Gas phase acidity of substituted benzenes

Gas phase acidity of substituted benzenes

Chemical Physics Letters 506 (2011) 167–174 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/lo...

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Chemical Physics Letters 506 (2011) 167–174

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Gas phase acidity of substituted benzenes Guy Bouchoux ⇑ Laboratoire des Mécanismes Réactionnels, Ecole Polytechnique, CNRS, 91128 Palaiseau, France

a r t i c l e

i n f o

Article history: Received 9 February 2011 In final form 9 March 2011 Available online 12 March 2011

a b s t r a c t Deprotonation thermochemistry of benzene derivatives C6H5X (X = H, F, Cl, OH, NH2, CN, CHO, NO2, CH3, C2H5, CHCH2, CCH) has been examined at the G3B3 level of theory. For X = F, Cl, CN, CHO and NO2, the most favorable deprotonation site is the ortho position of the phenyl ring. This regio-specificity is directly related to the field/inductive effect of the substituent. G3B3 gas phase acidities, DacidH° and DacidG°, compare within less than 4 kJ mol1 with experimental data. A noticeable exception is nitrobenzene for which tabulated acidity appear to be underestimated by ca. 120 kJ mol1. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Thermochemical parameters related to the Bronsted’s concept of acidity and basicity are essential tools for evaluating chemical and biochemical reaction processes. Gas phase acidities and basicities afford direct measurement of thermochemical quantities devoid of solvent effect. They consequently bring fundamental information on intrinsic acid–base properties. Moreover, they also allow to understand and quantify the various ionization-fragmentation processes occurring in a mass spectrometer. Due to the recent development of atmospheric pressure ionization techniques, protonation and deprotonation are indeed methods of choice for the analysis of molecules bearing basic or acidic functions. During the last decades, electrospray and atmospheric pressure chemical ionization have been mainly used in the positive ionization mode. Nowadays, there is increasing interest in negative ion analysis by mass spectrometry, particularly in the analysis of pesticides, pharmaceuticals and peptides. As a result, knowledge of the corresponding thermochemical parameters, gas phase acidity, DacidH° or its free energy counterpart, DacidG°, is obviously essential. Unfortunately, the number of experimental estimates of DacidH° or DacidG° is limited, and, for some of them, the reliability of the results is questionable [1]. Moreover, the most favored deprotonation sites cannot be always easily deduced from experimental data. It is clear that quantum chemistry may offer an alternative access to the thermochemical parameters DacidH° and DacidG° while providing also detailed structural information on the neutral and deprotonated species. Accordingly, in recent years, computational chemistry methods were developed to provide accurate thermochemical data for gaseous species containing first and second row elements. Several procedures are presently available which are expected to provide thermochemical data at ‘chemical’ or even

⇑ Fax: +33 01 69 33 48 03. E-mail address: [email protected] 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.03.032

‘benchmark’ accuracy, i.e. within a few kJ mol1. These procedures are based either on (i) composite ab-initio based methods [2–4] or on (ii) the most recent density functional theory models [5,6]. The present study intends to deliver consistent and accurate calculations of the gas phase acidities, DacidH° and DacidG°, of archetypal aromatic compounds. The considered C6H5X molecules include heteroatomic acidic centers (X = OH, NH2) and, mostly, carbon centered acids (X = H, F, Cl, CN, CHO, NO2, CH3, C2H5, CHCH2, CCH). These molecules and their various possible deprotonated forms were investigated using the G3 composite method [2,3] which offers good compromise between accuracy, computation time and computer memory size. Comparison with re-evaluated available experimental DacidH° and DacidG° values is discussed. Finally, computed and experimental heats of formation of the neutral molecules were also considered.

2. Computational section All structure optimization and thermochemical calculations were performed using the GAUSSIAN03 suite of programs [7]. The G3B3 [2] composite method use geometry optimized at the B3LYP/6-31G(d) level, zero point vibrational energy is obtained from vibrational frequencies calculated at this level and scaled by a factor 0.96. In a second step, single points quadratic configuration interaction are performed using (i) the frozen core QCISD(T)/ 6-31G(d) approximation, (ii) MP2(full) computation using the G3large basis set and (iii) MP4(FC) computations using the 631G(d) and larger basis sets such as 6-31+G(d) and 6-31G(2df,p). Finally, a high level correction is introduced to account for remaining deficiencies in the energy calculation. Since Gaussian doesn’t correctly identify the local symmetry point group, a correction of the computed entropy has been included using the symmetry number r of the species of interest (quoted in Table 2) in order to obtain the correct S298°. A particular case is toluene for which the internal rotation around the r CAC bond is associated with

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Table 1 Experimental deprotonation thermochemistry of molecules M (kJ mol1). M

