FEATURE  ARTICLE
Molecule Matters
van der Waals Molecules
1. History and Some Perspectives on Intermolecular Forces
E Arunan
E Arunan is an Associate
Professor at the Inorganic
and Physical Chemistry
Department, Indian
Institute of Science,
Bangalore. His research
interests are in molecular
and van der Waals
spectroscopy, chemical
kinetics and dynamics,
hydrogen bonding and
other molecular
interactions.
Keywords
van der Waals forces, hydrogen
bonding.
v a n d e r W a a ls in tro d u c e d h is e q u a tio n o f sta te in
1 8 6 7 m o d ify in g th e id e a l g a s e q u a tio n . H is e q u a tio n h a d tw o c o n sta n ts a a n d b , w h ich a c c o u n te d
fo r th e fa c t th a t m o le c u le s h a v e a ttra c tiv e fo rc e s
a n d ¯ n ite siz e . T h e c o n sta n t a a c c o u n te d fo r th e
a ttr a c tiv e fo r c e s, th e n a tu re o f w h ich w a s n o t
o u tlin e d b y v a n d e r W a a ls. T h is se rie s o f a rtic le s
w ill d isc u ss a b o u t th e n a tu re o f in te rm o le c u la r
fo r c e s .
1 . A to m ic H y p o th e s is
If o n e w ere to select a sin g le m o st im p o rta n t d iscov ery
o f scien ce, th e a to m ic h y p o th esis, i.e., th a t a ll th in g s
a re m a d e o f a to m s, w o u ld b e a stro n g co n ten d er. O f
co u rse, a to m s w ere p o stu la ted sev era l th o u sa n d y ea rs
a g o in In d ia , a n d G reek p h ilo so p h ers h a d d iscu ssed it
a s w ell. T h e ren ow n ed T a m il p o et sa g e T h iru va llu va r's
T hiru ku ral (litera l tra n sla tio n m ea n in g h o n o ra b le sh o rt
v erse) p rov id es g u id elin es to b e fo llow ed u n d er a ll p o ssib le circu m sta n ces. It is m o re th a n 2 0 0 0 y ea rs o ld a n d
it rem a in s a n d w ill rem a in releva n t. It p a ck s so m u ch
in fo rm a tio n in a co u p let (tw o -lin e p o em ), th a t a n o th er
ren ow n ed T a m il p o et, L a d y A u w a iy a r, co m p a red it to
p a ck in g sev en o cea n s in to o n e a to m (an u in T a m il, th e
w o rd p o ssib ly h av in g its o rig in in S a n sk rit). W h a t is
in terestin g a b o u t A u w a iy a r's co u p let o n T h iru k u ra l is
Kural packs so much in a couplet that it is
equivalent to
piercing an atom and inserting the seven
oceans into it.
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FEATURE  ARTICLE
th a t it ta lk s a b o u t `p iercin g a n a to m a n d in sertin g sev en
o cea n s in to it'. H ow ev er, it w a s o n ly in th e la st cen tu ry
th a t in d isp u ta b le ex p erim en ta l ev id en ces fo r a to m s w ere
o b ta in ed . T o d ay, a to m s ca n b e v isu a lized w ith so p h istica ted m icro sco p y tech n iq u es th a t a re p u sh in g th e reso lu tio n o f m ea su rem en t to su b n a n o m eter (A n g stro m )
lev el (1 n m = 1 0 ¡9 m = 1 0 ºA ), th e a to m ic d im en sio n [1 ].
A to m s p h y siso rb ed o n a su rfa ce ca n n ow b e p ick ed a n d
m ov ed a ro u n d !
Figure 1. Structure of H2O:
red sphere represents oxygen atom and the grey ones
represent hydrogen atoms.
