather than the reverse path to R4. The rate constants for R4 and R5, calculated with the improved canonical variational theory (ICVT) considering the thermochemical data from Figure 2, are presented in Table 1. Table 1: Thermal rate constants for R4 (cm3 ·molecule−1 ·s−1 ) and R5 (s−1 ) reaction paths.

T (K) 250 300 400 700 1000 2000

TST 8.0 ×107 4.4 ×104 4.1 ×100 3.5 ×10−5 4.1 ×10−7 3.7 ×10−9

T (K) 250 300 400 700 1000 2000

TST 1.3 ×10−22 1.3 ×10−16 4.8 ×10−9 3.7 ×101 3.9 ×105 2.3 ×1010

R4 ICVT 5.8 ×107 3.4 ×104 3.4 ×100 3.2 ×10−5 3.9 ×10−7 2.5 ×10−9 R5 ICVT 9.5 ×10−23 9.7 ×10−17 3.5 ×10−9 2.5 ×101 2.4 ×105 1.1 ×1010

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ICVT/SCT 5.8 ×107 3.4 ×104 3.4 ×100 3.2 ×10−5 3.9 ×10−7 2.5 ×10−9 ICVT/SCT 1.6 ×10−22 1.4 ×10−16 4.3 ×10−9 2.6 ×101 2.5 ×105 1.2 ×1010



These rate constants are larger than the values calculated for the reactions on the quartet surface and also larger than the measured ones. However, the concentration of duplet atomic nitrogen in active nitrogen is very small, since the mechanism for its formation starts with reaction 1 and the concentration of (A3 Σ+ u )N2 is estimated to be low. In this sense, we are inclined to believe that even a small concentration of duplet atomic nitrogen is able to accelerate the nitrogen consumption, and consequently, increase the value of the measured rate constant. The kinetic isotope effects (KIEs) were calculated for the whole temperature range considering the rate constants obtained with ICVT/SCT for each reaction path. The resuls are presented in Table 2. Table 2: Kinetic isotope effects (KIEs) for the reaction paths R1 to R4. CH3 OH/CH3 OD R1 R2 R3 8.7 1.1 1.3 6.2 1.1 1.2 4.3 1.1 1.1 2.5 1.0 1.0 2.0 1.0 1.0 1.5 1.0 1.0 CH3 OH/CD3 OD T(K) R1 R2 R3 250 12.5 2.8 26.1 300 8.3 2.1 15.0 400 5.2 1.6 7.3 700 2.8 1.2 2.9 1000 2.1 1.1 2.1 2000 1.6 1.0 1.5 CH3 OD/CD3 OD T(K) R1 R2 R3 250 1.4 2.5 19.9 300 1.3 1.9 12.2 400 1.2 1.5 6.4 700 1.1 1.1 2.8 1000 1.1 1.0 2.0 2000 1.0 0.9 1.5 T(K) 250 300 400 700 1000 2000

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R4 3.7 3.1 2.5 1.9 1.7 1.0 R4 3.2 2.8 2.3 1.8 1.6 1.3 R4 0.9 0.9 0.9 0.9 1.0 1.2