DacidH°a

298 DacidS°b

DacidG°a

Methodsc

References

Benzene

1678.5 ± 0.8 1674.4 ± 1.3 1680.3 ± 2.1 1680.7 ± 2.1 1678.5 ± 2.9

37.1

1641.4 ± 0.6 1637.3 ± 1.3 1643.3 ± 2.1 1643.6 ± 2.1 1641.4 ± 2.9

FA HPMS (vH) THERM KIN average

[26,21] [26,9] [26,25] [23]

Fluorobenzene

1620.0(±4) 1624.5(±4) 1620.5(±4) 1620.0(±10) 1621.2 ± 2.2

34.5

1585.5(±4) 1590.0(±4) 1585.8(±4) 1585.5(±10) 1587.4 ± 2.2

HPMS (vH) ICR ICR KIN average

[10] [14] [17] [23]

Chlorobenzene

1624.2 1620.4 ± 2.3 1614.4(±12) 1625.1 ± 1.8 1621.0 ± 4.8

36.4

1587.8 1584.0 ± 2.3 1578(±12) 1588.7 ± 1.8 1584.6 ± 4.8

KIN ICR ICR FA average

[23] [17] [18] [30]

Phenol

1464.9 ± 1.7 1461 ± 4 1456 ± 4 1459.0 ± 2.9 1460.2 ± 3.71

29.6

1435.3 ± 1.7 1431 ± 4 1429 ± 4 1429.4 ± 2.9 1431.2 ± 2.9

HPMS (600 K) ICR TCID THERM average

[12] [15] [28] [25,29]

Aniline

1541.8 ± 2.6 1542.2 ± 2.4 1538.5 ± 0.1 1540.8 ± 2.0

37.1

1504.7 ± 2.6 1505.1 ± 2.4 1501.4 ± 0.1 1503.7 ± 2.0

ICR HPMS (vH) HPMS (vH) average

[16] [10] [13]

Benzonitrile

1603(±4)

34.3

1568.7(±4)

HPMS (vH)

[10]

Benzaldehyde

1619.6 ± 18

32.6

1587 ± 18

ICR

[19]

Nitrobenzene

1473(±4)

37.1

1436(±4)

HPMS (vH)

[10]

Toluene

1597.0 ± 1.0 1587.0 ± 2.8 1600.5 ± 2.5 1598.3 ± 0.8 1598.6 ± 1.8d

29.7

1567.3 ± 1.0 1557.3 ± 2.8 1570.8 ± 2.5 1568.6 ± 0.8 1568.9 ± 1.8d

ICR ICR THERM FA average

[16] [20] [25] [22]

Ethylbenzene

1597.6 ± 0.7

31.5

1566.1 ± 0.7

ICR

[16]

Styrene Ethynylbenzene

1633.7(±4) 1550.4

32.2 31.5

1601.5(±4) 1518.9 ± 1.0

HPMS (vtH) ICR

[10–26] [16]

a

Errors are standard deviation when several experiments are reported, errors indicated into parentheses are arbitrary estimates as given in Ref. [1]. From S298° corrected using the symmetry numbers quoted in Table 2. c FA = flowing afterglow; KIN = kinetic method; HPMS (vH) = high pressure mass spectrometry (van’t Hoff plot); ICR = ion cyclotron resonance; TCID = threshold collisioninduced dissociation; THERM = thermochemical cycle using Eq. (2). d Without considering the data of Ref. [20] b

essentially no rotational barrier. The contribution to vibrational entropy was thus calculated within the free rotor approximation using a symmetry number of 3. The total 298 K Gibbs free energies presented in Table 2 includes these corrected entropies which were also used when interconnecting DacidH° and DacidG° in Table 1. Gas phase acidity is defined by the thermochemical quantities DacidG° and DacidH° which are Gibbs free energy and enthalpy of the reaction (1) at 298 K, respectively:

AH ! A þ Hþ

ð1Þ

The computed estimates are consequently obtained from the atomization enthalpies and Gibbs free energies, H298° and G298° by:

Dacid H ðAHÞ ¼ H298 ðA Þ  H298 ðAHÞ þ 6:2 kJ mol

1 1

and Dacid G ðAHÞ ¼ G298 ðA Þ  G298 ðAHÞ  26:3 kJ mol

where the last numerical terms correspond to the enthalpy and Gibbs free energies of the proton at 298 K. The procedure used in the present study to calculate heats of formation consists in obtaining heat of formation at 0 K based on the computed atomization energies combined with the experimental gas phase 0 K heats of formation of the constituent atoms. Temperature correction to 298 K is obtained using the theoretical correction for the species of interest and the experimental contribution for the constituent elements [8].