2 . T w o Im p o r ta n t M o le c u le s: W a te r a n d D N A
A to m s h av e rem a in ed a d o m a in o f p h y sics, ra th er th a n
ch em istry, a s m u ch a s o n e ca n d raw th ese a rti¯ cia l b o u n d a ries b etw een d iscip lin es. A to m ic p h y sics is a sta n d a rd
su b -d iv isio n o f p h y sics a n d a to m ic ch em istry d o es n o t
ex ist. C h em istry is a m o lecu la r scien ce. It is so m u ch
so th a t recen tly th ere w a s a su g g estio n th a t A m erica n
C h em ica l S o ciety b e ren a m ed S o ciety fo r M o lecu la r S cien ce a n d E n g in eerin g [2 ]. M o lecu les, a s w e a ll a re aw a re,
a re m a d e u p o f a to m s; d i® eren t a to m s co m b in e o n ly in
select p ro p o rtio n s a n d m u ch o f it w a s esta b lish ed b y th e
tu rn o f th e 1 9 th cen tu ry b y J o sep h P ro u st [3 ] a n d J o h n
D a lto n [4 ]. A s w e a ll k n ow , w a ter is a m o lecu le m a d e u p
o f tw o h y d ro g en (H ) a n d o n e ox y g en (O ) a to m s a n d rep resen ted b y th e m o lecu la r fo rm u la H 2 O a n d p icto ria lly
a s in F igu re 1 .
T h e m o lecu le o f life, D N A , o n th e o th er h a n d is m a d e u p
o f th o u sa n d s o f a to m s o f H , C , O , N a n d P . It is im p o ssib le a n d a lso m ea n in g less to w rite a m o lecu la r fo rm u la
o r stru ctu re m a d e u p o f a to m s fo r D N A a n d b io lo g ists
h av e b etter w ay s o f rep resen tin g D N A (see F igu re 2 ).
D N A ex ists a s a d o u b le h elix a n d th e rea d er is referred
to th e b o o k b y S in d en fo r d eta ils [5 ]. T h ere is a m o re
im p o rta n t d i® eren ce b etw een sin g le m o lecu les o f w a ter
a n d D N A . In w a ter, ox y g en is cova len tly b o n d ed to th e
tw o h y d ro g en a to m s a n d th ere a re tw o O -H cova len t
b o n d s. A co va len t b o n d b etw een tw o a to m s im p lies th a t
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Atoms have
remained a domain
of physics, rather
than chemistry, as
much as one can
draw these artificial
boundaries between
disciplines. Atomic
physics is a
standard subdivision of physics
and atomic
chemistry does not
exist. Chemistry is a
molecular science.
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FEATURE  ARTICLE
Figure 2. The double helix of DNA is shown
along with details of how the bases, sugars
and phosphates connect to form the structure of the molecule. DNA is a double-stranded
molecule twisted into a helix (think of a spiral
staircase). Each spiraling strand, comprised
of a sugar-phosphate backbone and attached
bases, is connected to a complementary
strand by non-covalent hydrogen bonding
between paired bases. The bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A and T are connected by two hydrogen bonds. G and C are connected by three
hydrogen bonds. Moreover, the bases interact through stacking schematically shown
by the dashed line in color.
(Figure taken with permission from the following
website: http://www.accessexcellence.org/RC/VL/GG/
dna_ molecule.php).
ea ch a to m co n trib u tes a n electro n a n d th e tw o electro n s
rem a in la rg ely b etw een th e tw o a to m s h o ld in g th em to g eth er. T h o u g h th e tw o electro n s w ith th e sa m e n eg a tiv e ch a rg e rep el ea ch o th er, th ey a cq u ire o p p o site sp in s
a n d sh a re a sp a tia l o rb ita l lo cated m o stly in b etw een th e
tw o a to m s, h o ld in g th e tw o n u clei to g eth er in a b o n d .
3 . In te rm o le c u la r In te ra c tio n s in W a te r a n d D N A
Hydrogen bonding
occurs in water as
well and in fact it
is the hydrogen
bonding that
makes water what
it is, i.e., the
molecule without
which life as we
know would not
exist.
