Experimental and Computational Thermochemical Study of Maleic

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Experimental and Computational Thermochemical Study of Maleic Anhydride and Vinylene Carbonate Clara Sousa, M. Agostinha R Matos, and Victor M.F. Morais J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07175 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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The Journal of Physical Chemistry

Experimental and Computational Thermochemical Study

of

Maleic

Anhydride

and

Vinylene

Carbonate Clara Sousa,1,2 M. Agostinha R. Matos,1 Victor M. F. Morais1,3 1

CIQ – Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, Porto, Portugal

2

3

LAQV/REQUIMTE, Departamento de Ciências Químicas, Faculdade de Farmácia, Universidade do Porto

ICBAS – Instituto de Ciências Biomédicas Abel Salazar, Universidade do Porto, Porto, Portugal

ACS Paragon Plus Environment

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ABSTRACT: The standard molar enthalpies of formation of maleic anhydride and vinylene carbonate in gaseous phase, at T = 298.15 K, were derived from the standard molar enthalpies of formation of the compounds in condensed phase combined with the phase transition enthalpies. The standard molar enthalpies of formation in condensed phase were obtained from the enthalpies of combustion measured using mini-bomb combustion calorimetry and static bomb combustion calorimetry for vinylene carbonate and maleic anhydride, respectively. Phase transition enthalpies were obtained by Calvet microcalorimetry. High level quantum calculations were performed at the composite G3 level of theory in order to estimate the standard molar enthalpies of formation of both compounds in gaseous phase. Good agreement was obtained between experimental and computational results. In addition, analysis of the factors affecting the relative stability of both systems has been carried out in the framework of the ab initio Valence Bond (VB) theory in order to clarify the aromaticity / antiaromaticity issues involving these molecular systems.


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1. INTRODUCTION The vinylene carbonate/maleic anhydride pair meets the formal structural change requirement, viz. the inversion of the carbonyl and the ether fragments, which has been 1

conjectured to result in dramatic interchange between putatively antiaromatic and aromatic character. It was in the context of the study of such structural and stability interplay that we found that the available experimental data for the vinylene carbonate evidenced large discrepancy with our computational chemistry calculations, thus prompting us to conduct a detailed experimental and computational study of this pair of molecules.

Vinylene carbonate

Maleic anhydride

The present work reports the results of calorimetric and computational thermochemistry of maleic anhydride (2,5-furandione or cis-butenedioic anhydride) and vinylene carbonate (1,3-dioxol-2-one or cyclic vinylene ester). Maleic anhydride is known for its importance as a co-monomer in the synthesis of polyester resins, surface coatings, lubricant additives and plasticizers and as a preservative in oils and fats.

2-4.

Vinylene carbonate was tested with 5

success as an additive to electrolyte solutions for Li-ion batteries.

In the experimental part of the work we performed the determination of the standard molar enthalpies of combustion, in oxygen, at T= 298.15 K, using a static bomb calorimeter for vinylene carbonate and a mini-bomb calorimeter for maleic anhydride. The phase transition standard molar enthalpies of the compounds were assessed with a Calvet microcalorimeter.


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These values allowed the calculation of the standard enthalpies of formation in the gas phase, at T = 298.15 K. The computational part of this work includes the estimation of the standard molar enthalpies of formation in gas phase using high level ab initio quantum calculations.

2. EXPERIMENTAL DETAILS 2.1. Materials and purity control. Both compounds were obtained from Aldrich Chemical Co. at a stated mass fraction of 0.990 and 0.970 for maleic anhydride (CAS 108-31-6) and vinylene carbonate (CAS 872-36-6), respectively. Maleic anhydride was purified by sublimation and vinylene carbonate by distillation under reduced pressure. Their final molar fraction purity was found to be better than 0.999 by gas chromatography performed on an apparatus Agilent 4890D equipped with an HP-5 column, cross-linked (5% diphenyl and 95% dimethylpolysiloxane) and a flame ionization detector, using nitrogen as carrier gas. The purity of vinylene carbonate was also confirmed through the carbon dioxide recovery ratio obtained in combustion experiments. Actually the average ratio, together with the standard deviation of the mean, of the mass of carbon dioxide recovered to that calculated from the mass of sample of vinylene carbonate used in each combustion experiment was 0.99980 ± 2 x 10-5 (data collected in table 1). We took special care to store the title compounds in nitrogen atmosphere at low temperature conditions (T ~278 K) and to conduct experiments as soon as possible after purification, in order to avoid spontaneous polymerization. The values assumed for the density, at T= 298 K, of maleic anhydride and vinylene carbonate are, respectively, 1.4980 g·cm-3

6

and 1.355 g·cm-3.

7

The atomic weights of the elements were those 8

recommended by the IUPAC commission in 2016.


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2.2. Static bomb combustion calorimetry. The standard massic energy, ∆ , of vinylene carbonate in the liquid phase was obtained from combustion calorimetry with a static bomb 9,10

calorimeter. The apparatus and technique have been described previously.

The energy

equivalent of the static bomb calorimeter, = (15995.3 ± 2.0) J·K-1, was obtained from calibration experiments using benzoic acid (NIST SRM 39j) with a certified massic energy of combustion of −(26434 ± 3) J·g-1 under bomb conditions. The calibration results were corrected to give the energy equivalent corresponding to the average mass of water added to the calorimeter, 3119.6 g. The compound was enclosed in bags made of Melinex®, using 11

the technique described by Skinner and Snelson

who determined the massic energy of

combustion of dry Melinex® as ∆ = −(22902 ± 5) J·g-1. This value was confirmed in our laboratory. The mass of Melinex used in each experiment was corrected for the mass fraction of water (0.0032) and the mass of carbon dioxide produced from it was calculated using the 11

factor previously reported.