3. Results and discussion 3.1. Experimental thermochemistry Experimental DacidG° values have been obtained previously from proton transfer equilibrium constant measurements involving the A ion of interest and a reference acid BH. Two types of experimental apparatus were generally used: pulsed high pressure mass spectrometry, HPMS [9–13], and ion cyclotron resonance mass spectrometry, ICR [14–20]. The possibility of variable temperature experiments in HPMS allowed the determination of enthalpy and entropy changes by a van’t Hoff plot treatment. Proton transfer equilibrium constant has been also obtained by using the ratio of forward and backward reactions rates measured in a flowing afterglow device [21,22]. Finally, kinetic method and collision threshold determination were also used to determine several DacidH° values [23,24]. Another, indirect, method of determination of gas phase acidity consists to consider a thermodynamic cycle associated with reaction (1) involving homolytic AAH bond dissociation and electron exchange between the two resulting radicals [25]. This treatment leads, at 298 K, to:

Dacid H ðAHÞ ¼ D298 ðAAHÞ þ IE298 ðH Þ  EA298 ðA Þ

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G. Bouchoux / Chemical Physics Letters 506 (2011) 167–174 Table 2 G3B3 computed H°298 and G°298 of neutral molecules M and their conjugated bases [MH]. M Benzene Fluorobenzene

Chlorobenzene

Phenol

Aniline

Benzonitrile

Benzaldehyde

Nitrobenzene

Toluene

Ethylbenzene

Styrene

Ethynylbenzene

a b

neutral

H°298 (Hartree)

DacidH°a (kJ mol1)

rb

G°298 (Hartree)

DacidG°a (kJ mol1)

232.051696 231.415300

1677.1

12 2

232.082373 231.447747

1640.0

331.294005 330.677925 330.668603 330.664696

1591.3 1615.8 1626.0

neutral ortho meta para

331.259476 330.642616 330.633267 330.630035

1625.8 1650.3 1658.8

2 1 1 2

neutral ortho meta para

691.518984 690.900805 690.896647 690.894082

1629.2 1640.1 1646.9

2 1 1 2

691.55485 690.938213 690.933625 690.930213

1592.8 1604.8 1613.8

neutral oxo ortho meta para meta0 ortho0

307.244107 306.689485 306.629416 306.612954 306.608296 306.609986 306.61425

1462.4 1620.1 1663.3 1675.5 1671.1 1659.9

1 2 1 1 1 1 1

307.279842 306.724142 306.664854 306.648735 306.644255 306.64599 306.650186

1432.8 1588.4 1630.7 1642.5 1638.0 1626.9

neutral amino ortho meta para

287.374753 286.790820 286.746039 286.737287 286.734583

1539.3 1656.9 1679.9 1687.0

1 1 1 1 1

287.410975 286.826476 286.782221 286.773765 286.771365

1508.4 1624.6 1646.8 1653.1

neutral ortho meta para

324.262951 323.654657 323.650763 323.651173

1603.3 1613.5 1612.4

2 1 1 2

324.299968 323.69238 323.688626 323.68825

1569.0 1578.9 1579.8

neutral aldo ortho meta para meta0 ortho0

345.322581 344.704874 344.710105 344.703663 344.705235 344.701188 344.698696

1628.0 1614.3 1631.2 1627.0 1637.7 1644.2

1 1 1 1 1 1 1

345.360688 344.744509 344.748248 344.742041 344.743331 344.740012 344.737064

1591.6 1581.7 1598.0 1594.6 1603.4 1611.1

neutral ortho meta para

436.472615 435.867302 435.86124 435.86346

1595.4 1611.4 1605.5

2 1 1 2

436.511914 435.908405 435.901227 435.902754

1558.3 1577.1 1573.1

neutral CH3 ortho meta para

271.326732 270.718388 270.691424 270.689783 270.689495

1603.4 1674.2 1678.5 1679.3

1 2 1 1 1

271.365072 270.754257 270.729904 270.727913 270.727524

1573.7 1641.4 1646.6 1647.7

neutral CH2 CH3 ortho meta para

310.597475 309.988857 309.947359 309.964344 309.961593 309.961279

1604.1 1713.1 1668.5 1675.7 1676.5

1 1 1 1 1 1

310.638172 310.029582 309.987433 310.004625 310.002399 310.002011

1571.6 1682.3 1637.2 1643.0 1644.0

neutral CH CH2(a) CH2(b) ortho meta para meta0 ortho0

309.388668 308.767573 308.756165 308.75957 308.760505 308.757776 308.758484 308.756919 308.759994

1636.9 1657.9 1668.8 1655.4 1662.6 1660.7 1664.9 1656.8

1 1 1 1 1 1 1 1 1

309.428198 308.807005 308.794805 308.797795 308.799702 308.797058 308.797575 308.798469 308.798733