348
In D N A , w e n o t o n ly h av e n u m ero u s su ch cova len t b o n d s
b u t a lso h av e o th er in tera ctio n s w h ich a re ca lled h y d ro g en b o n d in g a n d ¼ { ¼ in tera ction s o r ¼ sta ck in g (see F igu re 2 ), th e d eta ils o f w h ich w e w ill a d d ress in th is series
o f a rticles. T h ese h av e b een ca lled n o n -cova len t in tera ctio n s in th e litera tu re to d i® eren tia te th em fro m th e
cova len t in tera ctio n s m en tio n ed a b ov e [6 ]. T h e D N A is
o n e g ig a n tic m o lecu le (co u ld b e a s lo n g a s 3 cm !) a n d
m o lecu la r b io lo g y is a n esta b lish ed ¯ eld n ow . H en ce th e
h y d ro g en b o n d in g o ccu rrin g in D N A ca n b e lo o sely co n sid ered a s in tra -m o lecu la r h y d ro g en b o n d in g . H y d ro g en
b o n d in g o ccu rs in w a ter a s w ell a n d in fa ct it is th e
h y d ro g en b o n d in g th a t m a k es w a ter w h a t it is, i.e., th e
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FEATURE  ARTICLE
Figure 3. Structure of the
isolated water dimer:
(H2O)2.
(Figure taken from: http://
www.lsbu.ac.uk/water/
hbond.html)
m o lecu le w ith o u t w h ich life a s w e k n ow w o u ld n o t ex ist.
H ow ev er, in w a ter h y d ro g en b o n d in g is in term o lecu la r,
i.e., h y d ro g en b o n d in g lin k s tw o m o lecu les o f w a ter. In
liq u id a n d so lid p h a ses, ea ch w a ter m o lecu le is ty p ica lly
h y d ro g en b o n d ed to fo u r w a ter m o lecu les fo rm in g a
n etw o rk . T h e stru ctu re o f w ater d im er is sh ow n in F igu re 3 a n d th a t fo u n d ty p ica lly in liq u id w a ter is sh ow n
in F igu re 4 .
4 . v a n d e r W a a ls E q u a tio n
T h a t in term o lecu la r fo rces ex ist m u st b e o b v io u s, a s
w ith o u t th em a ll su b sta n ces w o u ld h av e rem a in ed g a ses
a t a ll tem p era tu res. In d eed , w h en co o led to low er tem p era tu res a ll g a ses w ith o u t ex cep tio n co n d en se to b eco m e liq u id s o r so lid s. H ow ev er, so m e g a ses b eco m e
liq u id s a t m u ch h ig h er tem p era tu res th a n o th ers. F o r
ex a m p le, w a ter va p o u r co n d en ses a t 1 0 0 ±C w h ile h y d ro g en su lp h id e (H 2 S ) co n d en ses a t th e m u ch low er tem p era tu re o f { 6 0 o C . E v id en tly, th e in term o lecu la r fo rces
Figure 4.Typically each water molecule has four hydrogen bonds in liquid and
solid phases of water as
shown by the dotted lines
forming an extended network. The oxygen atoms
form a tetrahedron with
one at the center and the
other four at the corners as
shown in the inset. In ice
the hydrogen bonds lead
to a rigid structure. However, in liquid water these
hydrogen bonds can be
broken and formed easily
and it is important to realize that a static structure
cannot represent the liquid water at all.
(Figure taken from: http://
www.lsbu.ac.uk/water/hbond.
html)
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in w a ter a re m u ch stro n g er th a n th o se in h y d ro g en su lp h id e. In term o lecu la r fo rces lea d to d ev ia tio n fro m th e
id ea l g a s law , w h ich w a s p u t fo rw a rd b y C la p ey ro n in
1 8 3 4 [7 ]. T h e id ea l g a s law a ssu m es th a t m o lecu les a re
p o in t m a sses a n d th ey d o n o t in tera ct w ith ea ch o th er.
It rela tes th e p ressu re (P ), v o lu m e (V ) a n d tem p era tu re
(T ) o f n m o les o f a g a s th ro u g h th e g a s co n sta n t R a s
P V = n R T . va n d er W a a ls rea lized th a t th e ¯ n ite size o f
th e m o lecu les a n d in term o lecu la r a ttra ctio n w o u ld ca u se
d ev ia tio n fro m id ea l g a s b eh av io u r. T h e ¯ n ite size o f th e
m o lecu les red u ces th e ava ila b le v o lu m e in a co n ta in er.
