Fluorine and hydrogen nitrate: rotational barriers and resonance structures

Journal of Fluorine Chemistry, 48 (1990)

378-393

FLUORINE AND HYDROGEN NITRATE:

379

ROTATIONAL BARRIERS AND

RESONANCE STRUCTURES*

NANCY J.S. PETERS+ Natural Science Division, Long Island University/Southampton Campus, Southampton, NY 11968

(U.S.A.)

and JOEL F. LIEBMAN Department of Chemistry and Biochemistry, University of Maryland, Baltimore County Campus, Baltimore, MD 21228 (U.S.A.)

SUMMARY While the magnitudes of the rotational barriers for HON02 and FON02 have been known for over 20 years, the conformation of FON02 has been established as planar only recently.

The

larger rotational barrier for FON02 (42 kJ/mole vs. 32 for HON02) seems counter-intuitive in light of the easily rationalized planarity of the HONO FON02.

and the controversial planarity of

To provide insight into this phenomenon, the rotational

barriers in HONO

and FON02 have been investigated using &

initio molecular orbital calculations including correlation. The calculated barriers agree well with the experimentally determined values.

An explanation is offered for the relative

and absolute magnitudes of the barriers.

* Presented in preliminary form at 9th Winter Fluorine Conference, St. Petersburg, Florida, 29 January 1989, Abstract No. 19, and the 12th Austin Symposium on Molecular Structure, Austin, Texas, 29 February - 2 March 1988, Abstract No. P48. 0022-1139/90/$3.50

OEIsevier Sequoia/Printed m TheNetherlands

380

BACKGROUND HONO

[l] has been well-established as planar.

The ra-

tionalization for this conformation's preference is the stabilization derived from the 6+/s- attraction between the H and one terminal 0 (la below).

While resonance structure lc is

expected only to have a minor effect, it does support planarity. Also, the destabilizing effect of two formal positive charges adjacent to one another in lc is mitigated by the stabilizing effect of H6+ - 06- attraction.

+y” O-N H la

0

+/ H/o- “\\,

+/O H,i=“\;

lb

1c

Structural determination of fluorine nitrate has proven a formidable task for experimentalists [2] and theorists [3] alike.

While FON02 is now unequivocally known to be planar, it

is nonetheless hardly obvious why it should be.

Indeed, simple

resonance structure reasoning suggests the opposite.

In par-

ticular, consider the three primary resonance structures for FON02, 2a-c, analogous to those for HON02.

F/o- “\, 2a

+

0

+/O

+/

F/o- “lo 2b

F

/

O=N \; 2c

Arguments against planarity have cited the destabilizing influence of 6-/S- repulsion between F and a terminal 0 (2a) and the minimal contribution of 2c because the positive oxygen is adjacent to both the positive nitrogen and an electron-withdrawing fluorine.

Earlier calculations have shown [3] the planarity to

be the result of very weak r-type bonding, both in the plane and perpendicular to the plane of the molecule. The above reasoning would seem to suggest that the planarity of HONO

is more stabilized than the planarity of FON02.

381

One would then predict that the rotational barrier, the energy difference of the planar and perpendicular structures, 3 and 4, should be greater for HON02 than for FONO2.

/” ,/O-“\,

/” xd ‘0

3

O-N

4

Experimental studies show this conclusion to be false -Erot(FON02) = 42 f 4 kJ/mol[2c]and E,,t( HON02) = 32 + 0.4 kJ/mo [41*

It is also surprising that the rotational barriers are so This paper presents ab initio calculations of the rota-

high.

tional barriers of FON02 and HONO

to examine why that of FON02

is larger than that of HON02. Comwutational Method All calculations were performed on the DEC VAX 11/780 using the GAUSSIAN 82 series of programs [5]. The planar and (perpendicular means FON perpendicular forms of FON02 and HONO or HON plane perpendicular to NO2 plane) were fully optimized The 6-

using the 4-31G basis [6] and the 6-31G** basis [7]. 31G** basis was used for HONO

because it provides p functions

on H as well as the d functions on heavy atoms provided by 631G*.

It has been shown that the 4-31G basis gives reliable

geometries, i.e. bond lengths and angles close to the experiThe

mental values, for molecules containing N, 0, and F [8]. results for both bases are collected in Table I.

A single point calculation was done at the MP2/6-31G** (This calculalevel [9] using the 4-31G optimized geometries. The barrier to rotation tion is noted as MP2/6-31G**//4-31G.) is then calculated as the difference in total energy between These

the optimized planar and perpendicular conformations. results are collected in Table II.