Combustion experiments were performed in oxygen at p = 3.04 MPa, with 1.00 cm3 of water added to the bomb. The bomb was previously purged twice with oxygen at 1.5 MPa. For 12 the cotton-thread fuse, empirical formula CH1.686O0.843, ∆ = −16250 J⋅g-1 was used. This

value has been confirmed in our laboratory. The energy corrections for nitric acid formation, ∆ HNO , were based on the value −59.7 kJ⋅mol-1

13

for the molar energy of formation of

0.1 mol⋅dm-3 HNO3 (aq) from N2 (g), O2 (g) and H2O (l). The mass of compound, m(compound), used in each experiment was determined from the total mass of gaseous carbon dioxide, m(CO2, total), produced after allowance for that formed from the cotton thread fuse and Melinex. An estimated pressure coefficient of specific energy: ⁄ = −0.2 J ∙ g ∙


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MPa, at T = 298.15 K, a typical value for most organic compounds, was assumed.

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14

As

samples were ignited at T = 298.15 K, ∆ IBP = −$ + ∆& H' O ∙ () H' O, + + , -∆./ + ∆ 012

(1)

where ∆U(IBP) is the energy associated to the isothermal bomb process; ∆m(H2O) is the deviation of the quantity of water from the average value, 3119.6 g, to which calibration experiments refer; cp(H2O, l) = 4.18 J.g-1. K-1 is the heat capacity of liquid water, at T = 298.15K; εf is the energy of the bomb contents after ignition and ∆Tad is the adiabatic 15

temperature raise calculated using the program LABTERMO. To derive the standard massic energy of combustion, ∆cuo, the corrections to the standard state, ∆UΣ, were made by the procedure given by Hubbard et al.

16

2.3. Mini-bomb combustion calorimetry. The standard massic energy of maleic anhydride in crystalline phase was obtained from a mini-bomb combustion calorimeter (aneroid isoperibol calorimeter). A detailed description of the calorimetric system can be found in 17

literature

so only a brief description of the main components and procedure will be given.

The mini-bomb is made of stainless steel with 18.185 cm3 internal volume and electrodes and crucible support of platinum. The compound was burnt in a pellet form contained in a Melinex bag. The platinum crucible containing the pellet was placed in its support in the head of the mini-bomb with a 20 mm long platinum wire attached to each electrode terminal for electrical discharge and sample ignition. A platinum sheet was placed with an inverted ‘‘U shape” above the crucible to promote complete combustion. A volume of 0.050 cm3 of water was placed inside the body of the mini-bomb and the head with a Viton O-ring was then tightly adjusted to it by means of a stainless steel screw-ring. The bomb was three-times filled with


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ultra-pure oxygen and purged before being filled with oxygen at a pressure of 3.04 MPa. Afterwards, the bomb was introduced in a cylindrical copper block. The block was sealed, evacuated and filled with ultra-pure helium up to 0.2 MPa. This copper block is confined in a cylindrical cavity surrounded by a 9 dm3 thermostatic water bath, regulated at constant temperature T = 298.420 ± 0.001 K by a TRONAC temperature controller (model PTC-40). A multimeter (Keithley, model 2000) interfaced to a PC was used for data acquisition containing a bean type thermistor (R = 4 kΩ at T = 298.15 K) for temperature measurement. The charging, firing circuit and voltage measurement across the 2000 µF discharger condenser were done automatically by means of a set of Advantech acquisition/automation modulus, series 4000. The electrical energy for ignition, ∆ 345 , was determined from the change in potential difference across a capacitor when discharged through the platinum ignition wire. Samples were ignited at T = 298.15 K, and the energy associated to the isothermal bomb process, ∆ IBP, was calculated including the ignition energy. The energetic corrections for carbon formation ∆ carb were based on the massic energy of combustion, ∆ = -33 kJ·g1 12

.

The adiabatic temperature raise ∆.9: was calculated using the program LABTERMO.

15

The energy equivalent of the mini-bomb combustion calorimeter, = 1946.71 ± 0.84 J.K-1, was obtained from calibration experiments, made with benzoic acid (NIST SRM 39j) with a certified massic energy of combustion of −(26434 ± 3) J·g-1, under bomb conditions.

2.4. Calvet microcalorimetry. The standard molar enthalpies of sublimation of maleic anhydride and of vaporization of vinylene carbonate were measured using the “vacuum sublimation” drop microcalorimetric method.

18,19

The microcalorimeter was calibrated


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previously for these measurements using the reported enthalpy of sublimation of 20

naphthalene and of vaporization of n-undecane.

21

Samples, of about 3 to 5 mg of the crystalline maleic anhydride or of 7 to10 mg of the liquid vinylene carbonate, contained in thin glass capillaries tubes sealed at one end, were dropped, at room temperature, into the hot reaction vessel, in a high temperature Calvet microcalorimeter (SETARAM HT 1000D) held at T = 319 K for maleic anhydride and at T = 320 K for vinylene carbonate and then removed from the hot zone by vacuum evaporation. The thermal corrections for the glass capillary tubes were determined in separate experiments and were minimized, as far as possible, by dropping tubes of nearly equal mass into each of the twin calorimeter cells. From six independent experiments for each compound a mean value was obtained for the observed standard molar enthalpy of sublimation and vaporization, 1, ?

∆;,,' ? @A , which was then corrected to T = 298.15 K, ∆'B HA , using the following

equation: ?

gD. ∆'B @A

g = C' ? (),A

(2)

g is the molar heat capacity of the where . is the temperature of the hot reaction vessel, (),A

compound in the gas phase and was obtained from statistical thermodynamics using the vibrational frequencies obtained from the RHF/6-31G(d) calculations after being properly 22

scaled by the factor 0.8929 to properly account for anharmonicity and correlation effects: Maleic anhydride g⁄ J (),A ∙ mol ∙ K = −0.000212 .⁄K' + 0.361 .⁄K − 2.102

(R2 = 0.999)

(3)

Vinylene Carbonate g⁄ J (),A ∙ mol ∙ K = −0.000189 T⁄K' + 0.329 T⁄K − 9.678

(R2 = 0.997)

(4)


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3. COMPUTATIONAL DETAILS Optimized geometries have been obtained for all molecules using density functional theory 23

24

(DFT) with the Becke three-parameter hybrid exchange and the Lee-Yang-Parr 25,26

functionals (B3LYP) together with the 6-31G*

and the cc-pVTZ+d

27-31

correlation correlation

consistent basis set. The optimum structures so obtained have further been confirmed as true minima by constructing and diagonalizing the corresponding Hessian matrices. These geometries were used as reference structures in further single-point calculations of the magnetic properties as well as in the Valence-Bond calculations. The very accurate energy calculations used to obtain reliable estimates of the energetics of the studied molecules have 32

been carried out at the composite G3 level. All ab initio molecular orbital calculations have been performed using the Gaussian 03 series of programs calculations were conducted using the XMVB program 36

initio package.