In term o lecu la r a ttra ctio n red u ces th e p ressu re ex erted
b y th e m o lecu les o n th e w a lls o f a co n ta in er, a s th ey
a re p u lled tow a rd s ea ch o th er b y th is a ttra ctio n . H en ce,
th e rea l v o lu m e is less th a n th a t o f th e co n ta in er a n d
th e rea l p ressu re w o u ld h av e b een m o re th a n w h a t is
m ea su red b u t fo r th e in term o lecu la r a ttra ctio n . B a sed
o n th ese em p irica l rea so n s, va n d er W a a ls p ro p o sed th e
eq u a tio n n a m ed a fter h im , in 1 8 6 7 in h is P h D th esis [8 ].
³
a ´
P +
(V ¡ n b) = n R T :
n 2V 2
T h e co n sta n t a d ep en d s o n th e in term o lecu la r p o ten tia l
w h ich co n tro ls th e in term o lecu la r in tera ctio n a n d b is th e
ex clu d ed v o lu m e fro m th e co n ta in er th a t is n o t ava ila b le
fo r th e g a s m o lecu les to o ccu p y d u e to th e ¯ n ite size o f
th e m o lecu le.
5 . v a n d e r W a a ls M o le c u le s
T h e term `va n d er W a a ls m o lecu les' is u sed to d escrib e
th e w ea k ly -b o u n d co m p lex es th a t a re b o u n d b y `va n d er
W a a ls' fo rces. T h is sta tem en t h a s a circu ito u s lo g ic a n d
o n e o f th e m a in rea so n s is th a t va n d er W a a ls d id n o t
sp ell o u t th e n a tu re o f th e in term o lecu la r fo rces h e in v o k ed to a cco u n t fo r th e d ev ia tio n fro m id ea l g a s law .
H en ce, a ll in term o lecu la r fo rces ca n b e sa fely co n sid ered
to b e va n d er W a a ls fo rces. H ow ev er, o n e m u st a d d
th a t th ere is n o u n a n im o u s v iew a b o u t w h a t is m ea n t
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b y va n d er W a a ls fo rces [9 ]. In th is series o f a rticles,
w e ta k e va n d er W a a ls fo rces to im p ly a ll in term o lecu la r fo rces. A m o n g ch em ists, recen tly a n o th er term is
b eco m in g p o p u la r a n d th a t is `clo sed -sh ell in tera ctio n ',
¯ rst p ro p o sed b y B a d er [1 0 ].
A s m en tio n ed a b ov e, a cova len t b o n d is fo rm ed w h en
a n electro n fro m ea ch a to m is sh a red to fo rm th e b o n d .
T h e H 2 m o lecu le fo rm ed b y th e tw o H a to m s is a cla ssic
ex a m p le fo r a cova len t b o n d . T h is is ca lled `sh a red sh ell in tera ctio n '. In a n `io n ic b o n d ' su ch a s fo u n d in
K C l (in th e g a s p h a se), a n electro n fro m p o ta ssiu m is
tra n sferred to C l a n d so w e h av e K + a n d C l¡ fo rm in g
K C l m o lecu le. B o th K + a n d C l¡ h av e clo sed sh ells o f
electro n s a n d a n io n ic b o n d w o u ld b e a clo sed -sh ell in tera ctio n . O f co u rse, b o th o f th ese a re in tra -m o lecu la r
in tera ctio n s. Io n ic b o n d in g co u ld b e fo u n d n o t o n ly b etw een tw o a to m s, b u t a lso b etw een tw o g ro u p s o f a to m s
su ch a s in a m m o n iu m n itra te N H 4 N O 3 , h av in g th e p o sitiv e a m m o n iu m io n a n d th e n eg a tiv e n itra te io n . In v a n
d er W a a ls m o lecu les w e n eith er h av e tw o electro n s fro m
tw o a to m s fo rm in g a b o n d o r a n electro n fro m o n e tra n sferred to a n o th er. va n d er W a a ls m o lecu les a lso h av e a
clo sed -sh ell in tera ctio n , a s th e electro n s fro m b o th th e
in tera ctin g m o lecu les a re la rg ely reta in ed w ith in th em selv es a n d still th ey ex p erien ce a n a ttra ctiv e in tera ctio n .
