Photoelectron spectroscopy of some biological molecules

JournaI of Electron Spectroscopy and Related Phenomena, 13 (1978) 379-393 @ Elsewer SclentlficPubhshmg Company, Amsterdam - Printed m The Netherlands

PHOTOELECTRON MOLECULES*

D DOUGHERTY,

SPECTROSCOPY

E S YOUNATHAN,

OF

SOME

BIOLOGICAL

R VOLL, S ABDULNUR

and S P McGLYNN

Departments of Chemrstry and Blochemrstry, Lourszana State Unrversrty, Baton Rouge, Louzsrana 70803 (USA) Umverszty of New Orleans, New Orleans, Lourszuna 70122 (U S A )

(First received 6 September 1977, m final form 13 October 1977)

ABSTRACT

The ultravlolet photoelectron spectra (UPS) of the followmg blologlcallyactive compounds are reported and/or assigned 2,4-dlmtrophenol, mcotmlc acid, mcotmamlde, barbltunc acid, xanthme, hypoxanthme, uric acid, uracll, thymme, cytosme, adenme, guamne, p-carotene, menadlone, purme and pynmldme The Importance of UPS data 1sexemphfied rn two ways First, by mvestlgatmg the vahdlty of the Pullman k-index approach1 4 to lomzatlon energies, and, second, by generating an expenmental scale of electron-donatmg ablhty INTRODUCTION

It 1s surpnsmg that so little work has been mvested m the ultraviolet photoelectron spectroscopy (UPS) of moderately-large blologcal molecules’ - l3 In contrast, a great number of correlative studies between electromc structure (usually deduced quantum-chenucally) and blologlcal “function” are avallablel 4 UPS data, via the intermediacy of Koopmans’ Theorem15, provide a very direct test of quantum-chemical computations’ 6 Therefore, the motlvatlon of this work 1s threefold (1) to demonstrate that one may obtain UPS data for a diverse selection of blologcal molecules, (11) to provide assignments for the UPS data, and (m) to compare the results with the extensive calculations of the Pullmans’4 EXPERIMENTAL

Spectra were obtamed on a Perkn-Elmer

PS-18 photoelectron

spectrometer

* This work was supported by contracts between the U S Energy Research and Development Admuustratlon-DiMsion of Biomedical and Environmental Research-Physics and Technologcal Program, The National Science Foundation (Grant No BMS-72-02266) and the LouIslana State University

380 This mstrument 1s equipped with a heated sample probe which can sustain temperatures as high as 350°C with an accuracy of f 2 “C The resolution normally attamable’ 7 1s 20-25 meV However, for T > 140 “C, the resoIutlon decreases becoming 75-100 meV at 300°C Sample heating 1s mandatory for most blochemlcals because of then- Iow vapor pressures Many of them also decompose when heated so that, If care 1s not taken, a spurious spectrum may be recorded The general procedure consists of a slow temper1 O/mm) until the sample pressure 1s suitable for recording a specature increase (trum The sample 1s then checked for stgns of decomposltlon (e g , change of color, charring or time-dependence of the spectrum) Expenence, as documented m this work, suggests that most small to medlumsized blomolecules should be tractable Larger molecules are intractable because of decomposltlon at the temperatures necessary to generate adequate vapor pressure Examples of mtractablhty are provided by vltamm K I and protophorphyrmIX dlmethyl ester PHOTOELECTRON

SPECTRA

Smce all molecules dlscussed here can be categorized m the C, pomt group, their MO’s can be labelled a” and a’ We use the symbol representation, n for the a’ representation The n-deslgnatlon connotes and “lone pour” All lomzatlon energies and assignments are collected m mdlvldual assignments are discussed tersely m the followmg sections