34,35

33

while the ab initio VBSCF

coupled to the GAMESS-US ab-

Analyses of the wavefunctions in the context of the Bader’s atoms in

molecules (AIM37) theory have been conducted using the AIMALL38 computer program, while analyses and plots of the Fermi (exchange) correlation have been carried out with 39

Multiwfn program.

Unless otherwise stated the experimental enthalpies of formation of all

auxiliary compounds are taken from the Pedley’s compendium.

40


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4. RESULTS AND DISCUSSION 4.1. Experimental gas-phase enthalpy of formation. The values of the standard massic energy of combustion, ∆cu0, refer to the combustion reaction 5 for maleic anhydride and reaction 6 for vinylene carbonate:

C 4 H 2 O3 (cr) + 3O 2 (g) → 4CO2 (g) + H 2 O(l)

(5)

C3 H 2 O3 (l) + 2O 2 (g) → 3CO2 (g) + H 2 O(l)

(6)

The individual results of all combustion experiments for the compounds, together with the mean values and their standard deviations of the mean (0.68 level of confidence) are given in Table 2. Table 3 lists the derived standard molar energies and enthalpies of combustion and the standard molar enthalpies of formation for the compounds in the condensed phase, at T = 41

298.15 K. In accordance with normal thermochemical practice, the uncertainties assigned to the standard molar enthalpies of combustion are, in each case, twice the overall standard deviation of the mean (0.95 level of confidence), and include the uncertainties (standard deviations of the mean) in calibration and in the values of auxiliary quantities. To derive cr, ∆, HA l from ∆c @0m cr, l the following standard molar enthalpies of formation, at T =

298.15 K, were used for H2O(l), –(285.830 ± 0.042) kJ.mol-1 ;

kJ.mol-1.

42

CO2(g) and –(393.51 ± 0.13)

42

Table 1. Source, purification and analysis of the compounds studied


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Chemical name

supplier

Initial purityª

Purification method

Final mass fraction

Maleic anhydride

Aldrich Chemical Co

0.990

Sublimation

>0.999

Vinylene carbonate

Aldrich Chemical Co

0.970

Distillation

>0.999 0.99980 ± 2x10-5c

a

Values referred to GC analysis, as stated in the certificates of analysis of manufacturer

b

Determined in the present work using GC

c

Determined in the present work by CO2 recovery after combustion experiments

Table 2. Combustion Experiments of Maleic anhydride and Vinylene carbonate, at T = 298.15 K Maleic anhydride 1

2

3

4

5

6

m(cpd)/ g

0.02142

0.01992

0.01923

0.01921

0.02026

0.01600

m(melinex)/ g

0.00624

0.00571

0.00627

0.00649

0.00587

0.00633

∆Tad/ K

0.23025

0.21302

0.21461

0.21659

0.21750

0.19122

εf / (J⋅K-1)

0.7964

0.7932

0.7934

0.7938

0.7940

0.7890

−∆U(IBP)/ J

447.46

413.98

417.04

420.95

422.67

371.46

∆U(melinex)/ J 4142.91

130.77

143.60

148.63

134.43

144.97

∆U(carb)/ J

0.12

0.31

---

0.89

∆U(ign.)/ J

0.95

0.88

0.91

0.86

0.91

0.94

∆UΣ/ J

0.50

0.46

0.46

0.46

0.47

0.40

-∆cu0 / (J·g-1)

14200.28

14209.84

14195.68

14198.33

---

14203.85

1.06

14196.88

−/ (J.g-1) = 14200.8 ± 2.2


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Vinylene carbonate 1

2

3

4

5

6

m(CO2, total)/ g

1.21210

1.23092

1.23851

1.21550

1.21217

1.24090

m(cpd)/ g

0.70722

0.71737

0.72268

0.70920

0.70726

0.72318

m(fuse)/ g

0.00270

0.00300

0.00279

0.00289

0.00264

0.00279

m(melinex)/ g

0.05353

0.05473

0.05319

0.05239

0.05448

0.05240

∆Tad/ K

0.60767

0.61767

0.61841

0.60751

0.60918

0.61845

εf / (J⋅K-1)

15.58

15.60

15.49

15.02

15.03

15.81

∆m(H2O)/ g

0.0

0.0

0.0

0.0

0.0

0.0

−∆U(IBP)/ J

9728.41

9888.53

9900.30

9725.42

9752.34

9901.05

∆U(fuse)/ J

43.85

48.72

45.29

46.98

42.98

45.39

∆U(melinex)/ J

1225.85

1253.49

1218.25

1199.85

1247.55

1200.01

∆U(HNO3)/ J

2.10

2.10

1.98

1.87

2.35

2.15

∆U(ign.)/ J

0.91

0.91

0.95

1.05

0.89

0.99

∆UΣ/ J

12.70

12.91

12.55

11.99

12.03

12.87

-∆cu0 / (J·g-1)

11939.63 11948.23 11930.91 11935.63 11943.86 11948.15 −/ (J.g-1) = 11941.1 ± 2.9

the mass of melinex used in the experiment; m(CO2, total) is the mass of recovered CO2 after experiment; ∆Tad is the corrected temperature rise; εf is the energy equivalent of contents in the final state; ∆U(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; ∆U(IBP) includes the ignition energy, ∆U(ign.); ∆U(melinex) is the energy of combustion of melinex; ∆U(carbon) is the energy correction for carbon formation; ∆UΣ is the energy correction to the standard state; ∆U(carbon) is the energy correction for the carbon eventually formed and present inside the crucible; ∆cu° is the standard massic energy of combustion.