T h u s w h en th ese in tera ctio n s resu lt in co n d en sa tio n o f
a m o lecu le, th e id en tity o f th e m o lecu le w ith in its liq u id a n d so lid p h a ses rem a in s in ta ct. H en ce th e n a m e
clo sed -sh ell in tera ctio n .
In van der Waals
molecules we neither
have two electrons
from two atoms
forming a bond or an
electron from one
transferred to another.
van der Waals
molecules have a
closed-shell
interaction, as the
electrons from both the
interacting molecules
are largely retained
within themselves and
still they experience an
attractive interaction.
6 . N a tu r e o f v a n d e r W a a ls F o rc e s
6 .1 D ip o le -D ip o le In te ra c tio n
W h a t is th e n a tu re o f va n d er W a a ls fo rces? A cco rd in g to D eb y e [1 1 ], o n ly in th e b eg in n in g o f th e 2 0 th
cen tu ry (sev era l d eca d es a fter th e va n d er W a a ls eq u a tio n ) d id p h y sicists sta rt w o n d erin g a b o u t th e in terp re-
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Figure 5. Structure of (LiF)2
is experimentally observed
to be cyclic as shown in the
right-hand side where the
twodipolesareorientedantiparallel. +/– represent a
partial positive/negative
charge.
ta tio n o f va n d er W a a ls' u n iv ersa l m o lecu la r a ttra ctio n .
T h o u g h m o lecu les d o n o t h ave a n y n et p o sitiv e o r n eg a tiv e ch a rg e, it w a s rea lized th a t th ey co u ld b e rig id
a ssem b ly o f ch a rg es w ith n o resu lta n t n et ch a rg e. W e
co u ld h av e a p a rtia l + v e ch a rg e in o n e en d a n d a n eq u iv a len t n eg a tiv e ch a rg e in a n o th er en d crea tin g a d ip o le
m o m en t a s sh ow n fo r th e m o lecu le L iF in F igu re 5 . T h e
lith iu m a to m ca rries a p a rtial p o sitiv e ch a rg e in th is
m o lecu le (± + ) a n d th e ° u o rin e a to m ca rries a n eq u iv a len t n eg a tiv e ch a rg e (± ¡ ). If a n o th er L iF m o lecu le
co m es n ea r, it w o u ld p referen tia lly a lig n in su ch a w ay
th a t th ere is a n et a ttra ctio n as sh ow n in F igu re 5 . T h e
d ip o les a re o rien ted a n ti-p a ra llel fo r n et a ttra ctio n . A t
a h ig h en o u g h tem p era tu re, L iF m o lecu les co u ld ro ta te
freely lea d in g to a ll p o ssib le m u tu a l o rien ta tio n s a n d
h en ce n o n et a ttra ctio n . A t low er tem p era tu res, o rien ta tio n s w ith n eg a tiv e p o ten tia l en erg y w o u ld b e p referred .
W H K eeso m in 1 9 1 2 co n sid ered th is scen a rio a n d d eriv ed a n ex p ressio n fo r a ttra ctio n b etw een tw o d ip o les
[1 2 ]. T h is a ttra ctio n is electrosta tic in n a tu re a n d it is
p a rt o f electro sta tic in tera ctio n s { sp eci¯ ca lly a d ip o led ip o le in tera ctio n .
