Mass spectrometry: the mass spectrum of methanol. Part I. Thermochemical information

Jcu~nnI of hhts Speclrometry and Ion Physics Elsevier PubIkbing Company, fknsterdam - Printed in the Netberkmds

hzternatio~t

MASS

SPECTROMETRY:

THE

PART

I. THERMOCHEMICAL

MASS SPECTRUM

1

OF METHANOL.

INFORM_4TION

J. H. BEYNON, A. E. FONTAINE AND G. R. LESTER ImperiaI Chemical Induiries

Ltd., Dyestuffs

Dicision,

Manchester

9, E&and

(Received February lst, 1968)

The mass spectrnm of methanol contains “meta-stable ing to stages

of fragmentation

of the

mole&e

under

peaks”, correspond-

electron

which

impact,

enable an unusual wealth of thermochemical information to be obtained very simply if certain assumptions are made about the activated states involved. Although there are several published spectra’ none of these publications set out details of the “meta-stable peaks” or used them to illustrate the compIete stages in the fragmentation of the molecular ion. The mass spectrum also contains an anomalous diffuse peak at mass 30.10 which will be discussed in detail in the second part of this work2. TABLE

1

THE hfAS.S

SPECTRA

OF

%CTHAXOS_S

m CH~OH e

Identity

36 35 34 33 32 CH,O 31 CH,O 30 CHSO

Co, OH Abunaknce

Identity

Abundance

80 100 5.2

CD,HO CD,0 CDtHO CD,0 CDHO CD0

82 3.3 100 3.3 3.2 32

CHO co CD,

2.7 <5 22

CD,

3

29 CHO 33 2s co (4.5 18 Water impurity 17 16 15 CH* IO-2 14 CH: 1.7 13 CH 0.5 12 c 02

CD C

CD, OD

CH, OD

I

1

Identity

Abunhnce

CH,OD CH.OD CHDO CHLO CD0 CHO

79 IO0 5.1 0.8 6

co

<3

D+O and H,O

Identity CD,0

83

CD,0

100

CD:0

5

CD0

‘332

CH C J. Moss Spectrometry

27

13

co CDs CD2

CJ&

Abundance

<7 20

3

Ii,

1.8 0.4 0.2

CD

0.8

c

0.4

and Ion Physics,

1 (1968) 1-24

J_ H_ BEYNON,

2 TABLE CHART

Te_Xf ref.

A_ E_ FONTAINE,

G- R- LESTEE

2 SHOWIXG

THE

.MEl-A-STABLE

DECOMPOSITXOXS

Decomposicfon **

M

CHJOH

AND CD,OH

Nominal masses axd idenriries

A B C D E

(p-l)-

+


F G

(p-l)T

+ (p--2)--;-H-

H-

OBTAIXZ

Aburhnce

%

mx

nt,

32 CH,OH 32 CH,OH

31 CH,OH 30 CHrO

31 CHIOH

19 CHO

0.14

31 CH,O

30 CH,O

Obscured by heavy isotope

0.02

Obscured by heavy isotope ionsof31’+29-

USIXG

THE

MASS

Appearance and pasiGon in normal mass speclrzm

Obscured -

Wide m/e 27.12 0.02 %

ions of 30’ -+ 29+ H I J

(p-2)--

+ (p--3)-+-H-

K

(p-z)--

+ @--4)--$-Z

H.

30 CH:O

29 CHO

30 CH:0

2s co

<2-IO-+

0.017 % Too weak for detection

29 CHO

23 co

< 5 * 10-J

Obscured

0.12

Wide m/e 28.03

L NI @-3)1

N 0

+ (p--4)--TH-

. .-

p-- + (p-!)-tH~nomaIous

l

Tbe d,composition

products listed in this table (e-g- (p--3)‘.+H.+D-)

** p represents the parent ion.

Anomalous 0.013 % m/e 30.10

are not intended to denote the ~

present study uses methanol and the deuterated analogues CH,OD, CD3 OH and CD, OD. The mass difference Hz-D is 1.548 - 10m3 a.~. and therefore around mass 30 a resolving power of 20,(?00 is sufficient to separate the peaks due to isobars differing only in that some contain prolium and some deuierium. It also makes possible correction of the spectra made necessary by the presence of impurity methanols, principally CH3 OH and CD, OH in CH, OD and CD3 OD respectively, caused by exchange of protium for deuterium due to adsorbed water near to and within the source of the mass spectrometer. It is therefore possible to obtin a great deal of detailed information by mass measurement alone and much of this can be confirmed by inter-comparison of the The

r_ &Gss Ypectromerry and Ion Physics, 1 (1968)

1-24

~

THE

MASS

spEcl-RouETER Ih’

SPECTRUM

MET_+STABLE

OF METHANOL.

MODE

PART

3

I

*

CD,OH

Decomposition

Nominal masses and identities ml 35 35 35 35

p+- -f (p-1)--wp’-

-+

(p-_2)-+D-

p-- --+ (~-_~)T-+_H-‘D$- + (~~--4)‘.~2

D-

Abumiance

m, CDSOH CDJOH CD,OH CD,OH

34 33 32 31

CD,0 CD,HO CD,0 CDHO

3 - 104 0.016 <2-104 3 - 104

-

@-3)--+D-

34 CD,0

32 CD:0

@-I)(p-2)+

+ +

@-5)-+2 D(p-5)--,-H-+D-

34 CD,0 33 CDIOH

30 CD0 30 CD0

(p-2)-

+

(p-3pT-H-

(p-2j-

--f Q-6)-+2

33 CDeOH 33 CD,OH

32 CD,0 29 CHO

(6-10’ 3-10-L

32 CD,0

30 CD0

0.04

32 CD,0

28

31 CDHO

:O CD0

0.09

31 CDHO

29 CHO

0.006

30 CD0 29 CHO

28 co 28 CO

Q-3)--

+

(p-5)--!-D-

(p-3)--

+

(p-7)----2

@-4)--

-

@-5)-TH-

(p-4)=

-+ (p--6)-+-D-

(&)(p-6)-+

-+ (p-7)---Y-D@-7)L_+H-

p-- +

D-

(p-2)--D-

co

Obscnred Obscured Too weak for detection

<

(P-r)-

D-

Appearance and poiition in normal mass spectrum

%

Too weak detection Wide m/e Wide m/e 0.013 %Too weak I detection


0.11

2646 27.28 for

Gaussian m/e 28.12, O.OOS”_? Too weak for detection Wide m/e 29.03 0.007 % Obrrr:d

<6-lo-’

(1 o-5 < 5-101

‘I;0 keak .-or dete&ion

-

Anomalons 0.007 % at m/e 31.22

, state of combination

for

of the neutral fraSrnents, but only the total mass eliminated from the ion.

low-resolution spectra of the various methanols. Table 1 shows the mass spectra of CH,OH, CDsOH, CHJOD and CD30D. Further information concerning which hydrogens are taking part in the various decompositions is obtained by the detection of “meta-stable pea!.!.‘. The meta-stable decompositions in the mass spectrum of CH30H and CD,OH were recorded by operatirtg the double focussing A.E.L ~M.S.9 mass spectrometer3 such that the accelerating voltage was variable independently of the defkcting electric sector voltage. In this way the main ion beam is not transmitted and daughter ions from decompositions occurring in the portion of the Llight path in front of the electric sector, can be colkcted4-6. This mode of operation, hereafter called the J. Mass Spectrometry

and Ion Physics,

1 (1968) l-24

3. H. BEYNON,

4

A. E. FONTAINE,

G. R. LESTER

“meta-stable mode”, can provide unambiguou- identif%zationof the nominaI masses of the parent and daughter ions of a decomposition together with high detection sensitivity. Details of the decompositions occurring in CH3 OH and CD3 OH are given in Table 2*.