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Table 3. Derived standard (p = 0.1 MPa) molar values, for the energies and enthalpies of combustion, ∆ A and ∆ @A , and for the enthalpy of formation of the compounds in cr, l, at the reference temperature, T = 298.15 K condensed phase, ∆, @A −∆ A / kJ.mol-1

−∆ @A / kJ.mol-1

cr, ∆, @A l / kJ.mol-1

Maleic anhydride (cr)

1392.5 ± 1.4

1390.0 ± 1.4

469.9 ± 1.5

Vinylene carbonate (l)

1027.5 ± 0.8

1025.0 ± 0.8

441.4 ± 0.9

Compound

1

, of vinylene carbonate, and Results of the standard molar enthalpies of vaporization, ∆ @A 1

of sublimation, ∆; @A , of maleic anhydride are collected in tables 4 and 5, respectively, where

the uncertainties assigned to the mean values are in each case twice the standard deviations of the mean (0.95 level of confidence), and include the uncertainties in calibration (standard deviation of the mean).


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Table 4. Standard (p°= 0.1 MPa) molar enthalpy of vaporization of vinylene carbonate, at T = 298.15 K .

m

1, ? ∆,' ? @A

T

Exp.

?

gD. P (),A

' ?

1

∆ @A (298.15 K)

mg

K

kJ.mol-1

kJ.mol-1

kJ.mol-1

1

7.034

320.2

53.40

1.64

51.76

2

5.492

320.2

52.21

1.64

50.57

3

6.100

320.2

51.23

1.64

49.59

4

5.911

320.2

52.40

1.64

50.76

5

6.560

320.2

51.85

1.64

50.21

6

6.740

320.2

52.39

1.64

50.75

1

∆ @A (298.15 K) = 0.61 ± 0.76 kJ.mol-1

m is the mass of compound which is heated from T= 298.15 K to T =T and then vaporized at temperature, 1, ? T, leading to the observed molar enthalpy ∆,' ? @A which is corrected using the integral of heat capacity ?

of the gaseous compound C' ? Q) gD. .

Combining the values of the standard molar enthalpies of formation of the compounds in condensed phase and the corresponding values for the enthalpies of sublimation (maleic anhydride) and of vaporization (vinylene carbonate), the values of the standard molar enthalpies in the gaseous phase were derived. The derived standard molar enthalpies of formation, in both condensed and gaseous phases and the standard molar enthalpies of phase transition, at T = 298.15 K, are summarized in table 6.


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Table 5. Standard (p° = 0.1 MPa) molar enthalpy of sublimation of maleic anhydride, at T = 298.15 K

m Exp.

4, B ∆R,' B @S

T

?

gD. P (),A

' ?

1

∆; @A (298.15 K)

mg

K

kJ.mol-1

kJ.mol-1

kJ.mol-1

1

6.185

319.0

70.78

1.87

68.91

2

5.898

318.9

69.45

1.86

67.58

3

5.210

319.0

71.84

1.87

69.97

4

5.670

318.9

70.47

1.86

68.60

5

5.433

318.9

70.82

1.86

68.95

6

5.397

319.0

70.94

1.87

69.07

1

∆; @A (298.15 K) = 68.85 ± 0.85 kJ.mol-1

m is the mass of compound which is heated from T= 298.15 K to T =T and then vaporized at temperature, 1, ? which is corrected using the integral of heat capacity T, leading to the observed molar enthalpy ∆,' ? @A ?

of the gaseous compound C' ? Q) gD. .

Table 6. Derived standard (p° = 0.1 MPa) molar enthalpies of formation of the compounds in gaseous state, at T = 298.15 K 1

Compound

−∆f @0m cr, l

∆;, @A

−∆f @0m g

kJ.mol-1

kJ.mol-1

kJ.mol-1

Maleic anhydride

469.9 ± 1.5

68.85 ± 0.85

401.0 ± 1.7

Vinylene carbonate

441.4 ± 0.9

50.61 ± 0.76

390.8 ± 1.2


15


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Page 16 of 49

4.2. Computational Results and Discussion. As stated earlier both molecular systems we considered in this work are structurally interrelated by formally reversing their carbonyl and ether groups, a procedure which converts the putatively aromatic 6 π-electrons vinylene carbonate into a putatively antiaromatic 4 π-electrons, maleic anhydride. Our experimentally measured enthalpy of formation for vinylene carbonate, -(390.8 ± 1.2) kJ.mol-1, disagrees from previously reported experimental values: −418.61 kJ.mol-1 −418.6 ± 3.0) kJ.mol-1.

40

43

or

The discrepancy between our standard enthalpy of formation and the

previous estimate for vinylene carbonate amounts to 27.8 kJ.mol-1, a discrepancy which can be ascribed simultaneously to a difference of 18.5 kJ.mol-1 in the liquid combustion enthalpy mesurements and to a difference of 9.3 kJ.mol-1 in the standard molar enthalpy of vaporization. On the other hand, the standard gas-phase enthalpy of formation for maleic anhydride we obtained in the present work −(401.0 ± 1.7) kJ.mol-1, is very consistent with the experimental 40

data previously reported in Pedley’s tabulations,

−(398.3 ± 5.1) kJ.mol-1, but our

measurements considerably narrow the experimental error bar. To support the reliability of our experimentally derived values, we considered a number of chemical schemes involving the considered systems, as well as other, well studied experimentally, auxiliary systems. Such available experimental data combined with the highly accurate G3 composite method may hopefully provide very reliable estimates of the thermochemical properties of both maleic anhydride and vinylene carbonate. We started with the following reactions relating maleic anhydride and vinylene carbonate with some auxiliary species:


16

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(7)

(8)

(9)

(10)

(11)