1604.7 1628.9 1636.8 1623.9 1630.8 1629.5 1627.1 1626.4

neutral CH ortho meta para

308.160949 307.571524 307.535839 307.535035 307.535585

1553.7 1647.4 1649.5 1648.1

2 2 1 1 2

308.198545 307.608775 307.57457 307.573827 307.573767

1522.2 1612.0 1614.0 1614.1

DacidH°(M) = H298°([MH])  H298°(M) + 6.2 kJ mol1 and DacidG°(M) = G298°([MH])  G298°(M)  26.3 kJ mol1. Symmetry number used in the calculation of S°298 (see text).

where D, IE and EA mean bond dissociation enthalpy, ionization energy and electron affinity, respectively. Since the two latter

quantities are generally known from spectroscopic data, 0 K values of IE and EA are the most easily available. Introducing

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these latter quantities, the preceding relationship may be expressed as:

Dacid H ðAHÞ ¼ D298 ðAAHÞ þ IE0 ðH Þ  EA0 ðA Þ þ

Z

298

DC p dT

0 +







with DCp = Cp(H )  Cp(H ) + Cp(A )  Cp(A ). Most of the time, the difference in heat capacities DCp is negligible since ions and radicals are very close in structure, thus allowing to estimate DacidH° from:

Dacid H ðAHÞ ¼ D298 ðAAH Þ þ IE0 ðH Þ  EA0 ðA Þ

ð2Þ

These various procedures were utilized previously to determine the acidity of the studied molecules. The essential results are summarized in Table 1 and detailed below. 3.1.1. X = H Revision of the ammonia acidity allows Ervin and DeTuri [26] to correct the previous estimate of DacidG°(benzene) based on flowing afterglow experiments [21]. The authors of Ref. [26] proposed DacidG°(benzene) = 1641.4 ± 0.6 kJ mol1, a value 2.5 kJ mol1 lower than that originally published [1643.9 kJ mol1in Refs. [21,1]]. Using the recently reevaluated DacidH° of water (1632.9 ± 0.1 kJ mol1, Ref. [26]) and the enthalpy change associated with proton transfer with C6H5 ions in pulsed high pressure mass spectrometry experiments [9], it may be deduced that DacidH°(benzene) = 1674.3 ± 1.3 kJ mol1. In 1995, Wenthold and Squires [23], applied a kinetic method using a flowing afterglow apparatus to determine DacidH° of halogen substituted benzenes and benzene itself. They obtain for this latter DacidH°(benzene) = 1680.7 ± 2.3 kJ mol1 (by comparison they also obtain DacidH°(CH3OCH3) = 1703 ± 8.3 kJ mol1, i.e. exactly the value retained in the NIST tabulations [1]). This DacidH°(benzene) value is also in correct agreement with an estimate based on the thermochemical relationship (2). Accordingly, using BDE (C6H5AH) = 472.4 [[21] corrected by [26]], EA(C6H5) = 105.8 [25] and IE(H) = 1313.7 kJ mol1, one obtain DacidH°(benzene) = 1680.3 kJ mol1. Combination of these experimentally determined quantities with the entropy term 298 DacidS° = 37.1 kJ mol1 (G3B3 computation, corrected for symmetry numbers, see Table 1) lead to averaged values DacidH°(benzene) = 1678.5 kJ mol1 and DacidG°(benzene) = 1641.4 kJ mol1 with a standard deviation of 2.9 kJ mol1 (Table 1). It may be underlined that these averaged values exactly match those proposed by Ervin and DeTuri after careful revision of the gas-phase acidity scale [26]. 3.1.2. X = F This molecule has been examined by Meot-Ner and Kafafi [10] by pulsed high pressure mass spectrometry at variable temperature. van’t Hoff plots allows the authors to conclude that DacidH°(fluorobenzene) is higher than DacidH°(methanol) by 23.8(±8) kJ mol1 [10]. Using DacidH°(CH3OH) = 1596.2 ± 0.4 kJ mol1 [22], it may be deduced that DacidH°(fluorobenzene) = 1620.0(±8) kJ mol1. Ion cyclotron experiments were designed to determine the gas phase acidity of fluorobenzene from equilibrium constant determination [14,17]. It was found by Buker et al. [14] that DacidG° of fluorobenzene was 51.4 kJ mol1 lower than that of benzene. Using the averaged DacidG°(benzene) of 1641.4 kJ mol1 we conclude that DacidG°(fluorobenzene) = 1590.0 kJ mol1 with a probable uncertainty of ±4 kJ mol1. Similar experiments by Andrade and Riveros [17] show that fluorobenzene is 4 kJ mol1 more acidic than furan thus, using DacidG°(furan) = 1590.0 kJ mol1 [1], it results that DacidG°(fluorobenzene) = 1586.0 kJ mol1. Finally, it may be recalled that DacidH° of fluorobenzene in its ortho, meta and para positions have been estimated, using a kinetic method [23], to be 1620.0 ± 10.5, 1653.5 ± 8.4 and 1671.9 ± 3.8 kJ mol1, respectively. These results