DlSCUSSlON

The mass spectra of CH30H, dar

CH,OD,

CD,OH

and CD3QD

of mok-

weights 32, 33, 35 and 36 respectively, have their base peaks at masses

p- 1Fp- 1, p -2 and p-2, wherep represents the mass of the parent ion. Thus it is clear that in CHS OH, formation cf the base peak by decomposition of molecuhr ions in the ioniztion ch.zmber by a fast process, largely Involves elimination of one methyl hydrogen’. Ions CD30A in the mass spectrum of CD,OH (abundance of peak -3 7;) show that a minor process involves elimination of the hydroxyl hydrogen from the molecular i m. Meta-stable decompositions detected in the mass spectrum of CD,OH show that some at least of the ions causing the peaks at (m/e) 32 (CD&+) and (m/e) 3 I (CHD6)+) are produced directly from

the parent_ Distinguishing the hydroxyl hydrogen by Ho and c-O.-paring the mass spectra of CH30H and CD,OH, the decomposition of the parent ion of CH30H can be written: CH,=O+-H,

_--H

CH,-b’-H,

(m/e) 31 (Fast, high

‘B)

probability

-CH=OI-H,

processj 4

+x-

We) 30 (Low probability)

-

-HO

+nH~‘A’

CH3-O*: (Lo;~;.~;.dity)

CH,=Oi + energy (m/e) 30 (Low probability)

A, B, etc. refer to the transitions shown in Table 2. The wide, dished, “meta-

sr.abIe peak” centred at (m/e) 27.12 in the mass spectrum of CH30H

is caused by the dissociation of theion of (m/e) 31 (CHLOHf) to an ion of (m/e) 29 (CHOi) with the release of maSS 2 either as a hydrogen molecule or as two separate hydrogen atoms. The mass spectra of CH,OD. CD,OH and CD30D mntain corresponding svide “‘dished meta-stabie peaks” at (m/e) 26.28, (m/e) 27.27, and (m/e) 26.46 respectiveiy. Typical peak shapes are shown in Fig. 1. These peaks correspond to the meta-stable decompositions: * No detailsor discussionare includedfor deumpo;itions Wining heavy isotopes of carbon or oxygen. J_ Mas

Spectrometry and Zen Pk~sics, 1 (1968) 1-21

invoiving the decay of ions con-

THE

MASS

SPECTRUM

m 31* e CH2=O*H !!I 32’ e CH?=O’D

“2 33’ e

OF METHANOL.

-+z

PART

I

29’+2

3 CH=O+ -+:

;2

H- (or + Hz) in CH,OH

Fig. l(a)

29++3

-, CH=O’

;H-

tD-

(or + HD) in CH,OD

Fig. l(b)

-snI 30’f3 e

CD,=O’H-,

CM’+H--+D-

(or +HD)

in CD,OH

Fig. l(c)

and -?nM’ e CH2=O+D

+--

m_ e

,0’;4

+ CD=O--1-2

D- (or +D,)

in CH,OD

Fig. l(d)

Thus it is clear that in CH,OH som: of the ions of (p>l/e) 29 are formed by the elimination of the hydroxyl hydrogen and one of the methyl hydrogens from the ion of (m/e) 31. I+-_CEO*

t_

H

l

i-

Ho-

(or +HHo)

(G)

Alternative low-probability fragmentation pathways of the (p-2)’ and (p- l)+ ions in CD30H are indicated by other “meta-stable peaks”. For CH30H these can be written: -

2 H- 0’“; (1)

2 30 e

Ef 31 e

:C=O L-HO

f!f 29 e

and CH,=+

“z 30

e

H-GO+

i- ener-q

!ff 29 e J. Mass Specfrometry and Ion Physics, 1 (1968) 1-24

6

J. H. BEYNON,

27 ,A,

:

I_

._

_A *

A. E. FONTAINE,

--

~-

__

--

G. R. LESTER

THE

MASS

SPECTRUM

OF

METHANOL.

PART

7

I

The “meta-stable peak” at (nz/e) 28.03 in the mass spectrum widened by the release of energy in the decomposition “f 30 e (CH,O)f

is

II? d-29-i-l e +

CHO’+H-

The mass spectrum of CD,OH to the decompositions: ‘n 31 e

of CHxOH

contains

+

(CDp)-

i7Z -

-

“‘30+2 e

+

(CDO)-

peaks” corresponding

at E 29.03 e

-+If30+1 e

(CDOH)?

two “meta-stable

i-H-

and e

32

(CD,O)I

at ” 28.12. e +D-

The first peak is widened by energy release while the second is a normal “gaussian” shaped “meta-stable peak”. The mass sptytrum of CD,OD also show a wide “meta-stable peak” at (nzje) 28.12 caused by the decompositicn: 21132

e

(CD,O)*

4 +

;

,

30+2

(C30)’

A-D-

The mass spectrum of CH3 OD, which inevitably contains some CH3 OH formed by deuterium,‘protium exchange on the walls, shows a wide “meta-stable peak” at (nz/e) 27.12 due to the decomposition nZ -

e

31

(CHOD)? superimposed composition:

-+ “‘29-i-2 e +

(CHO)’

+D.

on the still wider “meta-stabIe

peak” at @z/e) 27.12

due to the de-

Fig. i_ Annlagous “meta-stable

peaks” in the mass spectra of methanols caused by: (L) The loss of 2 H- (or H,) from (CH,O)- in the mass spectrum of CH,OH; (b) The loss of H- and D- (or HD) from (CH,DO)- in the mass spectrum of CK,OD; (c) The loss of I-i- and D- (or HD) from (CHDLO)- in the mass spectrum of CD,OH; (d) The ioss 2 D- (x D:) from (CD,OY in the mass spectrum of CDSOD. Fig. l(b) also show the superposition of the “meta-stable peak” caused by the loss of D- from (CHOD)’ in the mass spectrum of CH,OD on that caused by the loss of 2 H- or F+ from (CH,O)-in the mass spectrum of CH,OH, both compounds being present due to protium/deuterium exchange on the source walls and inlets. J. Mass

Spectromr~ry atzdlon Physics, 1 (i968)

l-24

8

J. ii.

’ E31 e

BEYXON,

A. E. FONTAINE,

G. R. LESTER

--, :29+2

(CH, O)+ --, (CHO)* 1-2 H- (or H,) occurring in CH3 OH. This is ihustrated in Fig. 1 (b). From these facts the process causing reIease of energy may be written: HiC_ot_H

e5

H-C=Of

5 30

t

energy

(I)

” 29 e

The other probable process may be written: H, H>CSo+

-H:_

H-C&’

m 30 e

(J)

m 29 e

Again, alternative low-probability fragmentation pathways of the (p--4)? and @-3)f ions in CD,OH are indicated by other me&stable decompositions. For CH,OH these can be written:

H\+O’--Ho H\ H,c_ot

-n-

:C=OQ-&,

(Ml

(CO)?