Using the calculated G3 absolute enthalpies for all molecules and the available experimental data (Table 7) we got the estimates −404.4 kJ.mol-1, −405.6 kJ.mol-1 and −394.9 kJ.mol-1 for maleic anhydride, all these values being in good agreement with our own experimentally measured enthalpy of formation. For vinylene carbonate we similarly obtained the estimates −382.1 kJ.mol-1 and −389.0 kJ.mol-1, also in good agreement with our experimental data


17


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Page 18 of 49

Table 7. Experimental Gas-Phase Enthalpies of Formation and G3 Absolute Enthalpies, at T = 298,15 K

Compound Maleic anhydride (2,5-furandione) Vinylene carbonate (1,3-dioxol-2-one ) Succinic anhydride (dihydro-2,5-furandione) 1,3-dioxolan-2-one 4-methyl-1,3-dioxol-2-one 4-methylene-1,3-dioxolan-2-one 3-methylfuran-2,5-dione 3-methylenedihydrofuran-2,5-dione Acetic Anhydride Propanoic anhydride 1,2,4,5-Benzenetetracarboxylic dianhydride Diethylcarbonate Vinyl Hydrogen Carbonate Acrylic Formic Anhydride Diphenylcarbonate Benzoic Anhydride Tetramethoxymethane Tetraethoxymethane Methane Ethane Ethene Formaldehyde Butane (Z)-2-Butene Methanol Methoxymethane Propene Vinyl Alcohol Carbonic Acid Acetic Acid Cyclopentane Cyclopentene Cyclopentanone Indane 2-Indanone 1,3-Benzodioxole Benzofuran-2(3H)-one (2-coumaranone) 2H-pyran-2-one Benzene Acetone H(2S) C(3P) O(3P) H2 * 1 EH = 2625.50184 kJ. mol-1.

−∆, @A /kJ. mol

@W /EY

-527.9 ± 1.7 -503.0 ± 4.243; -508.4 ± 2.5 -572.5±1.5 -626.5 ± 2.1 -826.8 ± 3.144 -637.9 ± 0.7; -642.4 ± 2.243 -311. ± 8.843 -319. ± 4.5 -727.2 ± 1.543; -727.643 -860.7 ± 2.1 -74.4 ± 0.4 -83.8±0.3 52.5 ± 0.3 -108.6 ± 0.5 -125.6 ± 0.6 -7.7 ± 1.0 -201.5 ± 0.2 -184.1 ± 0.5 20.0 ± 0.7 -128. 43 -432.8 ± 2.5 -76.4 ± 0.7 34.0 ± 1.4 -192.1 ± 1.8 60.3 ± 1.7 -56.6 ± 4.853 -142.7 ± 2.9 -213.6 ± 4.154 82.6 ± 0.7 -217.1 ± 0.7 217.998 716.68 249.18 0.0

-379.080668 -340.980275 -380.291205 -342.187536 -380.261500 -380.254840 -418.361385 -418.353764 -381.474286 -460.015273 -833.200435 -421.924177 -342.175172 -380.247767 -726.579858 -764.681582 -498.247978 -655.344855 -40.453810 -79.718911 -78.503412 -114.427242 -158.257943 -157.048199 -115.624919 -154.880574 -117.777080 -153.693789 -264.862470 -228.934397 -196.336060 -195.130097 -270.342069 -348.677925 -422.680342 -420.486919 -458.609790 -343.141820 -232.046743 -192.990776 -0.498642 -37.825356 -75.028630 -1.164074


18

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The standard methodology for obtaining estimates of the enthalpy of formation in the context of Gaussian Gn theories involves atomization enthalpies; at the G3 level we evaluated these enthalpic variations as 3730.92 kJ.mol-1 and 4453.03 kJ.mol-1, respectively for the carbonate and the anhydride, which, combined with the experimental enthalpies of formation for the atomic species, allowed deriving the estimates of the enthalpy of formation of vinylene carbonate, −397.4 kJ.mol-1, and of maleic anhydride, −402.8 kJ.mol-1. Regardless of recognizing that atomization processes are as far as possible from meeting the isodesmicity criteria required for plausible cancellation of the calculational errors, we nevertheless obtain remarkable agreement between those estimates and our experimental observations. In order to further support our experimental measurements we additionally considered the pair of homologous non-isodesmic hydrogenation reactions:

(12)

(13)

We note the quality of the G3 calculations by observing that the first reaction is exothermic by 112.2 ± 4.3 kJ.mol-1, a value which must be compared with the G3 reaction enthalpy of


19


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Page 20 of 49

−113.4 kJ.mol-1, while the second is exothermic by 126.9 ± 2.4 kJ.mol-1, the G3 reaction enthalpy being −122.0 kJ.mol-1. Other possible reactions involving vinylene carbonate can be profitably considered in our comparisons,

(14)

(15)

since the required gas-phase experimental data of both orthocarbonates is available. 43 We thus obtain from our G3 calculations, the reaction enthalpies −315.88 kJ.mol-1 and −217.82 kJ.mol1

, respectively for the first and the second of those reactions. These estimates must be

compared with the experimentally observed reaction enthalpies, −314.2 ± 2.5 kJ.mol-1 and −217.3 ± 2.8 kJ.mol-1, evidencing once again a remarkably good agreement between both sets of data. Diethylcarbonate provides additional experimental data for the following reaction:

(16)


20

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Using any of the two experimental values reported in Pedley’s and NIST tabulations for the enthalpy of formation of diethylcarbonate, −637.9 ± 0.8 kJ.mol-1 and −642.4 ± 2.2 kJ.mol-1, together with our own value for vinylene carbonate we got for this reaction the exothermicity values 27.0 ± 1.4 kJ.mol-1 and 31.5 ± 2.5 kJ.mol-1, both being in close agreement with the G3 calculated exothermicity, 24.9 kJ.mol-1. A similar reaction involving maleic anhydride,