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Considering the kinetic isotope effects (KIEs), since the rate constants for R3 are larger than the values for R1 and R2 by at least 105 (300 K), the total KIE on the quartet surface (considering R1 to R3) is dominated by the result obtained for R3. The measured value is equal to 3.6±0.4 for CH3 OH/CH3 OD (around 303 K). 3 Comparing with the calculated values, it is closer to the result for R4 at 300 K, equal to 3.1, on the duplet surface, which is the suggested reaction path by Roscoe and Roscoe. 3 The hydrogen abstraction reaction from the OH on the quartet surface presents a KIE value at 300 K equal to 6.2 for R1 and equal to 1.2 for R3 (hydrogen abstraction from the CH3 group). Furthermore, the measured KIE for CH3 OH/CD3 OD (around 343 K) is equal to 8 ± 1. Our results at 300 and 400 K for R3 are 15.0 and 7.3 and the values for R4 are 2.8 and 2.3. For the CH3 OD/CD3 OD, our results at 300 K are 12.2 (R3) and 0.9 (R4) and the value reported by Roscoe and Roscoe 3 is equal to 2.2±0.5. As observed by the rate constants for the reaction with CH3 OH, the measured values are intermediate between the results for the duplet and quartet surfaces (R3 and R4), it reenforces the presence of duplet atomic nitrogen on the active nitrogen used in the experiments. An important remark is that the Roscoe and Roscoe 3 does not suggest as the measured reaction the mechanism involving the ( 4S)N radical, but the active nitrogen. In this context, we believe that the experimental and theoretical works 1,3 are not conflicting, but complementary. Also, we want to reinforce that our goal was to report rate constants for elementary reactions, and even this reply is a possibility among others to explain the large value observed for the rate constant. We acknowledge Prof. J. M. Roscoe for bringing this question that enriched the discussion about the methanol reaction with atomic nitrogen, and also Dr. L. F. A. Ferrão for fruitful discussions about this subject.

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References (1) Ferreira, R. M.; Roberto-Neto, O.; Machado, F. B. C.; Spada, R. F. K. Thermochemical and Kinetics of the CH3 OH+(4 S)N Reactional System. The Journal of Physical Chemistry A 2018, 122, 5905–5910. (2) Roscoe, J. M. Comment on “Thermochemical and Kinetics of the CH3 OH+(4 S)N Reactional System”. The Journal of Physical Chemistry A To be published. (3) Roscoe, J. M.; Roscoe, S. G. The Reactions of Active Nitrogen with Simple Alcohols. Canadian Journal of Chemistry 1973, 51, 3671–3679. (4) Sole, M. J.; Gartaganis, P. A. The Reaction of Active Nitrogen with Methanol. Canadian Journal of Chemistry 1963, 41, 1097–1103. (5) Foner, S.; Hudson, R. L. Mass Spectrometric Studies of Metastable Nitrogen Atoms and Molecules in Active Nitrogen. The Journal of Chemical Physics 1962, 37, 1662–1667. (6) Meyer, J. A.; Setser, D. W.; Stedman, D. H. Energy Transfer Reactions of N2 (A3 Σ+ u ). II. Quenching and Emission by Oxygen and Nitrogen Atoms. The Journal of Physical Chemistry 1970, 74, 2238–2240. (7) Taghipour, A.; Brennen, W. N(2p3 2 P◦ ) in the Nitrogen Afterglow. Chemical Physics 1979, 37, 363–368. (8) Ruscic, B.; Bross, D. H. Active Thermochemical Tables (ATcT) values based on ver. 1.122d of the Thermochemical Network. 2018; available at ATcT.anl.gov (Retrieved at November 26, 2018.). (9) Ruscic, B.; Pinzon, R. E.; Morton, M. L.; von Laszevski, G.; Bittner, S. J.; Nijsure, S. G.; Amin, K. A.; Minkoff, M.; Wagner, A. F. Introduction to Active Thermochemical Tables: Several “Key” Enthalpies of Formation Revisited. The Journal of Physical Chemistry A 2004, 108, 9979–9997. 8


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(10) Ruscic, B.; Pinzon, R. E.; Von Laszewski, G.; Kodeboyina, D.; Burcat, A.; Leahy, D.; Montoy, D.; Wagner, A. F. Active Thermochemical Tables: Thermochemistry for the 21st Century. Journal of Physics: Conference Series. 2005; p 561. (11) Linstrom, P. J., Mallard, W. G., Eds. NIST Chemistry WebBook, NIST Standard Reference Database Number 69 ; National Institute of Standards and Technology: Gaithersburg MD, 20899, 2005. (12) Kramida, A.; Yu. Ralchenko,; Reader, J.; and NIST ASD Team, NIST Atomic Spectra Database (ver. 5.6.1), [Online]. Available: https://physics.nist.gov/asd [2018, November 26]. National Institute of Standards and Technology, Gaithersburg, MD., 2018.

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