clearly demonstrate a larger acidity of the ortho position, moreover the corresponding DacidH° is in excellent agreement with the above mentioned experimental values. 3.1.3. X = Cl Following Bartmess and McIver [1], DacidG°(Chlorobenzene) is situated between DacidG°(H2O) (i.e. 1605.3 kJ mol1, Ref. [26]) and DacidG°(CH3OH) (i.e. 1569.5 kJ mol1, Ref. [1]). Bracketing experiments allows Wenthold et al. [30] to show that meta and para positions of chlorobenzene have comparable acidities, situated between that of H2O and furan (i.e. DacidG° between 1605.3 [26] and 1590.0 kJ mol1 [31]). Similarly, the gas phase acidity of the ortho position has been bracketed between that of fluorobenzene and furan, leading thus to DacidG°(Chlorobenzene) = 1588.7 ± 1.8 kJ mol1. Four years later, the same authors used a kinetic method to determine the gas phase acidities of a series of aromatic compounds, among which chlorobenzene [23]. The resulting DacidH° of ortho, meta and para chlorobenzenes are equal to 1624.2 ± 8.4, 1635.9 ± 7.9 and 1650.2 ± 5.4 kJ mol1, respectively. Ion cyclotron resonance mass spectrometry has been also used in order to obtain information on the gas phase acidity of chlorobenzene [17,18]. Equilibrium constant determination demonstrates that chlorobenzene is more acidic than furan by 6.0 kJ mol1 thus leading to DacidG°(Chlorobenzene) = 1584.0 kJ mol1 [18]. This results is in reasonable agreement with the previous determination which provided DacidG°(Chlorobenzene) = 1578(±12) kJ mol1 [18]. A large set of experimental determinations of equilibrium constants show that chlorobenzene is more acidic than fluorobenzene by DG° = 2.3 ± 0.6 kJ mol1 [17]. It is satisfying to note that using the averaged DacidG° values of fluorobenzene (1587.4 kJ mol1, Table 1) and chlorobenzene (1584.6 kJ mol1, Table 1) a very close DG° value of 2.8 kJ mol1 is obtained. 3.1.4. X = OH Phenol is a well known oxygen base and its gas phase deprotonation thermochemistry has been experimentally explored by several research groups [11,12,15,24,25,27,28]. DacidG°(phenol) was deduced from equilibrium constant determination during HPMS [12] and ICR [15] experiments while energy resolved collision induced dissociation provided 0 K energies easily converted to 298 K thermochemical quantities [28]. HPMS results [12] demonstrate DacidG°(phenol) larger than that of acetic acid and propionic acid by 6.7 and 10.9 kJ mol1, respectively. Using DacidG°(acetic acid) = 1429 kJ mol1 and DacidG°(propionic acid) = 1424 kJ mol1 [1,11], we deduce DacidG°(phenol) = 1435.3 ± 1.7 kJ mol1 (error bar includes internal consistency and averaging standard deviation). Concerning ICR data [15], DG values for proton transfer reactions involving phenol and three reference acids: acetic acid, formic acid and benzoic acid can be considered. The three experiments converge toward a DacidG°(phenol) = 1431 kJ mol1 (with a possible error of ±4 kJ mol1 on the reference data). Finally, the relationship DacidH° = BDE(AH)  EA(A) + IE(H) lead to DacidH°(phenol) = 1459.0 ± 2.9 kJ mol1 (with BDE(C6H5OAH) = 362.8 ± 2.9 [29,41,42], EA(C6H5O) = 217.5 ± 0.6 [25] and IE(H) = 1313.7 kJ mol1) and consequently to DacidG°(phenol) = 1429.4 ± 2.9 kJ mol1. 3.1.5. X = NH2 Pulsed ICR experiments show that DacidG°(aniline) is higher than DacidG°(CH3CHO) by 0.8 kJ mol1 and lower than DacidG°(CH3COCH3) by 7.5 kJ mol1 [16]. Using the DacidG° of the reference acids from the ICR [16] source, an averaged DacidG°(aniline) values of 1504.7 ± 2.6 kJ mol1 is obtained. High pressure mass spectrometry at variable temperature provided relative DacidH°(aniline) with respect to phenyl-acetylene, 1-methyl-naphthalene and azulene [10,13]. Using the two most accurately known DacidH°