(17)

leads to the additional estimate −399.00 kJ.mol-1 for the standard enthalpy of formation of the title molecule and, even though no analogous experimental data

is available for

dimethylcarbonate, the estimate of the gas-phase enthalpy of formation of vinylene carbonate resulting from the similar reaction (16) above, −392.88 kJ.mol-1, agrees very closely with our experimental measurement, as well. An alternative scheme leading to maleic anhydride involves benzenetetracarboxylic 44

dianhydride, a system we have recently measured experimentally,

(18)


21


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Page 22 of 49

Its experimental endothermicity, 2.4 ± 4.0 kJ.mol-1 is again very well reproduced by the −2.15 kJ.mol-1 enthalpy variation predicted by the G3 calculations. Finally the pair of homologous reactions,

(19)

(20)

allow us to estimate the enthalpy of formation of vinylene carbonate as −398.9 ± 8.7 kJ.mol-1 and that of maleic anhydride as being −403.2 ± 4.7 kJ.mol-1, both of which are in very good agreement with the experimental measured values, but disagree considerably from previously existing data. A particularly instructive homodesmotic scheme involving simultaneously both title molecules is,

(21)

and it is observed to be exothermic by 24.4 kJ.mol-1 at the G3 level. The excellent agreement with the experimental enthalpy variation obtained by using our own experimental value for the enthalpy of formation of vinylene carbonate and any of the alternative values reported in


22

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NIST tabulation for diethyl carbonate, −21.6 ± 3.1 kJ.mol-1 or −26.1 ± 3.7 kJ.mol-1, constitutes further assessment of our measurement of the enthalpy of formation of both systems. We adopt also the following reactions, the first of which is expected to be somewhat exothermic since it compares a putatively antiaromatic molecular system with another putatively aromatic system,

(22)

while the second is expected to be essentially thermoneutral in view of the comparison of essentially non-aromatic systems:

(23)

We calculate, indeed, for the first one a G3 exothermicity of 25.75 kJ.mol-1 which must be compared with the corresponding experimental estimate 24.2 ± 6.5 kJ.mol-1. On the other hand, we find the second reaction not to be thermoneutral as expected but, instead, to be exothermic by 17.15 kJ.mol-1 as computed at the G3 level, a value which, incidentally, agrees closely with the -14.9 ± 3.1 kJ.mol-1 enthalpy variation obtained from experimental data. Such slight exothermicity must really be attributed to differences in the ring strain energy of both cyclic systems involved: dihydro-2,5-furandione must be more strained than 1,3-dioxolan-2one, the difference being about 15 kJ.mol-1.


23


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We are thus naturally led to suppose that a similar relation must exist between the strain energies of maleic anhydride and vinylene carbonate, thus implying that only a fraction of the exothermicity of the first reaction shall be assigned to stability differences resulting from the antiaromaticity/aromaticity relations. We will return to this issue in a more detailed analysis later. The aromatic nature of a molecular system can be quantitatively investigated through the calculation and analysis of their magnetic properties, particularly the chemical shielding 45,46

tensor, leading to the so-called nucleus independent chemical shifts (NICS),

the criteria

being that negative values for such parameters indicate aromaticity while positive values, on the contrary, indicate antiaromatic behavior. We used the B3LYP/cc-pVTZ+d geometries and the RHF/6-311G** wavefunctions to obtain the isotropic and the out-of-plane components of the chemical shielding tensor, both evaluated at the geometrical center of the five-membered ring and 1.0 Å above that point, as 38

recommended

in order to take account of the effect of the π-electrons while minimizing the

effects of the σ-frame, over the chemical shielding. We thus obtained for the geometrical center of the ring the values −8.49 (+14.85) and −1.42 (+16.21) and 1.0 Å above that point −4.41 (−9.54) and −3.92 (−3.75), respectively for vinylene carbonate and maleic anhydride. All reported values are in p.p.m. units and the numbers in parenthesis refer to the out-of-plane components of the shielding tensors. Since the effects we are analyzing result from the ring currents associated with the delocalization of the electrons within the π-system, occupying molecular orbitals for which the ring plane is a nodal plane, the most relevant values are those calculated above the ring plane, from which we can easily conclude that maleic anhydride has a much lower aromatic


24

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stabilization or can even be regarded as a non-aromatic system. In any case we are not able to identify antiaromatic character from the calculated NICS values. It seems thus that the eventual antiaromatic character of maleic anhydride should be very tenuous from the magnetic viewpoint. Alternatively, a pictorial way of examining the effects of electron delocalization relies on plots of the spatial distribution of the Fermi (exchange) correlation, which, unlike Coulombic interactions or the other electrostatic interactions present in a system of electrons and nuclei, does not decay in a monotonic manner with distance, but instead is able to transmit locally enhanced electrical effects to atoms removed from the position of the reference electron. Such plots allow observing the manner in which the charge of a reference electron is spread out in space, thereby excluding the presence of an identical amount of same-spin density. In other words, as an electron moves through space it carries with it a Fermi hole of ever changing shape, the density of the electron being spread out in the manner described by its Fermi hole. Accordingly, the extent of localization or delocalization of the density of the electron is 47

determined by the corresponding localization or delocalization of its Fermi hole.


25


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Page 26 of 49

Figure 1. Plots of the Fermi hole density for vinylene carbonate (upper row) and maleic anhydride (lower row). Only the π-electron density has been considered. The reference electron is located immediately over the left carbon atom of the C = C double bond (left panel plots) or immediately over the ether oxygen atom (right panel plots) in each case. All plots use Angstrom units (Ǻ).