(or near C,) 7r for the (I” non-bonding Table 1 The

2,4-dmtrophenol (DNP) and 2,4-dmtroanuole (DNAn) The UPS of DNP and DNAn are shown In Fig 1 The two spectra are very smular The correlation diagram of Frg 2 mdlcates that the 1(I) and I(2) events of DNP and DNAn correlate du-ectly with srmllar events m phenoll’ The lomzatlon events associated with the mtro groups reside m the intense bands centered at - 11 7 eVm DNP, and - 11 4 eV m DNAn This conclusion 1s based on the observation of the two mtro group lomzatlons-one n and one 7t--m mtrobenzenel ’ at 11 15 and 11 30 eV respectively These intense lomzatlon bands of DNP and DNAn suggest a model with the followmg charactenstlcs (1) A total of four lomzatlon events occurs m each of these bands Each mtro group contrlbutes an I(n) and an I(n) event and, smce there are two such groups, it 1s expected that doubling wlli occur to yield I(n+), I(n_), I(Tc+) and I(z-) events The & symbols denote linear combmatlons of the m&vldual mtro group MO’s (2) The various (f)-lmear combmatlons will exhlblt a small energy sphttmg Thus, a11 four mtro group lomzation events will occur m a narrow energy band, Just as do the two mtro group lomzatlon events m nltrobenzenelg (3) Finally, If the energy pattern of the three I(‘(K) events m phenol holds also for DNP and DNAn, the UPS band at 12 8 eV (DNP) and 12 25 eV (DNAn) correlate

381 TABLE

1

IONIZATION ENERGIES FREQUENCIES (cm-l)

Compound

2,CD1nltrophenol Ionization energy

(eV)a, ORBITAL

I(3)

1(4)

I(5)

9 85 9 57(l)

1070

114fOl

11 7

117

type

n

7c

N1tro group 1onlzat1ons

9 55 9 30(l)

1040

111fOl

3z

az

N1tro group ionizations

100*01 9 38(1)

type

N1cot1n1c acid, methyl ester Ionization energy MO type Nlcot1naxmde Iomzation energy MO

type

Hypoxanthlne Ionization energy %ib MO type Xanthme Ionization energy MO

type

Unc acid Ionuation MO

type

energy

10 93

114

1190

[n and nd]

[n and n]

l600(f50) n

985+01 9 25(l) [n and n]

10 65

11 25

[n and n]

?c

10 35

10 75

[n and n]

n

90 8 65(l) [n and n]

type

114

10 77

9 85 9 18(1) [H and n]

N,N-d1ethylnlcotmamIde Ionization energy MO

VIBRATIONAL

I(2)

N1cot1mc acid Ionization energy %b MO

CATION

1260( +50) type

2,4-Dmitroamsole Ionization energy MO

AND

I(I)

%'ib MO

ASSIGNMENTS

94&02 ?c

103 &02

8 87 8 70(1) 13Wf50) 3c

9 98

10 54

n

?c

11 15

114fOl

8 85 8 55(l) 3L

10 1

104

10 9

11 5

8 55 8 15(l) n

to 5

10 7

11 2

II 8

382 TABLE

1 (contmued)

Compound

I(1)

Caffeine

8 25 7 95(l) Z

MO

type

Cytosme Iomzatlon energy MO

type

Guanme Iomzation energy MO

type

Barblturlc acid Iomzation energy MO type B-Carotene Ionization energy MO

type

Menadlon? Iomzatlon energy

I(2)

9 45

8 30 7 85(l) 32 1040 10 20(l) n 7 65 6 50) 3L

n

115)

I(4) 9 95

96

8 82 8 45(l) 7li

I(3)

118&02

9 90 Z

9 90

n

10 45

11 15

n

n

1105

1145

n

f02

[one 7c two n’s] 8 25

8 65

9 51

a The parenthesized symbol 1 denotes the point at which the lomzatlon event mltlates

with the third Z (7t) event Pullman calculations l4

m phenol

This assignment

IS also supported

by the

The UPS of NA Nzcotrn~ actd (NA) and nrcotmzc acid methyl ester (NAME) and NAME are shown m Frg 3 Five low-energy romzatlons are expected three Z(n) and two lone-pair Z(n) events Two of the 7~MO’s are located on the pyrldme nng while the third IS on the acid group The mtrogen lone par provides one of the n MO’s while the acid group carbonyl lone pan- furnishes the other The low-energy lomzatlon region m both NA and NAME consists of three bands with relative cross-sections 2 2 1 Thus, Z(1) and Z(2) he wlthm the first band, Z(3) and Z(4) he wlthm the second and Z(5) yields the third band A correlation diagram IS given m Fig 4 The n MO on mtrogen and the highest-occupied 7~MO of the rmg remam nearly degenerate m both NA and NAME, Just as in pyndme” Itself The large destablhzatlon of Z(5) caused by estenficatron (I e , NA to NAME) shows that Z(5) arises from the n MO of the acid group The

383

P

2 LQ

z -

G?..