Mulliken populations [lo] for the two molecules in each conformation are found in Table III and valence molecular orbital energies are shown in Tables IV and V.

I

lengths

Pauling,

angles

L.; Brockway,

L. 0. J. Am. Chem.

Phvs.

'02

Sot.

(1937) 59.

(1965) 42, 3106.

in degrees.

J. M. J. Chem.

in angstroms,

1.193 1.1695

1.194 1.17 1.29

1.2029 1.1774

1.219 1.188 1.199

NO2

13.

(X = H, F)

1.193 1.1695

1.466 1.4061

1.434 1.3644

4-31G opt 6-31G* opt

per-p.

'03 - N'

1.187 1.165 1.29

1.459 1.386 1.40

1.418 1.359 1.42

4-31G opt 6-31 * opt expt 3

planar

as shown:

1.2026 1.1774

1.406 1.3617

4-31G opt 0.9627 6-31G** opt 0.9507

per-p.

are numbered

1.194 1.173 1.211

1.373 1.332 1.406

NC1

C3N

4-31G opt 0.9606 6-31G** opt 0.9511 exptC 0.964

X03

planar

Basis

and FON02alb

Cox, A. P.; Riveros,

Bond

for HONO

Conformation

The atoms

FON02

HONO

Molecule

Geometries

TABLE

<01N03

114.87 114.88 115.9 115.63 115.50 109.50 109.5

114.05 114.17

<01N02

128.95 129.10 130.3 128.75 129.00 132.50 132.5 125 131.90 131.66

102.23 102.71

107.40 108.4 105.0

108.68 106.50

107.65 105.42 102.2


383

RESULTS

Geometrv.

As shown

parameters in excellent

agreement

the 03-N bond

argued

length

in another

the experimental

is larger

paper

is probably

value

CH30N02

yielded

discrepancy,

6-31G

value

The geometries to each other ence between bond

bond

character.

length

in HONO

is shown

The 6-31G** those

between

the 4-31G

0 bond results

Rotation

and 6-31G**

Barrier.

do not

erroneously For FON02, calculation conformation mation.

with

of

to the

similar

of some

loss of double

that would

be partially

are consistently

calculated

short-

differences

lengths

are the F-

in both molecules. other

lost

lc [12].

The largest

numerous

differ-

is in the

studies

These comparing

for F, 0, N molecules

[13].

The best

calculational

(MP2/6-31G**)

barriers

are within

values

bases

conformations

lengths

level.

and the 03-N bond

are consistent

experimental

bond

are very

The largest

structure

and the 6-31G**

for the rotational

that

bond

in resonance

at the 4-31G

in FON02

the 4-31G

suggestive

optimized

er than

of 1.46 A

as compared

level.

and perpendicular

The multiple

but we

03-N bond

i.e. the 03-N bond

of the two conformations

planar

for HON02,

[ll].

at each calculational

03-N

on rotation

of 1.360

for

to the F or H).

A calculation

is 1.402 A experimentally

in CH30N02 optimized

are

except

the 4-31G value

of 1.40 A.

1937 experimental

length

bonded

[3] that the correct between

geometrical

values

than that

and the

a similar

optimized

for both molecules

(03 is the oxygen

for FON02

in FONO2

I, the 4-31G

conformations

with

length

The discrepancy have

in Table

of the planar

for both HONO

include

correlation

reverse

the relative

the rotational improves

magnitudes

and includes more

for HONO

tion

to the MP2 calculation,

that

for FON02.

and FON02.

give poorer

barrier

is stabilized

The barrier

results

the uncertainty

increases

Calculations

quality

results

and

of the barriers. consistently

correlation:

from the 4-31G

but the increase

as the

The planar

than the perpendicular improves

of the

is much

conforcalculaless than

384

The anomalously at the

6-31G**

more

important

tion

than

greater

TABLE

level

large value suggests

for the representation

for the perpendicular.

role

obtained

in describing

for the HON02

that polarization

of the planar

Correlation

the perpendicular

barrier

functions

are

conforma-

appears

to play

a

conformation.

II

Total Energiesa and Rotational for HON02 and FON02

Basis

Molecule

HF/4-31G//4-31G

HF/6-31G**//6-31G**

MP2/6-31G**//4-31G

EXPT

a Hartrees

(1 hartree

Barriers b

Conformation

Total

E

Barrier

HON02

planar perp.

-278.992112 -278.979908

32.04

FON02

planar perp.

-377.578549 -377.570469

21.21

HON02

planar perp.

-279.450746 -279.435912

38.94

FON02

planar perp.

-378.148022 -378.133995

36.82c

HONO

planar perp.