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values of phenyl-acetylene (1551 kJ mol1 [16]) and 1-methylnaphthalene (1552.7 kJ mol1 [13]), we derive DacidH°(aniline) = 1542.2 ± 2.4 [10] and 1538.5 ± 0.1 [13] kJ mol1. 3.1.6. X = CN Benzonitrile was studied by Meot-Ner and Kafafi by high pressure mass spectrometry [10]. The authors concluded that the DacidH° of benzonitrile is higher than DacidH° of methanol by 7.5 kJ mol1 [10]. Using DacidH°(CH3OH) = 1596(±4) kJ mol1 [1], it may be deduced that DacidH° of benzonitrile is equal to 1603(±4) kJ mol1. 3.1.7. X = CHO No entry for the gas phase acidity of benzaldehyde is available in the NIST database [1]. To the best of our knowledge, there is only one report on an ion cyclotron resonance mass spectrometry study concerning this molecule [19]. It is shown that all hydrogen atoms of benzaldehyde are equally acidic and that its gas phase basicity is situated between that of water and methanol, thus leading to the rough estimate DacidG°(benzaldehyde) = 1587 ± 18 kJ mol1. 3.1.8. X = NO2 High pressure mass spectrometry experiments were interpreted by a DacidH° of nitrobenzene 10.0 kJ mol1 higher than that of 1,1,1-trifluoroacetone [10]. The DacidH° of the latter is situated between 1465.6 kJ mol1 [11] and 1461.0 kJ mol1 [1]. Using an average DacidH°(1,1,1-trifluoroacetone) of 1463 kJ mol1 we derive DacidH°(nitrobenzene) = 1473 kJ mol1 (a surprisingly low value as will be discussed latter). 3.1.9. X = CH3 Earlier determination of the toluene acidity from equilibrium constant measurements by ion cyclotron resonance mass spectrometry [16] indicated that DacidG°(toluene) is lower than that of methanol by 1.3 kJ mol1 and higher than that of ethanol by 11.3 kJ mol1. Using DacidG°(methanol) = 1569.5 kJ mol1 and DacidG°(ethanol) = 1555.0 kJ mol1 [1,9], a value of DacidG°(toluene) = 1567.3 ± 1.0 kJ mol1 results. More recently, Gal et al. [20] obtained a significantly lower value of DacidG°(toluene) = 1557.3 kJ mol1, with a standard deviation of 2.8 kJ mol1. Proton exchange between toluene and methanolate ion has been studied in a flowing afterglow/selected ion flow tube instrument [22]. The free energy change is deduced from the ratio of rate constants and a value of DacidG°(toluene) = 1568.6 ± 0.8 kJ mol1 was obtained (using DacidG°(methanol) = 1569.5 kJ mol1). Finally, beside these experimentally derived data, the use of Eq. (2), with EA(benzyl radical) = 88.0 ± 0.6 kJ mol1 [5] and D298(C6H5CH2AH) = 375.7 ± 2.4 kJ mol1 [22], provided estimates of DacidH°(toluene) 1600.5 ± 2.5 kJ mol1 [22,25].

3.1.12. X = CCH The DacidG°(phenylacetylene) may be obtained from the DG° of proton transfer with acetone, acetonitrile and benzyl alcohol as reference acids [16]. Values of 3.3, 10.0 and 1.7 kJ mol1 were determined by ion cyclotron resonance spectrometry thus allowing to derive an average DacidG°(phenylacetylene) value of 1518.9 ± 1.0 kJ mol1.

3.2. Theoretical results Most of the theoretical results of the present study are summarized in Table 2 which contains G3B3 computed DacidH°298 and DacidG°298 corresponding to all the possible deprotonation sites of the molecules M. As expected, the most acidic hydrogens of phenol, aniline, toluene and ethylbenzene correspond to the OH, NH2 or CH benzylic positions due to the conjugation of the negative charge in the [MH] ions. Similarly, the strong acidity of the acetylenic hydrogen (i.e. bonded to a sp hybridized carbon atom) of ethynyl benzene is not surprising. Apart from these five particular situation the 45 remaining DacidH°298 values presented in Table 2 concern C(sp2)AH acidities. As a rule, the ortho position appears to be more acidic than the meta and the para positions, and the present computations confirm previous expectations [10,23,40]. The lone exception is the CH position of the vinyl group of styrene (see Scheme 1 for the nomenclature used for this molecule in Table 2). Particular structural features are observed for several orthodeprotonated [MH] species as displayed in Figure 1. For example, deprotonation of chlorobenzene results in a CACl bond elongation (from 1.761 to 1.973 Å) and an opening of the ClCC() angle in order to account for the charge–charge repulsion between the chlorine atom and the negatively charged carbon C(). No such effect is observed for fluorobenzene because of a stronger CAF bond. In the case of phenol and benzaldehyde, the two orthodeprotonated structures exhibit very different stabilities. The ortho forms are more stable than ortho0 (Figure 1) by 40 and 30 kJ mol1, respectively. This difference may be explained by the possibility, in the ortho forms, of a stabilizing interaction between the hydrogen atom of the OH or CHO groups and the negative charge while, on the contrary, the ortho0 forms present a repulsive interaction between the oxygen atom of either the OH or CHO groups and the negative charge (see Figure 1). Concerning the computed DacidH° and DacidG° presented in Table 2, comparison with the experimental values (Table 1) provide important findings. The first observation is that, excluding nitrobenzene from the comparison, experimental and theoretical DacidH° show an excellent linear correlation as illustrated by Figure 2. The mean absolute deviation (MAD) and mean signed deviation (MSD) calculated on DacidH°calc  DacidH°exp values (excluding nitrobenzene and benzaldehyde from the statistic because of the large uncertainties on the experimental data) are equal to 3.3