% TT

.z

UJ

’

130

’

OH

IllI

I1

I

IO

12

I

14

IONIZATION

I

I

16

ENERGY

I

I

I8

20

(eV)

Flgure 1 The He(I) photoelectron spectra of 2&dnutrophenol and 2,Cdlmtroarusole Figure 2 Correlation diagram of the UPS data for phenol 18, 2,4-dnutrophenol and 2,4-dmltroamsole

I

I

IO

I

I

12

1ONIZATlON

I I4

I

I

I

I6

ENERGY

I

I

16

20

(ev)

COOH

I

Q

57

Ii COOMe

QfOOMe

HCooH

Figure 3 The He(I) photoelectron spectra of mcotmlc acid and nlcotmlc acid methyl ester Figure 4 Correlation diagram of the UPS data for pyndme20, nicotnnc acid, nicotmic acid methyl ester, methyl formatezl and fornw acldzl

384

-1

E i=

90

z 10.0 5

0

t

I

IO

I 12

IONIZATION

I

I 14

II

11

II 16

ENERGY

Figure 5 The He(I) photoelectron

18

CONH,

HCONtb,

20

HCONH,

(ev)

spectra of mcotmamlde

and N,N dlethylmcotmamlde

Figure 6 Correlation diagram of the UPS data for pyridmezO, mcotmamlde, amide, N,N-dlmethylformamlde21 and formamldezl

N,N-diethylnicotm-

second z MO of the pyrldme rmg as well as the IEMO of the acid carbonyl group are, therefore, responsible for the second UPS band Nrcotmamzde (N) and N,N-dlethylmcottnamzde (DEN) The UPS of N and DEN are shown m Fig 5 Five low-energy lomzatlons are expected three 1(n) and two I(n) events Two of the z MU’s are located on the rmg, wtile the third 1s largely on the amldlc mtrogen One of the n MO’s IS a G lone-pair of the pyrldmlc nitrogen, while the second possesses large amphtude on the carbonyl oxygen A correlation diagram 1s given m Fig 6 As m the case of NA and NAME, the highest-energy occupied MO’s of N and DEN are expected to be the nearlydegenerate, low-energy n and 72 MO’s of pyrldmlc nature J(5) 1s assigned to the second z MO of pyndme on the basis of its null destablhzatlon upon dlethylatmg N to DEN and by reference to the Pullman calculatlons22 The n and n MO’s of the amide group, therefore, are bracketed to low energy by n and 7~MO’s of pyndme, and to high energy by the second n MO of pyrldme Dlethylatlon of N to yield DEN segregates two bands at higher energy, leaving three still unresolved m a lower-energy band This low-energy band cannot contam I(n) of the carbonyl oxygen, this conclusion follows from Fig 6 Indeed, 1(4) of DEN must anse from the n MO of the carbonyl group Therefore the I(n) and I(Z) events of the amide group must all occur m the unresolved low-energy band The

385

I

I

I

I

12

IO

I 14

IONIZATION

IO

12

IONIZATION

I

16

I

I

~

20

(eV)

spectrum

16

16

ENERGY

(eV)

14

I 18

ENERGY

Figure 7 The He(I) photoelectron Xe and Ar reference gases

8

-

I

I

of barbltunc

acid

The sharp spikes are due to the

20 IONIZATION

ENERGY

Figure 8 The He(I) photoelectron spectra of xanthme (bottom) and caffeme (top) m the spectrum of caffeine are due to Xe and Ar reference gases

(eV)