-280.172479 -280.159613

33.25

FON02

planar perp.

-379.056977 -379.041304

41.15

HONO

32f0.4d

FON02

42+4e

= 627.5

kcal/mole

= 2625 kJ/mole)

b kcal/moles c A recent paper Jones, Y. Cao, (1989) 7071. d Ref.

2c

e Ref.

4

confirms this result: V. Morris, G. A. Walker, S. C. Bhatia, J. H. Hall, Jr., J. Phys. Chem.,

P. 93

385

Mulliken often trends

limited

charges,

charge

in Table

lc and 2c play

planar

conformations

should

be different

more

positive

the HONO

tions.

The

result

formal

that

is, however

action.

FON02

perpendicular

Isodesmic tion

FON02

and HONO

have

bond O-NO2

Reaction.

as well

The

on both in FON02

ard heats

less negative

result

suggests 2c.

previous

expecta-

F and 0 in FON02

in the planar

small.

or for

for the FON02

structure

conforma-

1, 4 interaction

The

conformation

of HONO

is larger

hydrogen-bond

type

1, 4 overlaps

for the

sides

inter-

the MP2/6-31G**

in FON02

isodesmic

energies

reac-

for FON02

[14]:

+ HOF

The nearly

zero hH suggests

of the equation

is similar

to that

and HONO

of formation

&i and support

hH for the following

using

as for H20 and HOF

+ H20 + HONO

bonds

on 03

The difference

on both

similar

the

the charge

with

a weak

struc-

in stabilizing

This

interaction

with

The net

conformations.

LXH is 1.86 kJ/mole.

bonds

charges

extremely

consistent

can be computed

and HON02,

larger.

the

the two

charges.

and that

are consistent

negative

The

If resonance

role

conformation.

H and 0 in the planar

and favorable,

Mulliken

from resonance

in a net antibonding

between

Likewise

interest.

slightly

1, 4 overlaps

The

tion

similar

is negligible

contribution

data.

is

and

III, are approximately

in the two conformations,

is only

a minor

by other

of the two molecules,

conformations

analysis

can be made

of HON02.

a significant

in the planar

conformations only

have

on 03 is of particular

tures

zero

are supported

shown

of FONO2

population

comparisons

for the two conformations

conformations

The

Mulliken

utility,

can be seen that

Mulliken same

Although

Charses.

of only

are equivalent,

in HOF

in strength

are similar

for gas phase

the conclusion

the

and the

in strength.

species

1151.

that

i.e. the OF

also give

Standa nearly

386

.-I

Lz +

387 -.

Molecular orbital correlations can

be constructed for HONO

and FONO2 as the X0 bond rotates from

the planar to the perpendicular conformation.

While most

orbitals increase in energy upon rotation to the perpendicular form, not all do (see Tables IV and V). Also, with the exception of two orbitals in each case, #12 and #13 for HONO and #15 and #17 for FON02, the change in energy is fairly small. While the total energy is not the sum of the individual orbital energies, changes in total energy are often reflected in changes in orbital energies.

The key to understanding the rotational barriers then appears to lie in understanding the changes in the two orbitals that undergo the largest change in energy.

These molecular orbitals are shown in Figures 1 and 2.

TABLE IV

Energies (in hartrees) of the 6-31G** valence molecular orbitals for HONO in the planar and perpendicular conformations

Energy

Orbital ta 16 15 14 13 12 11 10 9 8 7 6 5

Planar Conformation -0.49301 -0.52043 -0.55204 -0.58501 -0.69892 -0.79126 -0.81435 -0.83285 -0.96928 -1.46740 -1.48482 -1.72415

(3a") (gal) (8al) (2all) (i'a') (6a') (la") (5a') (4a') (3a') (2a') (la')

Perpendicular Conformation -0.48821 -0.50954 -0.54575 -0.61632 -0.66242 -0.78059 -0.80820 -0.83943 -0.96088 -1.45881 -1.47770 -1.71531

(5a") (4a") (7a') (3a") (6a') (2a") (5a') (4a') (3a') (2a') (la") (la')

(Perpendicular -Planar) + + + + + + + + + +

.005 .Oll .006 ,031 .037 .Oll .006 .007 .008 .009 .007 .009

a Upon rotation, the plane of symmetry changes and hence the orbital symmetry designations. However, there is no crossing in the correlation between the two conformations. Thus we refer to the orbitals by their number rather than their symmetry designation.