In Figure 1 we show plots of the density of the Fermi correlation hole for fixed positions of the reference electron illustrating how the density for such an electron is spread out over space and whether it is localized or delocalized relative to a chosen boundary. Only the π electron density has been accounted in the plots. The position of the reference electron for left panel of


26

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the Figure is immediately over the left carbon atom of the double bond while for the right panel it is located immediately over the ether oxygen atom. We note that the Fermi hole density plotted in those Figures for vinylene carbonate and maleic anhydride exhibits maxima at positions far away from the position of the reference electron which indicates preferred centers for the accumulation of delocalized charge, for both systems, thus evidencing delocalization over both neighboring atoms as well as significant “long-range” delocalization of the electronic density. We thus conclude that circular electronic delocalization seems to be important for both systems, which is consistent with the conventional view of one being aromatic and the other antiaromatic. In fact both types of systems should be stabilized by circular electronic delocalization, the difference between them being that while aromatic systems are more stable than they ought to be, anti-aromatic systems are less stabilized than they ought to be. An interesting reaction which can further help us deciding the stabilization/unstabilization relationships involving vinylene carbonate and maleic anhydride is,

(24)

and it is observed to be almost exactly thermoneutral, both experimentally, Δ; @ = −2 \ 10 kJ. mol-1, and computationally, at the G3 level, Δ; @ = 3.5 kJ. mol-1. Since this reaction is presumably well balanced with respect to both electron correlation and ring strain effects, a first conclusion may be that vinylene carbonate is very slightly more stabilized than maleic anhydride. An alternative scheme leading to a similar conclusion, expectedly being even more well-balanced than the previous one is,


27


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(25)

Both its experimental reaction enthalpy, −14.7 ± 5.0 kJ.mol-1, and the G3 estimate, −8.6 kJ.mol-1, agree with the previous conclusion that vinylene carbonate is somewhat more stabilized than maleic anhydride, a conclusion which, on the other hand, conforms with πelectron count in each ring. Other useful techniques to evaluate the effect of circular electronic delocalization on the molecular stability involve analyzing the energetics of hydrogenation reactions, considered above, reactions (12-13). The experimental enthalpy of hydrogenation of maleic anhydride, −126.9 ± 2.4 kJ.mol-1, can then be compared with the value, −110.3 ± 1.6 kJ.mol-1, observed for the enthalpy of hydrogenation of cyclopentene, which is unequivocally a non-aromatic analogous system, allowing the conclusion that maleic anhydride is presumably slightly unstabilized by the circular electronic delocalization. Since the experimental value for the enthalpy of hydrogenation of cyclopentadiene, a merely conjugated system, is −100.4 ± 2.1 kJ.mol-1, then the conjugation energy of cyclopentadiene may be calculated as − 100.4 + 110.3 = 9.9 kJ.mol-1 while, similarly, the enthalpy variation − 126.9 + 110.3 = −16.6 kJ.mol-1 reflects the destabilization of maleic anhydride resulting from the occurrence of circular delocalization of its 4-π electrons (antiaromaticity). For vinylene carbonate we obtain similarly an enthalpy of hydrogenation of −112.2 ± 4.3 kJ.mol-1, which, being very similar to the enthalpy of hydrogenation of cyclopentene, does not allow however to conclude that vinylene carbonate is a non-aromatic system likewise. Indeed,


28

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the relatively large experimental error prevents such conclusion and we prefer instead to consider it to have a very marginal aromatic stabilization, being in any case only about − 126.9 +110.3 = −14.6 kJ.mol-1 more stabilized than maleic anhydride. Alternatively, we can compare the relative stabilities of both systems through the calculation of the enthalpy variations of the following isomerization reactions,

(26)

(27)

48

which arguably

measure the relative effect of circular electronic delocalization on both

systems. Using G3 absolute enthalpies we find the first reaction to be endothermic by 17.49 kJ.mol-1 while the second one is endothermic by 20.00 kJ.mol-1. Thus, we reach again the conclusion that electronic circular delocalization has very similar effect on both systems. The above conclusion can be also essentially corroborated by the enthalpy variation of the reactions,

(28)


29


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Page 30 of 49

(29)

which can be obtained either experimentally, as 5.7 ± 4.5 kJ.mol-1 and −9.0 ± 2.7 kJ.mol-1, respectively or, from the G3 calculations as 6.5 kJ.mol-1 and −2.08 kJ.mol-1, thus allowing once more both to assess the quality of our calculations and to corroborate the above conclusion: the modest endothermicity of the first reaction and the modest exothermicity of the last one imply obviously that neither vinylene carbonate can be unquestionably classified as aromatic nor maleic anhydride as antiaromatic. 49,50

Recently

we suggested the following homodesmotic chemical reactions as useful means

of evaluating and comparing the effects of electronic delocalization,

(30)

(31)

The first of those reactions is found to be almost thermoneutral at the G3 level, Δ; @ = +0.27 kJ.mol-1, indicating that vinylene carbonate is not particularly stabilized by the circular electronic delocalization. On the other hand the reaction relating maleic anhydride involves an enthalpy variation of Δ; @ = −10.49 kJ.mol-1, a value which is compatible with the ACS Paragon Plus Environment

30

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experimentally observed enthalpy variation for that reaction, −11.6 \ 2.5 kJ.mol-1, and with a slight unstabilizatiom relative to (Z)-2-butene and acetic anhydride, i.e, a slight antiaromatic character. We must however be aware of some criticism relating possible strain energy contamination for the above reactions, and perhaps also for other reactions we used above to reach similar conclusions, which would prevent considering their enthalpy variations as being directly related to the effect of the cyclic delocalization of π electrons alone. We indeed recall that earlier, reactions (22) and (23), we identified a difference in the strain energy of both studied cyclic systems. In order to further clarify how electron delocalization affects the relative enthalpic stability of the title systems we were additionally led to conduct quantum chemical calculations in the framework of ab initio Valence Bond (VB) Theory. In such context we can reach a categorization of the importance assumed by the possible Lewis-type localized structures in the description of the really delocalized molecular system. According to common interpretation the farther away a molecular system is from being representable by a single localized Lewis-type structure the more important are the electron delocalization effects occurring. These calculations were conducted using the B3LYP/cc-pVTZ+d optimized geometries and the VB wavefunction was optimized within the DFT approach using the LYP correlation functional and on the 6-311G(d,p) basis set. All VB calculations were carried out 51,52

at the Valence Bond Self Consistent Field (VBSCF) coupled with the GAMESS-US ab-initio package.