6 .2 H ig h e r a n d In d u ced M o m e n ts
M o lecu les th a t d o n o t h av e a n electric d ip o le m o m en t
(su ch a s C H 4 , C 6 H 6 a n d C O 2 ) a lso co n d en se a t low
tem p era tu res. H en ce, in term o lecu la r fo rces m u st ex ist
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Figure 6. Schematic representation of a quadrupole
as two pairs of positive and
negative charges separated in space.
fo r n o n -p o la r m o lecu les a s w ell. T h ese m o lecu les co u ld
h av e h ig h er m o m en ts, i.e., b o th C 6 H 6 a n d C O 2 h av e a
q u a d ru p o le m o m en t w ith tw o p a rtia l p o sitiv e a n d tw o
eq u iva len t n eg a tiv e ch a rg es a rra n g ed a ltern a tely, lea d in g
to a n et resu lta n t zero ch a rg e (F igu re 6 ). C H 4 h a s a n
o ctu p o le m o m en t, n a tu ra lly in v o lv in g eig h t a ltern a tin g
p o sitiv e a n d n eg a tiv e ch a rg es. T h ese ca n ea sily b e v isu a lized a s a tetra h ed ro n h av in g a ll its fo u r co rn ers ca rry in g a p a rtia l p o sitiv e ch a rg e a n d a ll th e fo u r fa ce cen tres ca rry in g a n eq u iva len t n eg a tiv e ch a rg e (see F igu re 4
in set fo r tetra h ed ro n stru ctu re). H ow ev er, th e in tera ctio n s fro m q u a d ru p o le a n d o ctu p o le m o m en ts, th o u g h
rea l, a re m u ch w ea k er.
D eb y e rea lized th a t th e m o lecu les a re n o t sta tic a n d th e
ch a rg e d istrib u tio n w ith in th e m o lecu les co u ld b e d efo rm ed . A n o n -p o la r m o lecu le co u ld b e p o la rized b y in tera ctio n w ith a ch a rg e o r a d ip o le a n d th is ca n lea d to
a n et a ttra ctiv e in tera ctio n . If th e p o la rizin g so u rce is a
d ip o le, th is in tera ctio n w o u ld b e ca lled a `d ip o le-in d u ced
d ip o le in tera ctio n '. D eb y e d eriv ed a n ex p ressio n fo r th e
p o ten tia l en erg y fo r th is d ip o le-in d u ced d ip o le in tera ctio n in 1 9 2 0 [1 3 ]. A co m p lex fo rm ed b etw een th e in ert
g a s A r a n d H F , w h ich co u ld b e w ritten a s A rH F is a
cla ssic ex a m p le fo r th is in tera ctio n . N a tu ra lly, th e in d u ced m o m en ts co u ld a lso b e o f h ig h er o rd er su ch a s
q u a d ru p o le, o ctu p o le, etc.
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In 1923, London came
up with an explanation
for the interaction
between two rare gas
atoms. He reasoned
that these atoms
should have an
instantaneous
distribution of charges
that is not spherical,
though on average it
would be spherical.
6 .3 D isp e rsiv e F o rce s
E lectro sta tic in tera ctio n s b etw een p erm a n en t a n d in d u ced m u ltip o le m o m en ts co u ld ex p la in th e in term o lecu la r fo rces to a la rg e ex ten t. H ow ev er, th ey co u ld n o t
ex p la in w h y ra re g a s a to m s su ch a s A r a n d H e co n d en se. T h ese h av e sp h erica l ch a rg e d istrib u tio n s. In
1 9 2 3 , L o n d o n ca m e u p w ith a n ex p la n a tio n fo r th e in tera ctio n b etw een tw o ra re g a s a to m s [1 4 ]. H e rea so n ed
th a t th ese a to m s sh o u ld h av e a n in sta n ta n eo u s d istrib u tio n o f ch a rg es th a t is n o t sp h erica l, th o u g h o n a v era g e
it w o u ld b e sp h erica l. T h u s a n in sta n ta n eo u s d ip o le m o m en t in a n A rg o n a to m ca n in tera ct w ith th a t o f a n eig h b o u rin g A r a to m . T h ese in sta n ta n eo u s d ip o le m o m en ts
n eed to b e co rrela ted fo r a n et a ttra ctiv e in tera ctio n .
T h e fo rces th a t a rise fro m th e co rrela tio n o f in sta n ta n eo u s d ip o les a re ca lled `L o n d o n d isp ersiv e fo rces'.
