Ebook A Handbook of spectroscopic data chemistry Part 1

(BQ) Part 1 book A Handbook of spectroscopic data chemistry has contents: Ultraviolet spectroscopy, infrared spectroscopy, proton magnetic resonance spectroscopy. Please refer to the content. BQ) Part 1 book A Handbook of spectroscopic data chemistry has contents: Ultraviolet spectroscopy, infrared spectroscopy, proton magnetic resonance spectroscopy. Please refer to the content.

Edition 2009

Oxford Book Company

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1

Ultraviolet Spectroscopy

1.1 Calculating Absorption Maxima of Unsaturated Compounds

Dienes and trienes : If the compound is suspected to be a conjugated

or substituted diene, its wavelength of maximum absorption can be predicted with the help of Table I I To be able to use this table, one must first learn to recognize different types of dienes, conjugations, double bonds, etc These are as follows:

i) ) c=c< _ ) C=C< A linear conjugation; for example, 1,3,5

a double bond is a part of the ring under

v)

vi)

consideration, such a double bond is called

an exocycIic double bond

A homoannular diene is one in which the two double bonds are conjugated and are in

a single ring

Note that both double bonds are exocycIic to ring B

A heteroannular diene is a conjugated system

in which the two double bonds belong to two different rings However, these double bonds are also exocycIic, one of them being exo-

Table 1.1: Woodward's and Fieser's rules for Diene absorption (ethanol

iv) ExocycIic double bond (effect is two fold ifbond is

v) Substitutents on Sp2 hybridised carbon atom, per substituent

Ultraviolet Spectroscopy 3

, The following points are to be noted:

only For other rings the values are:

228 nm

241 nm

Amax value usually agrees with the observed value

Limitations

but not so for other rings in some cases

sometimes very greatly if the strain is high A simple example of this is 1,2 dimethylene cyclohexane, which gives a strong UV band

at Amax 220 nm (E 10,050) which is quite different from the calcualted value

Polyenes : The above rules (Table 1.1) holds fairly well for unsaturated compounds containing up to four conjugated double bonds However, for systems of extended conjugation, such as those found in carotenoid pigments, Fieser and Kuhn have suggested equations to calculate the basic Amax and

Em ofUV absorption

Where

>c=o (-one), as in

P a

with two double bonds (-cliene), such as

Parent enone (acyclic or rings larger than 5 members)

5-membered cyclic enone

Aldehydes

Acid and Esters

Increments for Double bond extending conjugation (for eacb one)

Homodiene component

Exocyclic double bond (or any >C=C< endocyclic to 5- or

7-member ring in a case of acid and ester)

+5nm +IOnm +12nm

+ISnm +35nm +30nm 50nm +35nm +30nm + 17nm 31nm

Solvent Corrections (Enones) Solvent

+30nm variable

There are two types of aromatic molecules: benzenoid and nonbenzenoid Their spectra show considerable resemblance In fact, the presence or absence of certain features in UV spectra, such as a low intensity band (known as a fine structure band) at or about 255 run, is often used to detect the aromatic character of an unknown substance

Benzene Chromophore

fine-structure band), with a series offine-structue bands between 230 and 270 nm)

(electron-donating or electron-withdrawing character) shift the primary band

(204 nm) to longer wave lengths With polar substituents, e.g -N~, -OH, etc which allow for the n ~ 1t conjugation and -C = 0 and -N02 where polarisability is of importance, absorption due to electron transfer transitions

is apparent These two types of transfer can be expressed as shown below:

Chromophore substitutents:

Auxochrome substituents:

Table 1.3: Scotts rules for calculation ofAmax ofthe ET (electron transfer)

band of aromatic carbonyl compounds

Ar-C-Z

II

o

Parent chromophore : Ar=C6HS

Z = Alkyl or ring residue, (e.g.; ArCOR)

Z = H, (Ar CHO)

Z = OH, OAlk, (ArCOOH and ArCOOR)

Increment for each substitutent on Ar:

- Alkyl or ring residue 0-,

m OH, -OAlk

0- (oxyanion) •

p-0-, p-o-m-p-

m-In heterocyclic chemistry model compounds are

246 run essential for the

inter-250 run pretation of most spectra

No rules are available for

I Dienes

(1) Abietic acid

Basic heteroannular diene

Calculated Amax Observed

214nm 05nm 20nm 239nm

241 nm [Chromophore is shown by heavy lines; numbers indicate substituents.]