-t

Finally the meta-stable de,ompositiqns (N) and (0) in Table 2 in CD,OH indicate that both possible configurations of the (m/e) 29 ion in CH,OH decay to CO”. H-CEEE* --H-

CO?

z

:C=&H,

0

THE DECOMPOSITION

(0)

-nz

e

30 + g 29 +

ENERGY

IN

CH,OH

The widths of “meta-stable peaks” can be used to obtain a measure of the energy released on separation of the fragments *=‘. It has already been shown that release of energy is invoIved in the process which may be written: H-&0+-H,, X Mass

Spectrometry

-Ho,

H-GO+

and Ion Physics,

-I- energy

1 (1968)

l-24

THE

MASS

SPECTRU3f

OF METHANOL.

The widths of the “meta-stable spectra of CH30H,

CD,OH,

PART

peaks” CH30D

tively 0.10, 0.10, 0.20 and 0.20+0.01

9

I

corresponding

to this process in the mass

and CD,OD

taken

at 8 kV are respec-

u. The released energies calcuiated

decomposition are respectively: 0.19, 0.18. 0.39 and 0.38 10.04 -I-

-i-

Ml

in

CH,OH

for each

eV. Thus;

Hea: of reaction AH,

m2

H<=O+-H

+ HCO+ +I<-

-0.19

eV

(1)

in CH,OD

H-&Of-D

+

HCO*

+D-

-0.39

eV

(2)

in CD,OH

D&O’-H

+

DCO’

+H-

-0.18

eV

(3)

in CD,OD

-=0+-D

+

DCO+ +D.

-0-38

eV

(44)

An enera equation may be written for the process in terms of the standard heats of formation and excitation energies of the species concernedLet E denote an excitation

ener_q. Then

AI& = AH”,(m2*)+AH”,(m,)+~(m,~)-AH”,(m,*)--E(m,”) m3 cannot be in an electronically

excited state because the 2s state of H- is some 10 eV above the IS ground state. Thus ~J,(HCO’)tAH;(M-)-_or(~H=~H)i AiY”r(HCOi)

+ AH”,(D-)

=

-0.19

eV

from (1)

+ (E~~-E~~) =

-0.39

cV

from (2)

= -0.18

eV

from (3)

= -0.38

eL7

from (4)

(sip-E,,)

- AW,(CH=&D)

AH”@CO‘)+AH”,(H-)-AH”,(CH=6H)+(~38-~3z) A-Wf(DCOt

) -i- AW,(D-)

The numerical ??1r- and j? to mtf_

- AH”,(eH=6D)

+ (cd8 --E.&

suffix indicates the energy equation

number. z pertains to

It must be remembered that we are dealing here with meta-stable ions, that is to say with ions which decompose within a narrow range around 10Ss --, IOm6 set after formation. In decomposing, the ion releases energy which appears as kinetic energy of separation of the fragments and it is pertinent to ask what determines the quantity ofenergy so released. The ion is highly excited, vibrationally and electronically. None of this energy can appear as excitation of the separating hydrogen or deuterium atom for the reason stated above. The energy released will con&in a considerable contribution from the extra stability gained by intra-molecular efectronic and/or atomic rearrangement in the fragment ion. Contributions from vibrational excitation are not to be expected since these transfer through the ion at very high rates. The formation of an excitedion by eIectron impact of a molecule in or near its ground state followed immediately by some degree of dissociation, is unlikely to be affected by the subst tutioa of deuterium for hydrogen since the electronic structure of the molecule is not changed. Cross-over J. Mass Spctromrtry

and Ion Physics,

1 (1968) l-24

10

J. H. BEYNON,

A. E. FONTAINE,

G. R. LESTER

from the reactant energy surface to the product energy surface occurs at the same rate for both species since their lifetimes are the same. Thus, this mass-spectrometric method of studying fmgmentations should give meaningful thermo-chemical information and it seems reasonable to equate the excitation energy terms (G~--~J and (E~~--Q,)- nu; (i)-(2) give= (Qp- Go). (%P-%,), AH”&H-) - AH”,(D-) - AH”&H=;;H)

+ AHD,(eH=&D)

= 0.20 eV

LIH, = -a If we write H- + +I&; C(g) --, C(sIt; AH, = -d De + fD,; As’l, = -b AH, = -c o- + to;?; and D{x-y) as the decomposition energy of the bond x-y. u = 52.09 kcal/moB:; b = 52.98 kcal/mole’O. Then, putting the heats of forrlation in terms of bond strengths and the heats of formation of atoms, we have: a-ti-[--(D(C-H)+D(C’-O’)+D(O‘-H)+(-2c)t(-c)i(--d)] +[-(D(C-H);D(C=Oi)+D(Oi-D)t(-a)+(--b)f(-c)+(--d)] = 0.20 eV. and thus D(O+-H) - O(O’-DJ SimiizzrIy(3)-(4) gives:

= 0.20 eV

a--btCD(O’-H)+(--_b+(-a)]and thus D(O’-H)-_(Ot-D)

-lHJZ DECOb%‘OSTION

”

e

31 --,

= 0.20 eV

[D(O’-D)t(-2b)] = O-20 eV.

%9-+-mmcv e

IN CH,OH

As has already been shown this decomposition involv,ec.the loss of a hydroxyl hydrogen and a methyl hydrogen from the ion of (mje) 31 in methanol. The widths of the wide “meta-stable peaks” observed for this transition in CK,OH, CH30D, CD3 OH and CD,OD are respectively: 0.380, 0.485, 0.435, and 0.525 F0.005 U. The corresponding energ% released, calculated from the widths of the “meta-stable peaks” are respectively: 1.42+0.04, 1.64&0.03, 1_27f0.04 and i -48 IO.03 eV. Thus: ml

+

m2

i

- 1.42 eV

(5)

W +D- (or HD)

-1.64

eV

(6)

3 DCO’ f

H-+D-

(or HD)

- I.27 eV

(7)

--, DCO++

2 D-

(or Dz)

- 1.48 eV

(8)

CH&H

+ HCO’i-

2 H-

CH&D

-+ HCO’+

CD2=6H CD&D J- Mass Spectrometty

Heat qf reaction AH,

m3

md

(ar HZ)

Ion Physics, 1 (1568) 1-S

THE MASS

SPECTRUM

OF METHANOL.