TABLE V Energies (in hartrees) of the 6-31G* valence molecular orbitals for the FONO2 in the planar and perpendicular conformations

Orbital #a 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6

Planar Conformation -0.52106 -0.55514 -0.57184 -0.59277 -0.69204 -0.76079 -0.76915 -0.82057 -0.86306 -0.86792 -0.97265 -1.39805 -1.52304 -1.70177 -1.78827

(4a") (llal) (lOa') (3a") (gal) (8a') (2a") (7a') (la") (6a') (5a') (4a') (3a') (2al) (la')

Perpendicular Conformation -0.51552 -0.54135 -0.56185 -0.61749 -0.70589 -0.73849 -0.77982 -0.82865 -0.84803 -0.86032 -0.96918 -1.38877 -1.51801 -1.69969 -1.78196

Energy (Perpendicular -Planar)

(6a") (5a") (gal) (4a") (8a') (7a') (3a") (2all) (6a') (5a') (4a') (3a') (la") (2a') (la')

+ .006 + .014 +. 010 -. 025 - .014 + .022 - .Oll - -008 + -015 + .008 + .003 +. 009 +. 005 + .002 +. 006

a Upon rotation, the plane of symmetry changes and hence the orbital symmetry designations. However, there is no crossing in the correlation between the two conformations. Thus we refer to the orbitals by their number rather than their symmetry designation.

For HON02, the conformational change from planar to perpendicular causes the non-bonding R orbital #13 (Figure la) to become a N-O bonding orbital (Figure lb).

The change from

non-bonding to bonding results in lower energy.

The bonding

(N-O and O-H) orbital #12 (Figure lc) in the planar conformation retains O-H bonding upon rotation (Figure Id), but loses N-O bonding and therefore rises in energy. For FON02, orbital #17 in the planar conformation shows K antibonding interactions O3 -F and 03-N (Figure 2a).

Upon rota-

tion to the perpendicular form (Figure 2b), the O-F antibonding interaction is maintained, but the 03-N interaction is lost. Also lost is the N-O2 bonding.

The net result is lower energy. For orbital #15, the planar conformation (Figure 2c) exhibits weak N-O3 and N-O2 u bonding as well as O-F antibonding inter-

389

Fig. 1. Jorgensen plots of the molecular orbitals for HONO using the 6-31G** basis. a. #13 in the planar conformation: b. 113 in the perpendicular conformation: c. #12 in the planar conformation: and d. #12 in the perpendicular conformation. Produced by Lynn Read at Princeton University and drawn by Kristy Askam at Long Island University, Southampton Campus.

Fig. 2. Jorgensen plots of the molecular orbitals for FON02 using the 6-31G* basis. a. #17 in the planar conformation: b. #17 in the perpendicular conformation: c. #15 in the planar conformation: and d. #15 in the perpendicular conformation. Produced by Lynn Read at Princeton University and drawn by Kristy Askam at Long Island University, Southampton Campus.

391

actions. weak

The perpendicular

01 -N-O2

r bonding,

characteristics. results

conformation

O-F o bonding,

The conversion

in a net

increase

(Figure and N-03

2d) exhibits antibonding

from u to R interactions

in energy.

DISCUSSION

For both molecules, rotation

and one decreases

are significant,

#17 is also

smaller, give

significant

greater

in magnitude

However, 31G**

than

6-31G* that

change

energy

#17, making er higher orbital

#15, when

perpendicular, increases

(since

planar.

form decreases

As orbitals

perpendicular gap

conformation,

appears orbitals

larger

that

control

6-

a larger

FON02

If the total individual

upon

orbit-

more

upon

is reversed in energy

than

barrier

the perpen-

correlation more

changes

decreasing for FON02

the magnitude

conformations

the

in the

and the

the barrier. compared

to HONO

to the key molecular

of the barrier. with

than

in energy

the gap for #13 increases,

barrier

For

the

rotation).

because

with

barri-

rotation).

and the rotational

in energy

from

is larger

less and the rotational

are antibonding

in both

is

than perpendicular

from the F contributions

#17, the F orbitals contributions

both

rotational

to result

for these

#13 and #12 decrease

for #12 decreases, The

more

form of

form.

form decreases

#15 increases

on the

calculation

the planar

increases

lowering

is based

in energy

the gap

also would

for FON02.

in energy

them

#15 is

erroneously,

#17 decreases

For IiON02, the situation dicular

discussion

in the change

the gap between (since

interactions

changes

for the planar

more

for orbital

the energy

to the correlation

#17 decreases

changes

raising.

for the perpendicular

is reflected

als, planar

These

because

than

change

lowering

for orbital

do in fact give,

for HONO

calculation change

orbital

both

upon

for non-bond-

and antibonding

the change

than the energy

which

barrier

The total the

the energy

for FON02

the molecular

calculations

rotational

For HONO%,

c for r bonding.

barrier

in energy

interactions

as bonding

However,

exchanging

a smaller

bonding

For FON02,

interchanged.

increases

in energy.

exchanging

ing and vice-versa.

are

one orbital

respect

whereas

In orbital

to the O3

the H in orbital

392

#13 does not contribute at all in either conformation, i.e., is non-bonding.