The sharp spikes

Figure 9 The He(I) photoelectron spectra of hypoxanthme (bottom), xanthme, and uric acid (top) The sharp spikes m the spectra of uric acid and hypoxanthme are due to Xe and Ar reference gases

386 amidic I(x) I(Z) event

event IS presumably

of slightly tigher lomzatlon

energy than the pyrldmlc

Barbzturzc aczd (BA) The UPS of BA IS shown m Fig 7 Thus molecule should exist only m the keto form’ 4 Thus, the set of low-energy MO’s should consist of five three IZMO’s, one for each carbonyl, and two 71MO’s, one for each nitrogen lone-par orbltal of n-type z3 CNDO/s computations for BA yleld24 n, - 10 71, n, -11 13,n, -11 60, rr, -11 7l,n, - 11 78 eV The agreement between computation and experiment (See Table 1) IS excellent Hypoxanthzne (HX), xanthzne (X), urzc aczd (UA) and caflezne The spectra are shown m Figs 8 and 9 The resolutron for HX, X and UA IS poor (75 meV) because of the high temperatures requrred The low-energy lomzatlon event, however, 1s quite dlstmct m all four cases Calculatlons14 on relative stablhtles mdlcate that the keto form IS favored The slmllanty of the UPS of X and caffeine suggests that keto and enol forms do not co-exist m the gas phase If they did, the spectrum of X would be more complex than that of ctieme, and the lowest-energy band of X, being a composite of at least two lomzatlon events, would be broader than that of caffeme Since this IS not so, rt can be assumed that the keto form predommates m the gas phase Huckel calculations’ 4 predict one, Isolated, low-energy a MO m all molecules This prediction agrees with the UPS data Hence, I(1) IS assigned as I(n) m all four cases No assignments of any validity are feasible m the higher-energy region Cytoszne (C) and guanzne (G) The UPS of C and G are shown m Figs 10 and 11, respectively The vertical lomzatlon energes of the DNA/RNA bases are shown m Fig 12, and the CND0/2 results25 m Frg 13

I 8

II

I

IO

I 12

IONIZATION

I1

I

14

I, 16

ENERGY

F~gurc 10 The He(I) photoelectron reference gases

I I6

,

I 20

(eV)

spectrum of cytosme

The sharp spikes are due to Xe and Ar

387

I,

I

I

8

I

I

IO

I

I

IONIZATION Fig 11 The He(I) photoelectron reference gases

U

T

I

I

C

I

I

I

16

18

ENERGY

(eV)

14

12

I

spectrum of guamne

A

I’

20

The sharp spikes are due to Xe and Ar

G

Figure 12 Plot of the vertlcal lonlzatlon energies and MO asagnments of the four or five highest occupied MO’s for uracll (U,W, thymlne (T)ll, cytosme (C), adenme (A)13 and guamne (G)

The verbcal locations of the three lowest-energy lomzatlon events rn C are recognizable even though I(2) and I(3) do overlap Smce I(1) IS well-resolved, relative band area conslderatlons ehmmate the posslblhty of a fourth lomzatlon event under the band contammg I(2) and I(3) Band area conslderatlons do not dlscrlmmate between the presence of one or two lomzatlon events m the 11 8-eV band However, comparison of the 11 S- and 13 0-eV bands suggests the presence of two lomzatlons m the former band A comparison of Figs 12 and 13 IS of little help since the energy level patterns do not match very well Nevertheless, because of the absence of better dlscrlmmants, the assignments are taken from the CNDOJ2 results

388

s

-9

(u

3

0‘ 0 -100

5.

t

ho3

- 7r

3 - ‘IT

g

n

-Q 3 -120CD ii

0

I

x

-130-

1

-140

-

- 7r n -y

n

-E

T

77

LE

J

J

n U

T

C

n 7

A

G

Figure 13 Plot of the MO elgenvalues (from CND0/2 cytosme (C), ademne (A) and guanme (G)

IONIZATION

ENERGY

computatlons9

for uracd (U), thymme (T),

(eV)