11

PART I

Writing an energy equation for the process: AH,

= hU”,(m,+)+AN=,(nr,)+~(m,‘)--Luro,(m,*)--~(m,*).

With the same arguments as have been used above. we may write: -I-AIP,(2H-

AH”,(HCO+)

-!- AIT’,(H- +- D- or HD) - AF’,(CHz=dD)

+ (ass - as,J =

A.H”JDCO‘)

+ AZ’,(H-

+ (E,~ -E,~) = - 1.27 eV from (7)

AH’&DCO’)+-

or Hz)-

AH”,(CH,=&H)

+ (csa -es=) = - 1.42 eV from (5)

AH3@COf)

+ D- or HD) - AW,(CD,=6H:

AHCt(2D-

or D,)-

AH”,(CD,=6D)

+ (~g~-es~)=

-1.64eV from (6)

- 1.48 eV from (8)

Equating the excitation ener_q terms and *,ubtracting (5) and (6) gives AJP,(2H-

or H,) - AW,(H-

Subtracting AW,(H-

+ D- or HD) _- AH”,(CH2=6H)

+- AW,(CH,=&D) = 0.22 eV

(9)

(7) and (8) gives:

-t-D- or HD) - AH”,(2D-

Putting the heats of formation cf atoms, then (9) becomes: (2~ or 0)-(a+b

or Di ) - IpSI”,(CD,=bH)

+ AH”,(CD2=6D) = 0.21 eV

(10)

in terras of b nd strengths and heats of formation

or O)+- [D(O+-H)+(-33a)-_(Of-D)-(-~)-(-_)] = 0.22 eV

and (IO) becomes (afb

orO)-((2C

or O)t[D(O+-H)i-(--2b)+(--a)--D(Of-D)-(-336)] = ‘3.21 ev

If two neutral radicals are formed: (9) and (IO)

gives

D(O’-H)-_(O’-D)

= 0.22 eV

gives

D(O’-H)-B(OC-D)

= 0.21

eV

If a neutral molecule is formed:

(9) gives . -_ (10) gives

D(O’-H)-II(O*-D)+b-B(O’-H)-II@+--D) D(O+-H)-D(O*-D)+b-r

= 0.22 eV = OX-(b-a) = 0.22-0.039

2: 0.18 eV

= 0.21-0.039 N 0.17 eV J. Mass Specrromctry

and Ion Physic& 1 (1968) I-24

J. H. BEYNON, I

12

A. E. FONTAINE,

G_ P_ LESTER

Equations (5), (7) and (6), (8) may a30 be combined. (7~(5) gives: A WJDCO+)-

AXP,(HCO’) -

(8~(6)

i AW,(H-

+ D- or HD)-

AH”,(CD,-6H)

AH0,(2H-

t AH”,(CHz=hH)

or H-J

= 0.15 eV

(II)

gives:

AEP,(DCO’)

- AH”,(HCO’)

4

t AHGf(2D- or Dt) - AH”,(H- -ED. or HD)

- AH”,(CD,=&D)

i AF~(CHz=6D)

= 0.16 eV

(12)

(1 I) becomes -[o(C-D)~(--b)J-[~(C-H)t(--a)]f(~af-b

or 0)-((2a

tW(C-D)t(-%)-I-(-~a)]-

or 0)

[2D(C-H)t(-33a)]

= O-15 eV

(12) becomes, - [D(C-D)+(--b)Jt

[D(C-H)+(--a)]+@ f [2D(C-D)i(-33b)]-

If two neutral

radicah

or 0)-(sib [2o(C-H)i-(-22a)+(

-b)]

= 0.16 eV

are formed:

(11) gives

D(C-D)--(C-H)

= 0.15 eV

(12) gives

D(C-D)--(C-H)

= 0.16 eV

If a neutral molecule

or 0)

is formed,

(11) gives

D(C-D)--D(C--H)

= 0.15 eVf(b--a)

E 0.19 eV

(12) gives

D(C-D)

=

O.I6 eVt(b-a)

N 0.20 eV

-O(C-ii)

The iso-electron% neutral species corresponding are methyIeneimine (CHa=NH) and hydrogen cyanide heat af formation of HCN (gas) is j-32.3 kcal mole-‘.

to CHI&H and CHO’ (H/-N) respectiveiy. The 1-n estimate of the value

of the heat of formation of methyIeneimine (gas) gives -2.4 kcal mole-‘. This assumes a value of 5 keal mole-l resonance energy, the value obtained by Coates and Sutton for N-benzy’ideneaniline (C,H,-CH-N-C,H,) after correction for the resonance energy of the two benzene rings”. Thus the heat of the ma&ox CH2=NH

(g) -2.4 A&

-+ = =

HCN (&+-I-I, (8) t32.30 0 +34.7 kcal mole-’ i-1.5OeV

and for the reaction: L Mas

Spertrcmerry

and Ion Physics,

1 (1968)

l-24

(endothermic)

THE

MASS

SPECTRUM

CH2=NH

(g) AI&

OF METHAXOL

PART

13

I

+ = =

HCN+2 W (g) + 13S.7 kcal mole-’

+ =

HCO* + Hz or 2 H- (exothermic) - 1.42 eV

(endothermic)

+6_0 eV

This compares with: CH,=O’H A&

It appears impossible to reconcile these vaIues urdess the reveF:crl in sign of the heat of reaction is attributed to excitation energy of the precursor ion_ The “zero-point” energy of the vibrational energy mode of a bond containing a heavy isotope is lower than that of the bond containing a lighter isotope. For a bond containing deuterium the energy is lo-yer than that of a bond containing hydrogen by a factor of l/,,i2 to a very close Ltpproximation. This difference, in the absence of other factors. means that the dissociation energy of the bond containing deuterium is greater than that of the bond containing hydrogen. An estimate of the difference in zero-point energies and hence the difference in dissociation energies can be obtained from IR data. The C-H bond stretching frequency v1 is 2920 cm-‘, and the two bending frequencies vy and v2 are both closely similar and may be taken to be 1400 cm-‘. Thus the zero-point energy jh(v, +v2 tvg) = 0.355 eV and therefore for the C-D bond the zero-point ener-q N 0.355 x l/,/2 = 0.251 eV. Hence the difference in zero-point energies in the ground state is -0.10 eV and the difference in dissociation energies to be expected is D(C-D)--((C-H) _ 0.10 eV. The N-H stretching frequency in amines and imines ir 3,350 cm-‘. The corresponding bending frequencies are both around 1,600 cm-‘. Thus the zeropoint energy N 0.405 eV. The N-D zero-point energy is therefore O./KG = l/,/2 N 0.287 eV and the difference in dissociation energies expected is D(N-D) --D(N-H) M 0.12 eV. This will be very similar to the value expected for I$-D>--D&H), provided the eIectronic states are comparable. The literature contains a little information from which values of some of

the quantities discussed in this work may be obtained. Although information

contains ambiguities

the literature and in many cases large uncertainties are in-

volved, the values quoted as “best” ones are used here for purposes of comparison. The difference in the strength of C-D and C-H bonds can be obtained from information

given by Field and FranklinL7:

Ion&&ion potential of CH,Appearance potential of CH3+ Appearance potential of CHsi

= 9.9 eV from CH, = 1430 ev from CH, D = 14.43 eV

Hence B(CH,-H) > 4.40 eV and D(CH,-D) --D(CH,-H) ‘v 0.13 eV. J. Mms

z#-4.53 eV. Thus D(CH3-D)

Spctrotnetry

and Ion Physics,

1 <1968) l-24

I4

J. Ii.

BEYNON,

A.