If a n a to m ca n h av e a n in sta n ta n eo u s d ip o le m o m en t,
th ere is n o rea so n w h y a m o lecu le ca n n o t. C lea rly, a ll
m o lecu les ca n h av e a ttra ctio n fro m electro sta tic (in tera ctio n b etw een p erm a n en t m u ltip o le m o m en ts), in d u ctiv e (in tera ctio n b etw een p erm a n en t a n d in d u ced m u ltip o le m o m en ts) a n d d isp ersiv e (in tera ctio n b etw een in sta n ta n eo u s m u ltip o le m o m en ts) fo rces. W h ile th ese
fo rces ca n b e a ttra ctiv e o r rep u lsiv e d ep en d in g o n th e
o rien ta tio n (see F igu re 5 ), a s th e m o lecu les a p p ro a ch
ea ch o th er, stro n g rep u lsiv e fo rces b etw een th em sta rt
d o m in a tin g d u e to th e ov erla p o f th e electro n clo u d
a ro u n d th em . T h is ex p la in s w h y th e m o lecu les d o n o t
co a lesce in to ea ch o th er b u t reta in th eir id en tity b y k eep in g a d ista n ce b etw een th em ev en w h en th ey a re co n d en sed in to liq u id s o r so lid s. T h e o p tim u m d ista n ce
b etw een th em is w h ere th e n et b in d in g en erg y is th e
h ig h est fo r a g iv en m o lecu le.
6 .4 H y d ro g e n B o n d in g
T h o u g h va n d er W a a ls fo rces o p era te in b o th co n d en sed
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FEATURE  ARTICLE
a n d g a s p h a ses, va n d er W a a ls m o lecu les ty p ica lly refer to th e w ea k ly -b o u n d co m p lex es fo rm ed a t v ery low
tem p era tu res in a su p erso n ic ex p a n sio n . T h e b in d in g
en erg y is ty p ica lly v ery low ra n g in g fro m a s lo w a s 1 k J
m o l¡ 1 to a b o u t 4 2 k J m o l¡1 , th o u g h th ere is n o h a rd a n d
fa st ru le a b o u t th ese lim its. C lea rly th e va n d er W a a ls
m o lecu les a re b o u n d b eca u se o f electro sta tic, in d u ctiv e
o r d isp ersiv e fo rces a n d m o re o ften d u e to a co m b in a tio n
o f a ll th ese fo rces.
W h a t is th e n a tu re o f th e in term o lecu la r fo rces in w a ter,
i.e., w h a t is h y d ro g en b o n d in g ? A lo o k a t th e stru ctu re
o f (H 2 O )2 a n d co m p a riso n w ith th a t o f (L iF )2 g iv es a n
im p o rta n t clu e! It is n o t sim p le electro sta tic in tera ctio n .
If it w ere, th e w a ter d im er w o u ld h av e h a d a stru ctu re in
w h ich th e d ip o le m o m en ts o f th e tw o H 2 O m o lecu les a re
a n ti-p a ra llel. In w a ter d im er w e h a v e o n e H a to m fro m
th e ¯ rst w a ter m o lecu le, w h ich h a s a n et p a rtia l p o sitiv e
ch a rg e p o in tin g tow a rd s th e electro n rich reg io n in th e
o th er w a ter m o lecu le, i.e., tow a rd s th e lo n e p a ir o f electro n s in th e ox y g en a to m . O n e co u ld co m p a re (L iF )2
w ith (H F )2 ; o b serva tio n s a re sim ila r. T h e H F d im er h a s
F -H ...F lin ea r a n d th e L iF d im er is cy clic. C lea rly th e
d ip o le-d ip o le in tera ctio n d o m in a tes th e la tter a n d so m eth in g m o re is in v o lv ed in th e fo rm er, i.e., h y d ro g en b o n d in g . B u ck h in g h a m h a d sh ow n so m e tim e a g o th a t th e
in clu sio n o f m u ltip o le m o m en ts o f H 2 O a n d H F in th e
in term o lecu la r in tera ctio n w o u ld lea d to th e ex p erim en ta lly o b serv ed lin ea r stru ctu res fo r th e resp ectiv e d im ers
[1 5 ]. H ow ev er, th ere h av e b een sev era l cru cia l ex p erim en ta l ev id en ces in th e la st d eca d e w h ich p o in t to th e
p a rtly cova len t n a tu re o f h y d ro g en b o n d in g , th o u g h it
w o u ld co n trib u te o n ly to a sm a ll ex ten t (a b o u t 5 { 1 0 % )
in th e to ta l in tera ctio n [1 6 ]. T h u s h y d ro g en b o n d in g in v o lv es a ll th e in term o lecu la r fo rces d iscu ssed a b ov e a n d
in a d d itio n th e ch em ica l fo rces in th e fo rm o f a p a rtia l
cova len t b o n d .