3.1.10. X = C2H5 Ethylbenzene has been subjected to proton transfer equilibrium measurement with CH3O anion in an ion cyclotron resonance cell [16]. From these experiments, it has been shown that the gas phase acidity of ethylbenzene was 2.9 kJ mol1 less than that of methanol and 1.7 kJ mol1 less than toluene thus leading to DacidG°(ethylbenzene) = 1566.1 ± 0.7 kJ mol1. 3.1.11. X = CHCH2 From their high pressure mass spectrometry experiments, Meot-Ner and Kafafi [10] concluded that styrene is less acidic than water by 0.8 kJ mol1. Using DacidH°(H2O) = 1632.9 kJ mol1 [26] we obtain DacidG°(styrene) = 1633.7 kJ mol1.

Scheme 1.

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G. Bouchoux / Chemical Physics Letters 506 (2011) 167–174

Chlorobenzene

Phenol

neutral

o-deprotonated

o'-deprotonated

neutral

o-deprotonated

Benzaldehyde o'-deprotonated

neutral

o-deprotonated

Figure 1. Geometries of selected neutral and ortho deprotonated molecules M.

1700

H

C6H5X: gas phase acidities (ΔacidH°) 1650

C2H3

Δacid H° exp (kJ/mol)

CHO F Cl CN CH3 C2H5

1600

1550

CCH

NH2

1500

(NO2) OH 1450 1450

1500

1550

1600

1650

1700

Δacid H° calc (kJ/mol) Figure 2. Experimental vs. G3B3 slope = 0.9986 ± 0.023, excluding NO2).

calculated

DacidH°

(Pr = 0.9977,

and 3.7 kJ mol1, respectively. These average deviations are close to the standard deviation on experimental data (Table 1) and thus no significant systematic error is seemingly operative.

From examination of Figure 2, it clearly appears a large disagreement between experimental (1474 kJ mol1, Table 1) and computed (1595 kJ mol1 Table 2) DacidH°(nitrobenzene) values. Expectation based on chemical sense would predict DacidH° of nitrobenzene close to that of a benzene substituted by comparable electron withdrawing group. For example Hammet’s r constants of NO2 and CN are very close together (0.80 and 0.71, respectively [33]) and thus a DacidH° of nitrobenzene slightly below that of benzonitrile (i.e. 1603 kJ mol1, Tables 1 and 2) is expected and indeed a value of 1595.4 kJ mol1 is computed. This observation, and argument based on the following paragraph, suggest that the experimental DacidH° (and DacidG°) of nitrobenzene is significantly erroneous and should be revised. It must be recalled that the comparison of experimental and theoretical DacidH° illustrated by Figure 2 involves the most acidic sites and consequently different positions on the molecular structure depending upon X. If we restrict the discussion to the CAH acidities, an interesting observation emphasized above is that the ortho position appears to be more acidic than meta and para. This difference may be tentatively explained by a better stabilizing effect of the substituent X when it is closer to the negatively charged site, i.e. when it is situated in ortho. This expectation may be quantified by using the Taft’s substituent acidity constants rR, rF and ra which include resonance, field-inductive and polarizability effects [34]. If the ortho proximity argument is correct, a linear correlation would be observed between DacidH° (or DacidG°) and the field-inductive constants rF. Figure 3 shows that it is indeed the case for both the theoretical and experimental DacidH°. The only

G. Bouchoux / Chemical Physics Letters 506 (2011) 167–174

1680

Δacid H° calc (kJ/mol)

1660

H CH3 C2H5

C6H5X: acidity of the ortho position

NH2

OH ortho'

C2H3 CCH

CHO ortho'

173

of nitrobenzene can be overestimated by 5–11 kJ mol1 and that a better DfH°(nitrobenzene) value will be 56 ± 5 kJ mol1 [39]. The results presented in Table 3 are in line with this proposal. Concerning ethynylbenzene the experimental enthalpy of formation is lower than the calculated G3B3 value by no less than 11.3 kJ mol1. Here also it may be suggested that the experimental value, which is originating from only one experimental study [36], should be revised.