E. FOHTAINE,

G.

R. LESTER

We can also obtain an estimate of the value of the energy ga_ia PM tion of the extra bond in the product ion CHOi. Field and Franklin give: Ionisation potential of CH30H Appearance potential of CHO’

forma-

= 10.8 eV from CK,OH

= 14.3 eV

They also give some dissociation energies for bonds in molecules (or radicals) and ions: D(CH,OH-H)

= 100 kcal mole-’

= 4.34 eV-

D(CK,

=

24 kcal mole-’

= 1.04 eV

D(CHO-K)

=

79 kcal mole-1

= 3.43 eV

D(CK,hK-H)

=

27 kcal mole-’

= 1.17 eV

D(CK;&K)

=

98 kcal mole-’

= 4.25 eV

D(CH&K)

=

61 kcal mole-’

= 2.64 eV

O-K)

and

or I.47 eV

or 34 Hation’

3 gives a value for the ion&&ion potential of CHO- as 9%

These data are only consistent if the vaIue 2.64 eV is taken for D(CHO’-H): D(CK,O*-K)tD(CHO’-K)

N 6.89 eVand wecansay:

i-H- + H- requires 6.89 eV. As 2K- + Hz yields 4.48 eV

then

f CH2-OK

+

CH,-&H C$I=6’

eV. Thus

+ c:H=a+ + Hz

f

requires

2.41 eV. If the formation of a third bond in CHO’ yieids x eV then CH2=6H + C&O’-!-H, yields (x -2.41 eV). The measured value of the enxgy released is 1.42 eV and thus x = 3.S3 eV. If the hydrogen is evolved as two radicais then x = 8.31 ev.

CONCLUSIONS

The calculated differences in the bocd strengths D(C-D)--D(C-1 if a neutraI hydrogen molecule is formed in the decomposition CKO’ i- Hz or N 0.15 eV if two radicais are formed.

:j = 0.20 eV CH,=hK

+

This di!Terence has the sign expected if it is due to a change of the zero-point energies when deuterium replaces hydrogen. Its magnitude is about twice as large as the zero-point energy change calculated for the ground-state ion. The magnitude of the calculated difference if two radicals are formed is very close to the value obtained from mass-spectrometric data given by Field and Franklin for methane and mono-deuteromethane (0. I3 eV). 3- Maxs Specrrometry afxi Ion Pitysicr, 1 (1968)I-24.

THE

MASS

SPECTRUM

OF

MEI

HANOL.

PART

I

15

The calculated difference in the bond strengths D(O’-H)-Zl(O’-D) is N 0.18 eV (neutral HZ molecule) or -0.22 eV (2 hydrogen atoms)_ This difference has a sign opposite enera

to that expected

if it is due to a change of the zero-point

when deuterium replaces hydrogen. Consider

first

the

exothermicity

of

the

observed

ion

decomposition

CH2=O’-H-, CHO’ +2 N (or Hz) compared with the endothermicity of the isoelectronic imine dehydrogenation. We must conclude that the energy balance that is being measured as a release of kinetic energy is not the same as that involved in a conventional thermochemical process, where the reactants and products exist in the ground state. Whilst the numerical magnitudes are closely similar, the signs of the energies involved are opposite. following reasons: If the process were then no release of ener=T at all would concerned with a simi!ar exothermic

This difference is hardly surprising, for the strictly comparable with the irnine reaction, be possible_ If, on the other hand, we were process, then the lifetime of the ion would

be comparable with that of a single vibrational period, i.e. about 10-i ’ sec. Dissociation from a repulsive energy state would certainly not be detected in the me&stable region. Thus, we must conclude that a reverse activation energy is involved when HZ reacts with HCO’, and herein lies the source of the kinetic energy release. Such reverse activation energy is to be expected on account of the loss of some degree of n-bonding in the ion complex in the transition state. It is also to be expected by comparison with analogous processes in the mass spectra of alkanes. Thus, in the case of the propyl ion the dissociation:

C3H,+

+ C,H,‘+H,

is considered14 to involve an endothermicity of 33.2 kcal mole-’ and a reverse activation energy of 16.5 kcal mole -I. The forward activation energy is, of course, the sum of these, i.e. 49.7 kcal mole-‘_ The source of the reverse activation ener=gy is the loss of the double bond in the ally1 ion CH2-CH=CH2. that the addition of HZ to &CH

involves a similar activation

Our results suggest that the reverse activation

It will be seen

of a multiple

bond.

energy of the reaction CH,=&H

+

and the endothermicity HCO+ + H2 or 2 H- is 32.7 kcal mole -’ (1.42 eV mole-‘) is about 34.7 kcal mo!e-’ (I .6 eV moie- ‘), values which now begin to appear entirely reasonable. It is not surprising that the reverse activation energy is as higher than for the ally1 ion addition reaction. Indeed, much as Z6.2 kcal mole-’ if one allows for the fact that the third bond in HC=N is some 11 kcal mole-’ stronger ihan the second bond in CH2=CH2, the correspondence becomes very much closer. It is therefore a highIy excited state of the precursor ion that dissoci&es with a release of kinetic energy. The question might legitimately be asked as to why this internal enerq is not redistributd according to an equipartition principle, thus implying that most of the internal excitation remains in the products. A secJ. Mass Spectrometty

and Ken Physics,

1 (1968) l-24

16

J. H. BEYNON,

A. E. FONTAINE,

G. R. LESTER

ond important question concerns the reproducibility of the energy release, which is apparently independent of the original impact energy, and has, therefore all the appearance of a release of bond energies du : to a different pairing of electrons in the reactant and product species. Both qu,sions can be answered if it is accepted that the reactant species is not to be understood as a viborationaily-excited ground state (the so-called harmonic oscillator model) but rather as a modified electronic state with correspondingly rearranged nuclear confi,~tions. There is consequently no question of equipartition of vibrational quanta because these are not present as such; rather must one visual&e a strict electronic process proceeding synchronously with the dissociation because of the instantaneous character of electronic motion. Thus, the ener=T release is precisely determined. This assumes, that the eIectronic energy of the excited state is accurately quantized, i.e. that the energy Ievels are not unduly blurred by a vibronic coupling effect. It may be the case that anharmonicity effects in the oscillator model or blurring of electronic energy levels in the transition-state mode1 both tend towards a similar intermediate type of system which is not too well-defined quantum mechanically. In such cases it will not be easy to interpret the ener_ey redistribution in precise terms, according to well-defined selection r&s_ But provided we restrict attention to a comparison of deuterated aAogues, these uncertainties can be regarded as self-cancelling and we can have confidence that bond-energy difference determinations are mean+&& Zero-point energies determined on the basis of mass-spectrometric results will necessarily refer to thti activated state and we can do little at present but make a rather crude allowance for the electronic activation of the ion. If we may again draw an analogy with the propyl ion, it has been suggested” that (at feast as far as the active hydrogen is concerned) the following are reasonable v&es of vibrational frequencies.