(2) Ergosterol

HO

Basic homoannular diene

253 nm lOnm 20nm

283 nm

282 nm

Calculated "'-max Observed 3.3, j3-Acetoxyergosta-5,7,14,22-tetraene

II Polyenes

homoannular and heteroannular double bonds, the diene system which requires least energy for excitation (i.e the one with the longer wavelength of absorption)

is used as a base

Basic homoannular diene

253 nm

15 nm 25nm 30nm Extra double bond in conjugation

Calculated "'-max Observed

323 nm 319nm

(1) All trans j3-carotene

Ultraviolet Spectroscopy 9

*

Basic Amax value

M = number of alkyl substituents, 5 x 10 add

n = number of conjugated double bonds,

114 nm 40nm

(Note: Double bonds at ends are not in conjugation with others)

substract substract Calculated Amax Observed

8 (l x 18) Calculated A EtOH max

add add add add

Observed

215 nm

24 nm 05nm 244nm

241 nm

215 nm 30nm 39nm

Ultraviolet Spectroscopy 11

Dicarbonyl compounds

Diosphenol Parent base

Ultraviolet Spectroscopy 13

However, in the case of acetyl acetone agreement with the calculated wavelength (257 nm) is indifferent This may be due to the fact that the strong internal hydrogen bond forces the carbonyl group and the double bond into a configuration different from that which is present in cyclic structures, e.g., diosphenol exists almost entirely in the enolic form

1,3-cyclohexanedione, absorbs at 253 nm (Emax 22,000) in ethanol

242 nrn and a weak R-band near 434 nm

IV Aromatic carbonyl compounds

Base value NMe2 inp

Base value NH2 in p

246nm 02nm

03 nm

251 nm

250nm 85nm

335 nm

230nm 58nm 288nm Another approach to predicting, the "-max of the primary band of substituted benzenes involves the use of Table 1.5 This table has been successfully used with disubstituted compounds when the following rules are used:

I Para substitution:

Only the effect of the group causing the larger shift is used For example, the "-max ofp-nitrobenzoic acid would be expected to be

alcohol solvent)

The shift in the primary band of such a disubstituted benzene is usually greater than the sum ofthe shifts caused individually by the two groups Such large shifts in p-disubstituted benzens are attributed to interaction resonance, as illustrated below:

Ultraviolet Spectroscopy 15

2 Ortho and Meta substitution: The shift effects are additive

Table 1.4: Absorption characteristics of some polycyclic aromatic

Table 1.5: Calculation of the Primary Band (1[ ~ 1[*Transition) of

BBand Emax 3,200 1.960 Submerged 4,500 1,450 Submerged Table 1.6: Absorption characteristics of Aromatic systems and their

Transition KBand

Transition BBand

A max E max

(nm)

Ultraviolet Spectroscopy 17

Compound Solvent Primary band Secondary band

n ~ n* n ~ n* n ~ n*

Transition Transition Transition

These weak bands may be due to impurities rather than a forbidden

Ultraviolet Spectroscopy 19

Spectra of nonbenzenoid aromatic hydrocarbons show considerable resemblance to spectra ofbenzenoid compounds, Tropolone and its derivatives

nm (Emax ca 8,000); the latter absorption is characterized by the group of fine structure bands typical of aromatic systems

Azulene and its derivatives have complicated spectra consisting of a number of relatively intense bands throughout most of the ultraviolet region (up to 360 nm) and a number of relatively weak bands throughout most ofthe visible region (500 - 700 nm) As a consequence of the latter, azulene and most of its derivatives are blue

azulene

1.4: Approximate lower cutoff wavelengths* for commonly used solvents

in UV-visible spectroscopy of organic substances

Cutoff wavelength, nm

Nitromethane

Cutoff wavelength, nm

Exercises and problems:

(2) Identify which one of the following two isomers has the electronic

Ultraviolet Spectroscopy 21

(1)

(3) Which structural features may produce a bathochromic or a chromic effect in an organic compound

hypso-(4) Aniline absorbs at 230 nm (E8600), however, in acid solution the main

benzene Explain

(5) o-Nitrophenol gives a UV band at t max 350 nm when the spectrum is recorded in 0.1 N HCl solution, but the band is observed at t max 415

nm in 0.1 N NaOH solution Explain

(6) Optical whiteners are used to make white clothes appear brighter Can you suggest an explanation for the brightening action?

(7) Explain the substitution pattern on the following enone and calculate the position ofK band

(8) The position of absorption of acetone shifts in different solvents: 279

nm (hexane); 272 nm (ethanol) and 264.5 (water) Explain

(9) At what wavelength the coloured compounds absorb?

derivative however, the absorption pattern becomes almost similar to o-xylene Explain

Biphenyl

o-xylene

Emax ~ 19,000

planar conformation

"-max 262, emax 270 (II) How will you confirm the presence of a-diketone system in the following steroid?