Fq 14 The He(I) photoelectron spectrum of B-carotene Ar reference gases

The sharp spikes are due to the Xe and

The lowest-energy band m G IS quote discrete The 9-12-eV band IS presumed to contam three events because of the observation of three well-defined maxima Comparison of Figs 12 and 13 mdlcates good agreement The UPS assignments for G are also based on the CND0/2 results Comparison of Figs 12 and 13 mdlcates only fan- agreement between the two data sets all the CND0/2 elgenvalues deviate by at least one eV, the MO order from the CNDO/Z algonthm2 5, correct for U and T, IS incorrect for A, and the observed pattern of lomzatlon energy levels IS not satlsfactorlly reproduced There IS some concern, therefore, that the MO ordenng predicted for C and G may be incorrect

389 Q-carotene (j?C) The UPS of j?C IS presented m Fig 14 Since j?C IS not ngld, It can assume many conformations The effect of this conformatlonal multlpllcity IS a loss of UPS resolution The a&abatrc lomzatlon energy, then, refers to the unknown conformation of lowest energy The spectrum may be subdivided mto lower- and higher-energy regions with the boundary at - 9 eV The low-energy region contains three distinct maxima Since no srngle lomzatlon event stands m isolation, relative band area consrderatlons are of no use Hence, the actual number of events m the low-energy region IS unknown, but three IS the lower hmlt Huckel calculations are of no help since the elgenvalue spectrum cannot be partitioned mto lower- and tigher-energy sets DISCUSSION

We now correlate the UPS data wrth the Pullman k-mdex14 The lomzatron energy IS expressed, via simple Huckel theory, asl4 I = CI+ k/l where I IS the lomzatlon energy, GC IS the Coulomb integral, /?1sthe resonance integral and k,the coefficient of /I, IS the Pullman index Since ccand j3 are constantsz6 for a series of related molecules, It IS k which determines the MO order Furthermore, since the lowest lomzatlon energy IS a dn-ect measure of electron-donor ability, the k-index of the hlghestoccupied MO provides a relative ordering parameter for electron-donor capability (I e , Lewis base strength”) of lfferent molecules The relative electron donor-acceptor ablllty m any “related” set of molecules IS important because of Its observed correlation with certain essential physlcochemical properties These properties are important for vatlous (blochemlcal) transformations, mcludmg, for example, oxldatlon-reduction, the formation of charge transfer complexes and semlconductlon m aggregates A particularly important use of the lowest lomzatlon energy from UPS, therefore, lies m its determination of relative electron-donor ability within the set of blologlcally-active molecules The vertical lomzatlon energies and the k-indices are tabulated m Table 2 and plotted m Fig 15 The best least squares regression lme IS also shown and, for comparison purposes, we also include a reference lme for some aromatic hydrocarbons2 ’ The lowest lomzatlon energies of BA and pyrlmldme are I(n), not I(n) These energies are denoted in Fig 15 by open circles For DC we also plot two points IS denoted by a solid square I(n) vertlca,IS denoted by an open square while ~~~~~~~~~~~~~ The vertical and adiabatic transitions for the reference hydrocarbons28 are usually comcldent, but this IS not true of P-carotene Several observations concernmg Fig 15 are pertinent (1) Despite some scatter, the blomolecule regression line IS a remarkable vmdlcatlon of the PullmanI attitudes (2) The blomolecule regression line lies much higher than that of the aromatic This IS of course expected The values of Ial for nitrogen and oxygen hydrocarbon

390 TABLE

2

THE k-INDICES8 OF THE HIGHEST ENERGIES FOR SOME BIOLOGICAL

OCCUPIED z MO AND MOLECULES

THE LOWEST

IONIZATION

Compound

k-Index from Huckel E = a + k/l expresson

Lowest vertlcul comzutron energy (eV)

Pm-me (Pu) Pynmldmeg (Py)

0 69 1 06

Adenme (A) Guanme (G) Cytosine (C) Thymme (T) Uracil (U) Hypoxanthme (HX) Xanthme (X) Unc Acid (UA) B-Carotene (PC) Barblturlc Acldg (BA)