Ground state

Actiuaied slate

C-H stretcJSng

3,MtO cm-’

C-H C-H

1,400 cm-’ 1,490 cm-’

Imaginary 3,000 cm-l 1,000 cm-’

bending (in plane) bending (out of plane)

If, for the moment, we ignore any contribution from the reaction co-ordinate itself, the relevant zero-ptiint energies will be related as follows: Zero-point

energy of CH bond in activated state = 0 7O

Zero-point

en-er=T of CH bond in ground state

m

Thus we shall calcillzte the effect of deuterium substitution as though the precursor isn were in the ground state, and reduce this by 30% if we wish to recognize that the zero-point energies refer to a distinct equilibrium configuration of the nuclei. It wi!l be additionally shown that the ef%ct of the imaginary frequency in the reaction co-ordinate can be given an interpretation as a negative zero-point eneragy,

THE

MASS

SPECTRUM

OF METHANOL.

PART

17

I

corresponding to tunnelling through the barrier. would be to evaluate the above factor empirically

An alternative from our data;

interpretation it will be seen

that the empirical value is actually somewhat larger than unity. Such increase in the zero-point ener,gy factor must be primarily dependent on the incidence of additional vibrational degrees of freedom in the transition state, for example, the hydrogeos might be fairly tightly coupled. This information might be said to provide badly needed factual data concerning the nature of the transition state. Because alternative energy surr’aci~ ;>ing close together may well arise (an example is the possible adiabatic dissociation wherein the nuclei rearrange simultaneously in order to adopt the energetically most favoumble form, or on the other hand the “vertical” process where the dehydrogenation occurs too rapidly for such steric rearrangement to be realised) there is the added complication that the dissociation path adopted might be isotopically dependent_ But we shall adopt the assumption that the main factor controlling the reaction path is not the isotopic mass but the rate of traversal of the cross-over between possible energy surfaces_ Such trdversal rate will be largely independent of any specific isotope effects, because of the requirement that meta-stable decompositions detectable in a mass spectrometer must have lifetimes lying within a fairly narrow range. This range varies from one instrument to another but the important feature is that under the same set of conditions in the same instrument we can be reasonably confident that the same energy surface is adopted for a given ion and its deuterated analogues. This assumption is obviously essential to our approach for deriving meaningful thermochemical data from “mera-stable peak” widths. There remains the additional question as to whether the excess energy in the activated ion is also independent of isotope effects. More specifically, we must be quite sure that the “kinetic &ift”xs is either negligible or cancels out whenever we compare a given process with that for the deutemted snalogue. Implicitly, and because it has been regarded as intuitively obvious, we have invoked the idea that such excess energy is eliminated in such comparisons. In the notation of the body of the paper, we can assume that (.erS-cl,) = (eZB-cZZ), etc. That the magnitude of the kinetic shift is likely to be either neghgible or the isotopic effect almost the condition:

self-cancelling

NN

can be inferred

from simple consideration

of

106 Xc-‘,

where E is the total amount of internal excitation activation energy and S is the number of oscillators.

ener,T,

E,

is the actual

For the (p- l)+ ion CH-$H, we have S (the nominal number of oscillators) =9. Not 211 these are likely to be effective if a more accurate statistical treatment is adoptedi6. In the extreme case of equal effectiveness of all oscillators (S = 9) J. Mass Spectrometry

ancf Ion Physfcs,

1 (1968) l-24

18

J. H. BEYNON,

A. E. FONTAINE,

G. R. LESTER

and taking the frequency factor as of the same order as a vibrational frequency, il is found that E/E, = 1.i 11 (hydrogen), or 1.117 (deuterium). Whilst the actual kineric shifts are by no means negligible the isotopic effect is certainly

small.

How-

ever, if only one third of the oscillators are effective, the corresponding values of E/E0 are 1_ooO1 for both hydrogen and deuterium. The kinetic shift E-E, is then small or negligibIe both in an absolute and in a relative sense. Thus we may concIude with some confidence that, at Ieast so long as we are dealing with precursor ions comparable in size and number of effmtive oscillators with the methanol ion, the excess energy may be ignored in the type of comparisons with which we are concerned in this work. It remains to consider the various contributions to zero-point energy. Clearly in the reactant ion itself, there are three important components corresponding to the stretching mode along the reaction co-oidinate and to two bending modes. In the excited state leading to dissociation, dne frequency (vl) becomes imaginary because the ener,gysurface contains a saddle-point and the cross-section along this direction may be regarded as an inverted pqrabola to the first approximation, i.e. the case of negative curvature_ For the bending modes the corresponding vibration frequencies (vl and v3) are real. At first sight it might be considered that the zeropoint contribution of an imaginary frequency should be put identically equal to zero, because it is meaningless to talk of energy +hv, when v is an imaginary quantity. But the meaning to be attached to the zero-point energy is now to be interpreted as the lowering of the energy of the barrier because of a tunnelling process depending on the isotopic mass. It is convenient, for purposes of exposition, to refer tc a negative zero-point enera in this context. This has been evaluated for the idealized case of a parabolic energy barrier I7 . An isotopic inversion effect will occur whenever the zero-point energy for the imaginary frequency lowers the effective peak height of the barrier rbr the lighter mass isotope_ Whereas we would normally expect a lower dissociation energy for the lighter species, on account of increased zero-point energy in the reactants, yet the accompanying tunnelling effect can reverse this expectation if the barrier is narrow enough. If the curvature is relatively low_ the classical case is approached because the imaginary frequency te:lds to zero. If it is imagined that the barrier curvature is progressively reduced the isotopic inversion effect will mt of all decrease in magnitude whilst preserving the same sign, i.e. with the hydrogen linkage stronger than the deuterium analogue. This follows directly from the reversal in sign of the zero-point ener_q for that direction in the energy hypersurface with negative curvature_ But in the limit where the curvature along this direction approaches zero, the zero-point energies in the bending modes will become numerically larger and determine the sign of the effect. Since the curvature of the hypersurface for these modes is of the positive sign, the order of bond energies is then the normal one_ There will presumably be a neutral region where bond energies are isotopic&y insensitive because the negative and positive zero-point energies in the transition state mutuay cancel.