(12) Why the Amax for the diene (I) is observed at lower run than (II)

0-0 (I)

(13) The following triene on partial hydrogenation gives three products Which are separated by glc How UV spectroscopy and application of Woodward-Fieser rules will help to identify the products

GO

UltravIolet Spectroscopy 23

Answers to the problems

( I) In compound (I) the unshared electron pair of the nitrogen atom in the

conjugation) While in compound (2) this conjugation cannot be complete owing to the methyl group at the ortho position

When methyl at the ortho position is replaced by a more bulky group, namely, isopropyl, the peak wavelength is shifted still further (owing to

280nm

(3) A bathochromic shift (red shift) may occur by a change of medium or

by the presence of an auxochrome A hypsochromic shift (blue shift) may be caused by a change of the medium or by such structural changes like removal of conjugation

~ ~ ') NH3HSO;

Due to the removal of conjugation of the lone pair of electrons on the nitrogen atom of aniline with the 1t-bond system of the benzene ring on protonation, the main absorption band is seen at 203 nm (e7500) and is comparable with benzene

(5) In 0.1 N HCI solution o-Nitrophenol gives a UV band at Amax 350 nm

due to n 1[ conjugation

Conversion of a phenol to the corresponding anion (i.e in 0.1 N NaOH

solution) results in a bathochromic shift of the E2 and B bands and an

increase in Emax because the nonbonding electrons in the anion are

available for interaction with the 1[ electron system of the ring Therefore

band is observed at 4 I 5 nm in 0.1 N NaOH soln

(6) Singlet excited state of the brightening agent is converted into triplet

state which emits radiation in the visible range

(7) One a-substituent, two (3-ring residues and one exocyclic double bond:

215 + 10 + 24 + 5 = 254 nm

(8) This is the expected shift of the n ~ 1[* transition of acetone to shorter

wavelength (blue shift) by changing to solvents of increased polarity

(9) Longer than 400 nm

(10) Although biphenyl is slightly twisted, the angle of twist is small,

therefore, conjugation between the rings is not affected Biphenyl thus

shows a very intense absorption band at 252 nm (K-Band) Biphenyl

derivatives with bulky substituents in the ortho positions are more stable

in twisted conformations than in the planar conformation, which suffers

serious non-bonded compressions from the juxtaposed substituents The

loss of conjugation in the twist conformation of2, 2-dimethylbiphenyl

is reflected in its UV spectral data, which now structurally is like two

Ultraviolet Spectroscopy 25

281 nm ( Emax 9,700) which matches with the Cal "-max (on enolisation the double bond becomes exocyclic to one ring) On acetylation the spectrum is restored to that calculated for the system now with OAC in the a-position Further confirmation will come from the measurement

of the spectrum of the enolised form in alkaline solution which will show

(II), the two double bonds are exocyclic, thus in it "-max will be higher

Infrarred Spectroscopy 27

Table 2.1a: Characteristic infrarred absorption frequencies of common

classes of organic compounds

Skeletal 1175-1140 8.51-8.77 s., br

840-790 11 90.12.66 m -C(CH3)3 C-Hdef 1395-1365 7.17-7.33 s, d ratio 1:2

Skeletal 1255-1200 7.97-8.33 s., br., d

750-720 13.33-13.89 s., br -CH2- C-H str., asym 2940-2915 3.40-3.45 m strong if

several C-H str., sym 2870-2845 3.49-3.52 m.-CH2-

present C-H, Scissoring 1480-1440 6.76-6.94 m C-H, twisting & Ca 1250 Ca 8.00 m wagging

Skeletal, if 750-720 13.33-13.89 s -(CH2)4 or more

See Table 2.2a & 2.2b

3 Alkynes and allenes C-H str 3310-3300 3.02-3.03 m Terminal/

Allenes (C=C=Cj c=c type str.,

antisym 1970-1950 5.08-5.13 m C-C type str., sym 1060 9.43 m

4 Aromatic hydrocarbons

See Table 2.3

5 Alcohols and phenols

Free-OH O-H str 3650-3590 2.74-2-79 V., sh Intermolecularly

hydrogen bonded

(Change on dilution)

Dimeric (Single bridge

compounds) O-H str 3550-3450 2.82-2.90 v., sh Polymeric O-H str 3400-3230 2.94-3.10 s., br Intramolecularly

hydrogen ,bonded

(no change on dilution)

single bridge

compounds O-H str 3570-3450 2.80-2.90 V., sh chelate compounds O-H str 3200-2500 3.1-4.0 W., br Primary C-O-H O-H def 1350-1260 7.40-7.94 s