In orbital #15, the F contribution changes from

antibonding to minimally bonding, whereas the H contribution in orbital #12 remains bonding in both conformations. In summary, we find that ab initio quantum chemical calculations with explicit correlation corrections are needed to reproduce both the relative and absolute magnitudes of the observed rotational barriers of HONO is seen that

and FON02.

However, it

no single effect nor single orbital seems to

dominate the explanation that FON02 has a higher rotational barrier than HON02. REFERENCES 1

A. P. Cox and J. M. Riveros, J. Chem. Phys., 42 (1965) 3106.

2a

L. Pauling, and L. 0. Brockway, J. Am. Chem. Sot., 59 (1937) 13.

b

K. Brandle, M. Schmeisser and W. Luttke, Chem. Ber., 93 (1960) 2300; A. J. Arvia, F. R. Cafferata and H. J. Schumacher, Chem. Ber., 96 (1963)

1187.

C

R. H. Miller, D. I. Bernitt and I. C. Hisatsune,

d

J. Shamir, D. Yellin and H. H. Claassen, Israel J. Chem.,

Spectrochim. Acta, A23 (1967) 223. 12 (1974) 1015. e

K. 0. Christe, C. J. Schack and R. D. Wilson, Inorg.

f

R. L. Odeurs, J. van der Veken and M. A. Herman, J. Mol.

Chem.,l;l (1974) Strut., 118 3

2811.

(1984) 81.

N. J. S. Peters and L. C. Allen, Inorg. Chem., 27 (1988) 755.

4

G. E. McGraw, D. L. Bernitt, I. C. Hisatune, J. Chem.

5

J. S. Binkley, R. A. Whiteside, R. Krishnan, R. Seegar, D. J. DeFrees, H. B. Schlegel, S. Topiol, L. R. Kahn and J.

Phys, 42 (1965) 237.

A. Pople, Quantum Chemistry Program Exchange, 71 (1980) 36. 6

R. Ditchfield, W. H. Hehre and J. A. Pople, J. Chem. Phys., 54 (1971) 724.

393

7

P. C. Hariharan (1973)

8

and J. A. Pople,

Theor.

N. J. S. Peters

and L. C. Allen,

'Structure

, Molecular

N, 0, F Compounds Fluorine-Containing Synthesis

Deerfield

Beach,

a. C. Moller

Structure

Molecules:

and Applications,'

and W. R. Dolbier,

9

Chim.

Acta,

28

213.

Jr.,

Structure,

b. J. S. Binkley

A. Greenberg,

VCH Publishers,

and New York,

and M. D. Plesset,

Phys.

and J. A. Pople,

in

Reactivity,

J. F. Liebman,

Editors,

Florida,

and Bonding

and Energetics:

Inc.,

New York,

Rev.,

46

1987.

(1934)

Int. J. Quant.

618;

Chem.,

9

(1975) 229. 10

R. S. Mulliken,

J. Chem.

2238,

2343;

ibid 36

known

to be very

have generally

basis

proved

Phys.,

a

(1962) 3428.

(1955)

set dependent more

1833,

Mulliken

1841,

charges

and 6-31G*

chemically

are

values

significant

than

4-

31G values. 11

12

C. W. Brock,

S. V. Krasnoschchiokov,

N. Panchenko

and Yu. A. Pentin,

J. F. Liebman

and A. Greenberg,

222, discusses

the rotational

L. V. Khristenko,

Chem.

Phys.

Biophys. barriers

106

Yu.

(1985)

Chem.,

1

69.

(1974)

of carboxylic

acids

and esters. 13

D. J. DeFrees, S. Binkley

B. A. Levi,

S. K. Pollack,

and J. A. Pople,

J. Am. Chem.

W. J. Hehre, Sot.,

101

J.

(1979)

4085. 14

H20 and HOF were hartrees

15

and ET

fully optimized. (HOF) = -175.103976

&if0 for FN03 = 10.460, HOF = -98.324

W/mole

H20 = -242,

ET

(H20) = -76.222449

hartrees. HN03

(from D. D. Wagman,

Selected

Values

of Chemical

National

Bureau

of Standards,

Thermodynamic Washington,

= -134.306, &.

and

al.,

Properties, D. C.,

1965.