34,35

using the XMVB

software

36

In the framework of VB Theory we used particularly the Block-Localized Wave Function (BLW) method which is primarily aimed at evaluating the electronic delocalization and charge-transfer effects in molecules. In this method the VB orbitals are optimized to self-


31


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consistency while being kept strictly localized. For vinylene carbonate we restricted the 6 ring π-electrons to VB orbitals describing the C=C double bonds and to the two ring oxygen lonepair orbitals with π-symmetry. Similarly, for maleic anhydride our BLW calculations restricted the 4 ring π-electrons to VB orbitals describing the C=C double bond and to the ring oxygen lone-pair orbital with π-symmetry. Thus, comparing the energy raise resulting from restricting the electrons as described above, relative to the energy of the full delocalized RHF wavefunction we were able to finally conclude that vinylene carbonate has larger delocalization energy than maleic anhydride, the difference amounting to ca. 50 kJ.mol-1. So, all our previous findings seem to coherently suggest that maleic anhydride is considerably less stabilized than vinylene carbonate, such conclusion being fully compatible with the traditional description of an antiaromatic 4 π-electrons system vs. an aromatic 6 π-electrons system. However, as we referred previously, such conclusion must be adopted with some care since both systems are presumably strained systems and the ring strain energies may well mask the overall appreciation of the relative enthalpic stability. We thus proceeded by trying to quantitatively evaluate the strain energy of both molecules. To do so we analyzed the energy variation of some chosen reactions at the BLW-DFT/6-311G(d,p)//B3LYP/cc-pVTZ+d level. Thus we considered the set of reactions (28) and (29) which we find to be exothermic by 54.1 kJ.mol-1 and 182.0 kJ.mol-1, respectively. Since at the BLW level the delocalization effects are excluded, the above exothermicities are likely to reflect the ring strain energy relation for both systems, this meaning that maleic anhydride is presumably a more strained molecule than vinylene carbonate. Finally, using the set of reactions, (32) and (33),


32

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(32)

(33)

we obtain, at the BLW-DFT calculational level, the exothermicities 93.2 kJ.mol-1 and 192.9 kJ.mol-1 which are again in qualitative agreement with the previous estimates. Thus, even though the reactions used in estimating the strain energies are structurally diverse, thus leading to a considerable variation in the estimated relative strain energies, it seems however reasonable to us the conclusion that maleic anhydride is unquestionably a considerably strained molecular ring system, its strain energy being somewhat larger than that of vinylene carbonate. As so, the tenuous unstability obtained earlier, either from our experimental measurements or from the accurate calculations, for maleic anhydride as compared with vinylene carbonate, e.g. 14.7 ± 8 kJ.mol-1 (reaction 25) or ca. 12 kJ.mol-1 (reactions 28, 29), must largely be attributed to a corresponding strain energy difference and to a much less extent to a putative antiaromatic character. The importance of ring strain in both vinylene carbonate and maleic anhydride molecules can be assessed from the curvature of the corresponding ring bond paths as obtained from an analysis of the respective 37

B3LYP/ccPVTZ//B3LYP/ccPVTZ wavefunctions in the framework of AIM theory

. Indeed,

we observe for vinylene carbonate the following bond paths angles: 176.3º (O-C(=O)), 178.0º (O-C(-H)) and 178.9º (C=C), and for maleic anhydride: 177.0 (C-O), 178.9 (C-C) and 177.6


33


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(C=C). Such bond path angles, which must be compared with the value 180º for the nonstrained homologous bonds, suggest indeed considerable and similar strains for both cyclic studied molecules. We will additionally resort to an energetic-based aromaticity model suggested some time ago,

55,56

which allows predicting virtually the same relative aromaticities for both title

systems: 27.2 kJ.mol-1 for vinylene carbonate and 29.7 kJ.mol-1 for maleic anhydride, thus supporting our conclusion about the difficulty of attributing any antiaromatic character to maleic anhydride. Our overall conclusion is thus that such character, if it exists at all, seems thus to be extremely “elusive” as has already been argued from different reasonings for maleic 57

anhydride and similar systems .

5. CONCLUSION. We have conducted accurate measurements of the standard enthalpy of combustion of vinylene carbonate and of maleic anhydride which after combination with their phase transition enthalpies allowed the determination of the gas-phase enthalpies of formation. The enthalpy of formation for maleic anhydride results corroborate the previous reported values but with a considerably narrowing of the corresponding experimental error bar. On the contrary, for vinylene carbonate we obtained a very different value for the corresponding enthalpy of formation. Very accurate quantum chemical calculations on the title systems provided further assessment of our newly obtained experimental data. Additionally we have investigated the aromatic or antiaromatic behaviour for both systems using theoretical approaches ranging from the analysis of their magnetic properties (NICS), to the behaviour of


34

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the exchange-correlation function (Fermi hole) and to the energetic stabilization or unstabilization. Our main findings seem to indicate that the comparative stability of both studied systems cannot be uniquely explained in terms of their putative aromatic or antiaromatic characters, but rather that ring strain plays an important role in determining the observed unstability of maleic anhydride. Thus while vinylene carbonate can be viewed as an aromatic system, it revealed impossible to ascribe an unequivocally antiaromatic character to maleic anhydride, despite its ring π-electron counting indicates antiaromaticity.

AUTHOR INFORMATION *Corresponding author e-mail: [email protected]; Phone: +351 220 428 334 http://orcid.org/0000-0001-8227-0845

ACKNOWLEDGMENTS Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, for the financial support to Project UID/QUI/0081/2013 and to FEDER (COMPETE 2020) for the financial support to Project POCI-01-0145-FEDER-006980. Clara Sousa was funded by a CIÊNCIA 2008 contract from FCT.

REFERENCES


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Figure 1. Plots of the Fermi hole density for vinylene carbonate (upper row) and maleic anhydride (lower row). Only the π-electron density has been considered. The reference electron is located immediately over the left carbon atom of the C=C double bond (left panel plots) or immediately over the ether oxygen atom (right panel plots) in each case. All plots use Angstrom units (Ǻ). 381x304mm (100 x 100 DPI)



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TOC Image 462x181mm (90 x 90 DPI)


Experimental and Computational Thermochemical Study of Maleic

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