THE

MASS

SPECTRUM

The indications

OF METHANOL.

PART

are that bonds involving

19

I

the neutral carbon atom show a “nor-

mal” thermochemical behaviour presumably because the barrier is flat-topped and the imaginary frequency is consequently relatively small. Thus, the measured energy release is determined

by directions of positive curvature as in a conventional

chemical reaction, where reactants and products are in the ground state. On the other hand, it would seem that the enera barrier for a byciroxyl dissociation in CHz-6H must be sufficiently narroq for isotopic inversion to occur, thus explaining the apparent occurrence of the deuterated form withadditional excitation. It would appear that the origin of the excited quantum is to be found in the tunnelhng effect of the lighter isotope, any such lowering of the barrier having the same effect observationally as higher excitation of the initial species. It is easy to see that in principle inversion is realisable if the barrier permeability becomes sufficiently large. Thus, if one takes the case of an energy barrier of height EO. then in the classical case, the unimolecular dissociation rate constant is E-E,

s-’

( >

v-

E

Suppose that a reduced energy E,’ is sufficient becauseof barrier permeability. Then if the permeability factor is unity, the rate is increased over the classical value to:

But an approximate I

+

exp

expression for the permeability

P7i(Eo- G)l

factor G(E’) is:

-’

hV, m is the cRective mass and 2a the barrier width at haif where v, = ~i’~ojxaj2m, height. BeII’ 7 has used the simpler form exp[2z(E0 -E&)/hvJ which is an adequate approximation for moderately large degrees of tunnelling. Instead or passing over the energy barrier (in which case we have already seen that the kinetic shift is exceedingly small) we shah suppose that the mean vibrational energy content exceeds appreciably the height of the barrier. In the absence of tun.neIling such ious wouId normally undergo dissociation within the ion source because the rate constant is much greater than i05 set-‘. On the other hand, it may be supposed that it takes much longer for the requisite energy E, to concentrate in the reaction co-ordinate than the lower ener-7 E& insufficient for passing ocer the barrier but sufficient for Ieakage through it. In such cases, where the barrier itself is very narrow, we may infer that tunnelling is the more likely mechanism. It may be supposed that the system is not disposed to delay dissociation 3. Mass Spectromerry

and Iox Phy+cs,

1 (1968)

1-24

20

J. H. BEYNON,

A. E. FONTAINE,

G. R. LESTER

until the till activation ener=T E, is available when it can do so by a more facile

leaka& process. Replacing the ordinary unimolecuIar rate equations for OH and OD,

and we have now:

0 ) (hydrogen) k” =

5

(y)‘-’

exp

(--2n~;Et]

(2)

(deuterium)

To simplify matters vve suppose that the mean internal energies I? are the same, although in a Franck-Condonprocess these will differ somewhat because of the different intersections of the zero-point energy levels with the potential energy curve. However, such difference will be neglected in the present argument. Moreover, there is a band of excitation energies which is here simplif?ed to a single representative mean vaIue corresponding to the effective “temperature” of the system if an Arrhenius factor had been introduced. From consideration of effective masses:

As possible illustrative values we take E = i-2 E,, E,‘, = 0-S E,,, v = lOI xc-‘, k = lo6 set-‘, S = 9, and write E,jhv: = x, a shape parameter reIating barrier hei&t and barrier curvature at *!L.zsummit. Then, from eqn. (I) we fYindx = 7.67. In eqn. (2) we find that k” >> k’ unless the tunnehing energy Ez above the hydrogen

value, i.e. we write E;

= yE6 where )’ >

is increased

1 and the source

of the excess ener,T in the OD bond can be explained as the additional energy (y-

1)Ei required to bring the tunnelhng dissociation rate for OD into the

same range as that for OH. From (2) we have: IO6 = T

(I -$y)*

exp f--2rr,i2x(l

-O.Sy)]

t-

which is satished by y = 1.12. Thus the additional excitation of OD is 0.8(-v- l)E, = 0.096 E,_ On the reasonable basis that E. is likely to be something in the order of IOhv, it would appear that the origin of the apparent additional may be thus interpreted. J. Afuss Spectrometry

and Ion Physics, 1 (1968) i-24

excitation in OD

THE

MASS

SPECTRU55

OF -METHANOL.

PART

I.

21

APPENDIX The

condifiorts

implicit

it2 Q

demkation of thermochemical

properties from %ma-

stable peal?’ analysis The classical

activation energy E, is measured from the lowest points on the

potential energy curves, which are themselves virtually independent of the particular isotope. E, is the true activation energy. The difference in electronic energy of products and precursor ion is denoted by EP, and the corresponding endothermicity of the overall reaction, including zero-point contributions, is AH. Ei is the reverse activation energy (Fig. 2.).

Fig. 2 Schematic representation of the energy relationships OF the potential ener_gy CUNeS precursor, transition state and product ions in an energetic meta-stable decomposition.

Following

E, = EC;

Glasstone

of

et a1_18 we have:

C ~hv: - C

9hvi

(1)

where vt , Vi are respectively transition-state and ground-state frequencies. The kinetic shift is measured by any non-fixed vibrational energy in the transition state. The kinetic ener,T released is

T = E, +x

fhv:-

EP -c

+hv, + (kinetic shift - vibrational energy of products)

in terms of the ckusical activation I = E, +x=$hv; -E,

.

-_Cfhv,+

(2)

energy, or

(kinetic shift -vibrational

energy of products)

(3)

in terms of the true activation energy. To a good approximation it is now considered that the terms in the last bracket can be equated to zero, since it may be supposed that any non-fixed energy in the transition state, which has been designated as a kinetic shift, wiii remain as vibrational energy in the product. At least, when isotopic species are compared, a cancellation of such second-order quantities seems likely. It is this interpretation J- Mass Specrromexry and Ion Physics. 1 (1968) l-24

22

J. H. BEYNON,

A. E. FONTAINE,

G. R. LESTER

that we give to the neglect of z+~-E~=, E~~-E~~, _ _ _ in the earlier part of this paper-

