Spectral PHC characteristics using Hy Logging Imaging Spectroscopy

2. Spectral PHC characteristics using HyLogging Imaging Spectroscopy

Polysaturated aromatic hydrocarbons are a distinctly separate class of chemical substances. Of course, they have spectroscopic features, typical for all hydrocarbons, but certain characteristics related to this class only, also exist. Typical hydrocarbon infrared absorption bands are usually attributed to C-C and C-H bonds stretching and vibration. Table 2 lists major spectral features used in IR/NIR spectroscopy for determining presence of hydrocarbons and aromatic compounds. The latter, in particular are rigid molecules that have very little opportunity for rotational isomerism, and unlike those of aliphatic compounds, arene (aromatic) absorption bands are needle-sharp (Mayo et al, 2003). Vibrational modes present in aromatics, associated with the phenyl ring are created by carbon-hydrogen bending vibrations – in-plane and out of plane bends, carbon-carbon ring bending and stretching modes, including out of plane ring puckering. If one of the ring components is substituted, additional modes arise: C-X stretching, C-X bending (in and out of plane).

The Carbon-Hydrogen stretching vibrations generally occur in 3200-3000 cm-1, with majority being closer to 3000, since the aromatic C-H stretch follows “the 3000 rule” – the sp3 hybridized atoms stretch well below 3000 cm-1. Particularly degenerate stretches can also occur, of first and second orders (see Figure 1 & Figure 2 ).

The most important aromatic ring group frequencies are summarized in Table 1. The 2000-1650 region is essentially a sum of weak bands, and can be used to support assumptions about substitution patterns in benzene ring system, especially at enhanced resolutions (like that of hyperspectral or surface-enhanced spectroscopic technologies).

Arene structureWavelengths

C-H stretch3200-3000

C=C ring stretches1600±10,1580±10, 1500±10, 1450±10

C-H out of plane bend 1-5H900-860, 860-800, 810-750, 770-735, 770-730

C-C ring out-of-plane bend (1-; 1,3-; 1,3,5-substituted)690±10

C-H out-of-plane bend sum tones2000-1650

Table SEQ “Table” \*Arabic 1 : Most important aromatic ring group frequencies. (Mayo et al., 2003)

WavelengthBondStructure

1343C-H second overtimeAromatic

1160C=O stretch 4th overtoneC=O

1170C-H second overtone.HC=CH

1195C-H second overtone.CH3

1215C-H second overtone.CH2

1225C-H second overtone.CH

1360C-H combination.CH3

1395C-H combination.CH2

1410O-H first overtoneROH, Oil

1415C-H combination.CH2

1417C-H combinationAromatic

1420O-H first overtoneArOH

1440C-H combination.CH2

1446C-H combinationAromatic

1450C=O stretch 3rd overtoneC=O, Water

1492N-H stretch 1st overtoneArNH2

1620C-H stretch 1st overtone=CH2

1685C-H stretch 1st overtoneAromatic

1695C-H stretch 1st overtone.CH3

1705C-H stretch 1st overtone.CH3

1725C-H stretch 1st overtone.CH2

1765C-H stretch 1st overtone.CH2

1790O-H combinationWater

1940O-H bend 2nd overtoneWater

2070O-H combinationOil

2140C-H deformationOil

2170Asym C-H stretch/C-H deformation combinationHC=CH

2310C-H bend 2nd overtoneOil

2380C-H stretch/C-C stretch combinationOil

Table SEQ “Table” \*Arabic 2 : Basic characterizing wavelengths for hydrocarbons, aromatics and water. (Shenk et al.,2001)

Regarding the mineralogy of rocks, certain complications emerge, since these also contain other minerals consistent of metals and non-metals, especially Si, carbonates, phosphates, borates, arsenates, etc. The rocks may also be contaminated with soil, bacteria and traces of vegetation – green (wet, photosynthetic), and dry. Green vegetation’s spectra are dominated with water-vibrational absorptions, which may be shifted to slightly shorter wavelengths because of hydrogen bonding. Chlorophyll from plants …
Posted by: Mackenzie Dahms

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