0 49 0 31 060 0 51 0 60 040 040 0 17 008 103

Menadlone (Ka) 2,bDuutrophenol (DNP) Nlcotnuc Acid (NA) Nlcotmarmde (N)

0 92 0 84 1 01’ 1001

9 52b 9 75(n)C 10 5(n) 8 4gd 8 30 8 82 9 20” 9 60e 8 87 8 85 8 55 7 65 10 40(n) 11 05(Z) 951 9 85 100 9 85

8 Ref 14 b Ref 6 C Ref 33 d Ref 13 e Ref 11 f Calculated using the Huckel parameters listed m ref 14 g HOMO 1s an oxygen lone-paw, n, orbital, not a 7~ MO

, 60’

/ ’

0

I

t

02

04

HUCKEL

I

I

I

I

06

06

IO

COEFFICIENTS,

k-

Figure 15 Plot of vertical z lomzatlon energies vs Huckel k coefficlents14 for the blochemlcals hsted in Table 2 The solid lme obeying the equatron I = 7 6 + 2 7k IS best least-squares regression line for the pomts mdlcated by solid circles Slmllar data for some aromatic hydrocarbons2* IS also plotted

391

----PC

UA---

-__

K3=,_ Vol---E’

_.-BIAC -__-U -P

Leu -_ I,e_-----Ala ----i-

-DNP

DHU----

UreaBA----

-T

10

---NA

- -PA - Glyoxol -PBA

Figure 16 Scale of electron-donor characterlstlcs from UPS data Iomzation energy (eV) 1s plotted down the center of the chart Abbrevlatlons are /3C for #?-carotene, Caff for ctieme, G for guanme, A for adenmel3, UA for unc acid, C for cytosme, X for xanthme, HX for hypoxanthme, T for thymme, Ka for menadlone (vitamin Ka), BlAc for blacetyl3, Val for I-vahne12, U for uracd, Leu for I-leucmel2, Ile for I-lsoleucme12, P for pyruvamldes, Ala for I-alanme12, N for mcotmamlde, DNP for 2,4-dmltrophenol, DHU for dlhydrouracll, NA for mcotmlc acid, Urea for urea, BA for barblturlc acid, PA for pyruvlc acid 3, Glyoxal for glyoxa134 and PBA for parabamc acid5

are larger than for carbon Srmllar regression hnes are available for other senes of unsaturated hydrocarbons Eland’ ‘, for example, found I = 6 37 + 2 70 k for a hydrocarbon set conslstmg of naphthalene, azuIene, blphenylene, mdene and blphenyl, and Schmidt” recently found I = 5 652 + 3 214 k for a very extensive set of 61 hydrocarbons The Eland and Schmidt hnes encompass the hydrocarbon hne of Fig 15 and all three lmes he conslderably fower than the blomolecule lme (3) Although 1(l) IS of n-type m most cases, 1(l) of BA (and possibly NA and mcotmamlde) IS of n-type In these cases, frontier MO conslderatlons based on the supposltlon that 1(l) = I(Z) are wrong

392 The importance of the UPS data does not consist solely of a vmdlcatlon of the Pullman attitudes The UPS data replace computed values and provide an absolute scaling of electron-donor capablhty In addltlon, these data can provide a better parametrlzatlon of semi-emplncal models which, m turn, can be used to compute lomzatlon energies for cases m which UPS data 1s unobtamable Electron donor and acceptor characterlstlcs are at the heart of blochemlcal processesi4, 30$ 31 Measured electron aflinltles are rare and, for blochemlcals, almost non-exlstentj’ However, the lowest lomzatlon energy 1s a direct measure of electron donor ablhty Figure 16 summarizes the electron-donor characterlstlcs of blomolecules for which UPS data are available This figure 1s the experimental counterpart of a theoretical diagram due to Pullman It provides a graphic demonstratlon of the power of the UPS techmque m biology