1640

4. Conclusion

Cl F OH ortho

1620

CHO ortho 1600

CN

G3B3 calculations Experiment

0.0

0.2

NO2

σF

0.4

0.6

Figure 3. Calculated DacidH° of the ortho positions vs. Taft’s field-inductive parameter rF (Pr = 0.985) (note that the experimental point for nitrobenzene is not included in the figure).

exception is nitrobenzene for which the experimental point would appear 120 kJ mol1 below the correlation line. Symmetry corrected entropies and heats of formation computed by the atomization method are presented in Table 3 and compared with experimental values. There is a clear agreement between the G3B3 computed values and experiment. This may be seen from the average signed and absolute deviations: for entropies MSD and MAD amount for only 1.6 and 2.2 J K1 mol1, respectively. For the heats of formation these quantities are equal to MSD = 0.9 kJ mol1 and MAD = 3.4 kJ mol1. We note however that deviations in heats of formation are close to 10 kJ mol1 for nitrobenzene and ethynylbenzene (Table 3). Excluding these two compounds the MAD is reduced to 2.0 kJ mol1. The large differences observed between computed and experimental DfH° for nitrobenzene and ethynylbenzene may be questioned. It has been recently suggested that the experimental enthalpy of formation

In summary, the present work presents G3B3 calculations of DacidH° and DacidG° of a series of aromatic molecules of general formula C6H5X (X = H, F, Cl, OH, NH2, CN, CHO, NO2, CH3, C2H5, CHCH2, CCH). These theoretical results compare within 4 kJ mol1 with carefully re-evaluated experimental data. This systematic computational study reveals that the experimental DacidH° and DacidG° values of nitrobenzene are manifestly erroneous (the tabulated values [1] are underestimated by ca. 120 kJ mol1). Several other experimental DacidH° and DacidG° are tainted with large, or unknown, uncertainties. This is the case of X = CN, CHO, C2H5, C2H3 and C2H. The present G3B3 calculations provide new estimates which increase the precision on gas phase acidity data previously derived from experimental results. In particular, the values DacidH°(benzaldehyde) = 1614 ± 4 kJ mol1 and DacidG° (benzaldehyde) = 1682 ± 4 kJ mol1 can be safely proposed while only a bracketing into a ±18 kJ mol1 range was until now available [19]. For X = F, Cl, CN, CHO and NO2, the most favorable deprotonation site is the ortho position of the phenyl ring. This regio-specificity is directly related to the field-inductive effect of the substituent as attested by the linear correlation observed between DacidH°(ortho) and the Taft’s rF constants. Two other important thermochemical parameters: third law entropies and heats of formation, were also investigated at the G3B3 level. For entropy, the agreement with experiment is excellent since the average deviation doesn’t exceed 2.2 J K1 mol1. Concerning the heats of formation, the average deviation between G3B3 and experimental DfH° is as low as 2.0 kJ mol1 if nitrobenzene and ethynylbenzene are exclude from the statistic. For these two molecules absolute deviations attain values close to 10 kJ mol1. This large difference suggests a revision of their experimentally determined heats of formation. References

Table 3 Experimental and G3B3 calculated entropies (J mol1 K1) and heats of formation (kJ mol1) of the studied molecules M.

a

M

S°298 calc.

S°298 exp.

DfH° calc.a

DfH° exp.

Benzene Fluorobenzene Chlorobenzene Phenol Aniline Benzonitrile Benzaldehyde Nitrobenzene Toluene Ethylbenzene Styrene Ethynylbenzene

270.3 304.2 316.0 314.8 319.1 326.1 335.7 346.2 325.3e 358.6 348.3 331.2

269.2 302.6 313.5 315.6 319.2 321.0 336.0 343.1 320.7 360.6 345.1 –

85.2 113.7 52.4 92.7 89.2 213.2 39.0(37.6)d 57.5 52.2 30.4 149.3 317.9

82.6 ± 0.7b 116 ± 1.4b 52.0 ± 1.3b 96.4 ± 0.9b 87 ± 1.0b 215.7 ± 2.1b 36.7 ± 2.9b 67.5 ± 0.6b 50.4 ± 0.6b 29.9 ± 1.1b 147.9 ± 1.5b 306.6 ± 1.7c

G3B3 calculations using the atomization energies method (see text). Ref. [35]. c Ref. [36]. d G3B3 calculations combined with homodesmotic reactions, Ref. [37]. e Using a contribution to S° of 14.8 J mol1 K1 for the internal rotation considered as a free rotor as given in Ref. [38]. f Ref. [32] b

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