C-O str 1075-1000 9.30-10.00 s Secondary C-O-H O-H def 1350-1260 7.40-7.94 s

C-O str 1120-1030 8.93-9.71 s Tertiary C-O-H O-H def 1410-1310 7.09-7.63 s

C-O str 1170-1100 8.55-9.09 s

C-O str 1230-1140 8.13-8.77 s

6 Ethers and epoxides

Acyclic CHJ-O-CHJ R-O-R str.asym 1150-1070 8.70-9.35 s

C-H str 2830-2815 3.54-3.55 m Aryl and aralkyl Ar-O-R str.,

Ar-O-Ar asym 1260-1200 7.98-8.33 v-s

= C-H str 3150-3050 3.18-3.28 w Conjugated C-O-C str 1275-1200 7.84-8.33

Bonded OH O-H str 3333-2500 3.00-4,00 w" br

fine structure bands2700-2500 3,70-4-00 w

Solid fatty acids CH2 vib 1350-1180 740-8.48 w

characteristic pattern -COOH C-O str plus

O-H def 1440-1395 6,94-7.17 w

1320-1210 758-8.26 s, Carboxylate ion O=C-O str., asym, 1610-1550 6.21-6.45 s

O=C-O str., sym, 1420-1300 7,04-7,69 m, c) Esters

Fonnates C-O str, 1200-1180 8,33-8.48

Acetates C-O str, 1250-1230 8,00-8.13 s, Propionates and

higher esters C-O str, 1200-1170 8 33-8,55 s,

primary, free NH N-H str 3540 - 3480 2.83-2.88 s

3420-3380 2,92-2.96 s bonded NH N-H str 3360-3320 2.98-3.01 m

3220-3180 3.11-3.15 m Free or bonded N-H def plus

(amide II band) 1620-1590 6.17-6.29 s (amide II band)

Secondary, free NH N-H cis str 3440-3420 2.91-2.93 s

N-H, trans str 3460-3430 2.89-2.92 s bondedNH N-H, cis str 3180-3140 3.15-3.19 m

N-H, trans str 3330-3270 3.00-3.06 m N-H, cis and

trans str 3100-3070 3.23-3.26 w N-H def Ca.700 Cal4.30 conc

(amide V band) dependent

N -H def plus 1305-1200 7.67-8.33 m C-N str (amide III band)

Acyclic compound N-H def plus 1570-1515 6.37-6.60 s

C-N str (amide II band)

1550-1510 6.45-6.62 s (amide II band)

8 Amines, amino acids and their salts see table 2.6

9 Unsaturated nitrogen compounds

b) Oximes, pyridines, quinolines purines, pyrimidies elc

10 Compounds containing nitrogen-oxygen bond

Nitrates (O-NOzl N02 str asym 1590-1500 6.29-6.67 s nitramines.nitro N02 str sym 1390-1250 720-8.00 \\

(compounds CC-N02) C-N vib 920-830 1088-12.05 m -s Nitroso compounds (R-C-N=O)

alkyl, aromatic N=O str 1550-1500 6.45-6.67 s (a-halogeno aliphatic N=O str 1620-1560 6 I 7-6.47 s Nitrites (R-O-N=O)

trans form N=O str 1680-1650 5.95-6.05 v • Os,

N-O str 815-750 1227-13.33 s O-N=O def 625-565 16.00-17.70 s Cis form N=Ostr 1625-1610 6.16-6.21 V., s

N-O str 850-810 1 I 76- I 2.35 s O-N=O def 690-615 14.49- I 6.26 s overtone 3360-3220 2.98-3.11 m Nitrosamines N=O str 1500-1480 6.67-6.76 s.Vapor

phase (R-N-N=O) 1460-1440 6.85-6.94 s solution

phase N-N str Ca 1050 Ca 9.52 s N-N = 0 def Ca 660 Ca 15.15 s Azoxy compo N-O str 1310-1250 7.63-8.00 moo-s (R-N-N-O)

Covalent sylfates S=O str., sym 1440-1350 6.94-7.41 S

(RO)z S02 S=O str., as)'m 1230-1I50 8.13-8.70 S

Covalent sylfonates S=O str sym 1420- 1330 7.04-752 S

(R,-O-S02-R2) S=O str asym 1200-II45 8.33-8.73 S

Sulfonyl chlorides S=O str., sym 1375-1340 7.27-7.46 S

(R-SOzCI) S=O str asym I 190-II60 8.40-8.62 s Sylfonamides S=O str sym 1370-1300 7.30-7.69

Inti-arred Spectroscopy 33

monochlorinated

14 Silicon and boron compounds

def = deformation asym.=asymmetric m.=medium absorption

Table 2.1b Correlations of infrared absorption and structure of organic

Oximes (R-C = NOH) R-OH, Ar-OH, intramolecular hydrogen bonded

Carboxylic acids, free OH R-OH, Ar-OH, dimeric Ketones, C=O overtone Primary amides, free NH Primary amines, free NH secondary amines Sec amides, free NH (trans) Nitrites (R-O-N=O) overtones

Sec ami des, free NH(cis) Pyrroles Imines R-OH, Ar-OI-{' polymeric

Bond

O-H str O-H str

O-H str O-H str O-H str C=O str N-H str

N-H str N-H str N=O str N-H str N-H str N-H str O-H str