REFERENCES 1

A

Padva, P

R

LeBreton,

R

J Dmerstem

and J N

A

Rldyard, Bzochem

Wophys

Res

Commun , 60(4) (1974) 1262 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

18 19 20 21 22 23 24 2s

T P Debies and J W Rabakus, J Electron Spectrosc Relat Phenom , 3 (1974) 315 S P McGlynn and J L Meeks, J Electron Spectrosc Relat Phenom , 6 (1975) 269 J L Meeks, J F Arnett, D B Larson and S P McGlynn, J Am Chem Sot ,97 (1975) 3905 J L Meeks and S P McGlynn, J Am Chem Sot, 97 (1975) 5079 N S Hush and A S Cheung, Chem Phys Lett , 34 (1975) 11 G Lauer, W Schafer and A Schwelg, Tetrahedron Lett ,4S (1975) 3939 H E Popkle, W S Koshl and J J Kaufman, J Am Chem Sot , 98 (1976) 1342 S P McGlynn and J L Meeks, J Electron Spectrosc Relat Phenom , 8 (1976) 85 A Padva, T J O’Donnell and P R LeBreton, Chem Phys Lett ,41 (1976) 278 D Dougherty, K Wlttel, J Meeks and S P McGIynn, J Am Chem Sot , 98 (1976) 3815 L Klasmc, J Electron Spectrosc Relut Phenom , 8 (1976) 161 S Peng, A Padva and P R LeBreton, Proc Nat Acad Scz USA, 73(9), (1976) 2966 B Pullman and A Pullman, Quantum Bzochemzstry, Wiley-Interscience, London, 1963 T Koopmans, Physzca, 1 (1934) 104 K Wlttel and S P McGlynn, Chem Rev , 77 (1977) 745 The term “resolution”, as used here, means the full width at half height (1 e , “half width”) of the Xe or Ar 2P,,~ lomzatlon band Most mvestlgators refer this term to the Ar band since it 1s slightly narrower than the Xe band J H D EIand, Int J Mass Spectrum Ion Phys , 2 (1969) 471 J W Rabalsus, J Chem Phys , 57 (1972) 960 R Glelter, E Hellbronner and V Hornung, Angew Chem , 82 (1970) 878 Part of a Ph D Thesis submitted to LSU by J L Meeks, 1974 The Huckel calculations on mcotmlc acid and mcotmarmde were done usmg the parametnzation of ref 14 The MO computer programs were obtained from the Quantum Chemical Program Exchange and are listed as QCPE-141, CNDO/Z and QCPE-174, CNDO/s-CI The geometrical parameters used m tlus computation are from W Botton, Acta Crystallogr , 16 (1963) 166 Geometrical parameters used are the followmg For A and U-K Tormta, L Katz and A Rich, J Mel Bzol , 30 (1967) 545, for T, the U geometry was used and a methyl group was added m the 5-posltlon using standard bond lengths and angles, for G-U Thewalt, G E Bugg and R E March, Acta Crystailogr , Sect B, 27 (1971) 2358, for C-E J O’Brien, Acta Crystallogr , 23 (1967) 92

393 26

27 28 29 30 31 32 33 34

The constancy of a and B IS, of course, merely approximate One of the obJectlves of this work was to test the vahdlty of this assumed constancy For the k-index to have any vahdlty, the constancy of a and /? must be assured A Lewis base IS defined as an electron pair donor, a Lewis acid IS defined as an electron pan acceptor E Hellbronner and H Bock, The WMO-Model-l, Wiley-InterscIence, London, 1976, p 367 W Schmidt, .7 Chem Phys , 66 (1977) 828 L Bnlloum, A Pullman, B Pullman and M Kasha m M Kasha and B Pullman (Eds ), Norlzons m Bzochemwfry, Academic Press, New York, 1962 A Szent-Gyorgyl, Electron Brology and Cancer, Marcel Dekker, Inc , New York, 1976 P M Colhns, L G Chrlstophorou, E L Chaney and J G Carter, Chem Phys Left, 4 (1970) 646 L Asbrmk, C Frldh, B 0 Jonsson and E Lmdholm, Int J Mass Specfrom Ion Phys ,8 (1972) 215 D W Turner, C Baker, A D Baker and C R Brundle, Molecular PhotoeZectron Specfroscopy, Whey-Interscience, London, 1970