Again, for isotopic species, E, is variable but E,, depending as it does only

on the potential ener_q scrface as such, is fixed. Thus it seems more appropriate to use (2) tnan (3) and to infer that the quantity we are concerned with is c+hv’ rather than c fhq. On the other hand, if electronic energy in the activated state were not well quantized, a larger part of the excitation energy of the precursor ion would have to he regarded as essentially non-fixed energy. It might then be considered that E, is not strictly a constant determined only by the form of energy hypersurface, but an energy quantity s:lbject to its own “hidden” vibrational component varying with isotopic mass. This would introduce some further uncertainty in the meaning to be attached to our calculated zero-point energies, but this effect must remain a subject for further exploration. It is clearly of fundamental importance to tsy to assess whether a transition state in the sense of Glasstone et al-l8 or the oscillator model of SIater’g is a more satisfactory physical conception as applied to the transitory ionic species encountered in mass spectra. We consider that the metastable ion technique can contribute signifkantly to our more detailed understanding of particular theoretical models. It is not difhzult to show that the inversion effect cannot be accounted for by lower energy of the zero-point IeveI in the products. Whilst it is quite true that z negative zero-point level in the reactant species or a larger positive zero-point ener_gy in the products produce an equivalent effect, it is easy to show that the magnitude of the latter IS insufficient, on its own, to lead to .eversal of sign in the differential release of kinetic ener-v. This may be seen on consideration of the extreme case where we disregard the react-,& zero-point energies altogether. Then: D(O’-H;--D(O‘-D)

= Zero-pokt

The product ion Cl-510~

energy of H-H-zero-point

energy of H-D.

is of course identical whether the precursor

is deuterated or not. From consideration

ion

of effective masses,

,3 Yn-u = 3 - vn-n 2 whence; D(Ot-H)-_(Oi-D) = +hvu-u(1 --JT/2). Taking the Raman frequeacy for HZ as 4160.2 cm-’ we find a value of 0.035 eV for this difference in dissociation energies_ Clearly this value would be much reduced by compensatin, = zero-point energy differences in the reactants and ti-ie observed 0.18 eV carrot be interpreted in any such straightforward manner. Further evidence, if it were needed, is provided by ion dissociations where a single atom is eliminated J. Mass Spechomehy

and where the order of magnitude and Zon Physics, 1 (1968)

l-24

of D(O*-H)--D(O’-D)

THE

MASS

SPECTRUM

OF METHANOL.

PART

I

23

is unaffScted to the fmt order. Thus in the reactions H-C=O’-H H--&0+-D

+ HCO+ f Hi HCO++Da

we still i5nd a difference of 0.20 eV despite the non-occurrence of any zero-point energy differences in the products. Thus, we feel that the evidence is very strong for a sign reversal effect in the excited state of the precursor ion and this can hardly be h.:erpreted in any other way than that of a pseudo negative zero-point Ievel when appreciable tunnelling is possible_ In terms of our energy diagram it is possible to give an inierpretation of the excitation ener,oy differences cIB-cIX, cZa-s2= _ _ and an indication of what is involved when we negiect such differencesNow e’SZ.= excitation energy of ml7 in the sth reaction, &ss = excitation energy of I+~ -t-m, in the sth reaction. Clearly, shift in reactant ion 5.x = E,tkinetic = EC+ c fhvt -c fhv, + (non-fixed vibrational

and

energy in reactant ion)_

&58 = non-fixed vibrational energy in products E~-.Q = Ectx +- ( non-fixed vibrational energy in reactants fh(v:-v,) -non-tied vibrational energy in products).

Taking the three components of .s,,- sSs in turn, the fixed quantity E, is eliminated whenever we make a comparison of two equivaIent reactions with a simple isotopic change. The second quantity is itself second-order since it involves a difference of zero-point energies in two states of the initial species; it is therefore even less important when we consider an isotopic change but must be considered when we wish to make a more detat:d analysis. The third quantity is considered seif-cancelling if there is, apart from fluctuations, a simple equipartition of aLI such non-fixed energy in the process of dissociation. Thus, with the proviso made, and to keep the interpretation of the results as simple as possible, we disregard these excitation energy differences in the thermochemical energy balances.

SUMMARY

The stases in the electron-impact induced fragmentation of the methanol molecuie-ion have been elucidated by the study, using a double-focussing mass spectrometer, of methanol and the three deuterated analogues CH,OD, CD,OH and CD,OD.

24

J. H. BEYNOK,

A. E. FONTAINE,

G. R. LESTER

The exothermic meta-stable ion decompositions (CH,O)’ + (CHO)+ and (CH,O)f + (CHO)-_ are of special interest since a caiculation of relative bond strengths in the ion from the widths of the “meta-stable peaks” accompanying the decompositions gives the apparent result that the bond strength of (O’-H) is greater than (O&-D) as well as the result that (C-D) is stronger than (C-H j_ This effect is discussed in det.aX An explanation of it in terms of an isotopically-dependent tunneliirg process which has the same effect observationally as different degrees of {xcitation of the decomposing species, is given.

REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

_-lass Spectral Data, American PetroIeum Institute, Research Project 44, Nos. 282 and 363. J. H. BEYSON. k E. Fosre~ XXD G. R -% to be published. R. D. Crurc, B. N. GREEN ASD J_ D. WALDXOX, Chhia, I7 (f963) 33_ J. H. FUTTRELL, K. R. RYAX ASD L. W_ SIECK, J_ Chem. Phys_, 43 (1965) 1832. J. H. BEYXON AND A. E. FoY.ir~!h~, Some newer phlsicai methods in structural chemistry, United Trade Press. London, 1967, p. 118. K. R. JENNINGS, 1. Chem. Ph)s., 43 (1965) 4176. H. BUDZIWEWICZ, C. DJERASSI ASD D. H. WILLIXXS, Inrer~rerafion of MassSpectra of Organic Cempoun& Holders-Day. San Frnncko. 1964. p_ 29_ J. H. BEYSON. R. A. SAUP~DERS ASP A. E. WILLIAMS, Z. Natur-rsch., 2Oa (1965) 180. 3. H. BEYSON AXW A_ E Fox~~\x, Z_ Naflirforsch., 2% (1967) 334_ Nan&auk of Chemistry and Physics, 48th ed., Chemical Rubber Publishing Co., Cleveland, Ohio, i967. G_ E. COAASD L E Srrrrox, J_ Chem. Sot., (1948) 1187. F. H. FIELD ?BD J. L. FRANKLIN, Efecrron Irnpg:r Phenomena. Academic Press inc., New York, 1957, pp. 143, 249. A_ G. Hxrtnrjon in F. W. MCLAFFERTY (Cd.), &lass Spectrometry of Organic Ions, Acndemic Press, New ‘fork, 1963, p. 207. H. M. ROSENSTOCK, Doctoral Thesis, University of Utah, 1952, pp_ 41, 74. W_ A. CHUF:X, i. Chem. Ph_rs_, 30 (1959: ‘91. M. L. VESTAL, A. L_ WAHRHA~G XE;C W. H. JOHXSTOX, J_ Chem. Phys.. 37 (1962) 1276. R. P. BELL, Trans. Faraday Sot., 55 (1959) I_ S. GLASSTOIE, K. J. J~AIDLER ASD H. EYR~KG, The Theory of Rate Processes, McGraw-Hill, New York, 1941. N_ B_ SLATER_ The Theory of Linimolecular Reactiottx, Methuenand Co., London, 1959.

J. Mass Specrromemy and Ion Physics, 1 (1968)

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