RESONANCE  April 2009
What is the nature of
the intermolecular
forces in water, i.e.,
what is hydrogen
bonding? A look at
the structure of (H2O)2
and comparison with
that of (LiF)2 gives an
important clue! It is
not simple
electrostatic
interaction. If it were,
the water dimer would
have had a structure
in which the dipole
moments of the two
H2O molecules are
anti-parallel.
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FEATURE  ARTICLE
7 . C o n c lu sio n
C lea rly h y d ro g en b o n d in g co u ld b e co n sid ered a s p a rt
o f th e va n d er W a a ls fo rces, th o u g h it is n o t a w id ely a ccep ted v iew a m o n g ch em ists. In th e su b seq u en t a rticles
in th is series, w e p la n to p resen t so m e ex a m p les o f v a n
d er W a a ls m o lecu les. O n e ex a m p le ea ch fo r sy stem s w ith
p red o m in a n tly d ip o le-d ip o le, d ip o le-in d u ced d ip o le, d isp ersio n a n d h y d ro g en b o n d in g in tera ctio n s w ill b e d iscu ssed in d eta il. F o r co m p leten ess, it is a lso p la n n ed to
in clu d e o n e ex a m p le ea ch fo r cova len t a n d io n ic b o n d in g w ith in a m o lecu le. T h ese a rticles w ill a p p ea r in th e
fo rth co m in g issu es.
Suggested Reading
[1]
[2]
C N R Rao, Understanding Chemistry, University Press, Bangalore,
2001.
R A Baum, Chemical and Engineering News, Vol. 82, p.5, 2004.
[3]
J L Proust, Ann. chim., Vol.32, p.26, 1799.
[4]
J A Dalton, New System of Chemical Philosophy, Manchester, Vol.1,
1808.
[5]
R R Sinden, DNA Structure and Function, Academic Press, New Delhi,
2006.
[6]
K Müller-Dethlefs and P Hobza, Chem. Rev., Vol.100, p.4253, 2000.
[7]
[8]
Source: Wikipedia http://en.wikipedia.org/wiki/Ideal_gas_law.
J D van der Waals, Ph.D. Thesis, Univ. Leiden, (1873) and J D Van
der Waals, Physical Memoirs, Physical Society of London, Vol.I,
Longmans Green, London, 1890.
[9]
E Arunan, P K Mandal, M Goswami and B Raghavendra, Proc. Ind.
Nat.Sci. Acad., Vol.71A, p.377, 2005.
[10]
R F W Bader, Atoms in Molecules: A Quantum Theory, Clarendon
Press, Oxford 1990.
[11]
B Chu, Molecular Forces, John Wiley, New York, (based on the Baker
Lecture series presented by P J W Debye at Cornell Uni-versity), 1967.
[12]
J Israelachvili, Surface and Intermolecular Forces, Academic Press,
London II Edition, Vol.28, 1991.
[13]
[14]
P W Debye, Z. Physik, Vol.21, p.178, 1920.
F London, Z. Physik, Vol.63, p.240, 1930. (See References 11 and 12
for more details.)
[15]
A D Buckingham and P W Fowler, Can J. Chem., Vol.63, p.2018, 1985.
[16]
E Arunan, Curr. Sci., Vol.77, p.1233, 1999.
Address for Correspondence
E Arunan
Department of Inorganic and
Physical Chemistry
Indian Institute of Science
Bangalore 560 012, India
Email:
[email protected]
[email protected]
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RESONANCE  April 2009